1. Temperature Scales

Temperature Scales

Temperature is a measure related to the average kinetic energy of the particles in a substance.

All matter is made of particles that are constantly moving. When particles have greater average kinetic energy, the temperature is higher. When they have less average kinetic energy, the temperature is lower.

Scientists and everyday users measure temperature using several different scales. The three most common are:

• Celsius (°C)
• Kelvin (K)
• Fahrenheit (°F)

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Temperature and Particle Motion

Particles in matter possess kinetic energy because they are moving.

Temperature is related to the average kinetic energy of these particles.

In general:

Higher temperature → Greater average kinetic energy

Lower temperature → Lower average kinetic energy

For example, particles in hot water have greater average kinetic energy than particles in cold water.

It is important to use the word average. Not every particle in a substance is moving at exactly the same speed.

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Temperature Is Not Heat

Temperature and heat are related, but they are not the same thing.

Temperature

A measure related to the average kinetic energy of particles.

Heat

Energy transferred from a higher-temperature object or region to a lower-temperature one because of a temperature difference.

For example, when a hot metal spoon is placed in cool water:

Hot spoon → Energy transfer → Cooler water

Energy moves from the hotter object to the cooler surroundings until they approach the same temperature.

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Measuring Temperature

A thermometer is used to measure temperature.

Different thermometers may use:

• expanding liquids
• electrical resistance
• thermocouples
• infrared radiation
• digital temperature sensors

Regardless of the method used, the measurement must be expressed using a temperature scale.

The most important temperature scales are Celsius, Kelvin, and Fahrenheit.

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The Celsius Scale

The Celsius scale is widely used for:

• weather
• cooking
• room temperature
• refrigerators
• everyday measurements
• many laboratory measurements

Its symbol is: ^oC

At standard atmospheric pressure, two familiar reference temperatures are approximately:

Water freezes: 0^oC

Water boils: 100^oC

$100^\circ\text{C}$

Some familiar temperatures are:

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The Kelvin Scale

The Kelvin scale is the standard temperature scale used extensively in science.

Its unit is the kelvin, with the symbol: K

Notice that we do not write a degree symbol.

Correct: 300 K

Incorrect: 300^oK

The Kelvin scale is particularly useful because it begins at the lowest possible thermodynamic temperature:

absolute zero.

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Absolute Zero

Absolute zero is: 0 K

which is exactly: -273.15^oC

It represents the theoretical lower limit of thermodynamic temperature.

At absolute zero, a system has the minimum possible thermal energy. In the simple particle model used in introductory science, particle thermal motion is described as reaching its minimum.

A more advanced understanding from quantum mechanics shows that particles do not literally become completely motionless.

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Why Can't Temperature Go Below Absolute Zero?

The Kelvin scale is an absolute temperature scale.

For ordinary thermodynamic systems: T ≥ 0 K

You cannot simply keep removing thermal energy until a conventional temperature of -10 K is reached.

Absolute zero represents the lower limit of the thermodynamic temperature scale.

This makes Kelvin particularly useful in scientific calculations.

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Celsius and Kelvin

Celsius and Kelvin use the same size temperature interval.

A change of: 1^oC

is the same size temperature change as: 1 K

The difference is where the scales begin.

Celsius uses: 0^oC

as the freezing point of water under standard reference conditions.

Kelvin uses: 0 K

as absolute zero.

Therefore: 0^oC = 273.15 K

and: 100^oC = 373.15 K

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Converting Celsius to Kelvin

The conversion is: T_K = T_C + 273.15

For many introductory problems, this may be approximated as:

T_K = T_C + 273

Example

Convert: 25^oC to kelvin.

T_K = 25 + 273.15

T_K = 298.15

Often this would be rounded to: 298 K

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Converting Kelvin to Celsius

To convert in the opposite direction: T_C = T_K - 273.15

Example

Convert: 310 K to Celsius.

T_C = 310 - 273.15

T_C = 36.85

Approximately: 37^oC

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Worked Example: A Cold Temperature

Convert: -20^oC to kelvin.

Use: T_K = T_C + 273.15

Substitute:

T_K = -20 + 273.15

T_K = 253.15 K

Notice that a negative Celsius temperature can still correspond to a positive Kelvin temperature.

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Worked Example: Absolute Zero

Convert: 0 K to Celsius.

T_C = 0 - 273.15

T_C = - 273.15

This is absolute zero.

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Comparing Celsius and Kelvin

The relationship between the two scales is linear.

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The Fahrenheit Scale

The Fahrenheit scale is another temperature scale.

Its symbol is: ^oF

It is used mainly for everyday temperature measurements in the United States and a few other contexts.

At standard atmospheric pressure:

Water freezes at: 32^oF

Water boils at: 212^oF

A typical comfortable indoor temperature might be around:

68^oF to 77^oF

which corresponds approximately to:

20^oC to 25^oC

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Comparing All Three Scales

Important reference points can be compared directly.

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Why Does Science Use Kelvin?

Kelvin is particularly important in physics and chemistry because it is an absolute scale.

Many scientific equations require absolute temperature.

For example, relationships involving:

• gases
• thermodynamics
• thermal radiation
• molecular motion

often use temperature measured in kelvin.

Using Celsius in these equations could produce incorrect results because 0^oC does not represent zero thermal energy.

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Choosing the Appropriate Scale

Different temperature scales are useful in different situations.

Everyday Weather

Usually: ^oC in most countries.

Example:

Today's temperature is 31^oC

Cooking

Usually: ^oC or ^oF

 depending on the country and recipe.

Scientific Calculations

Often: K particularly when using equations involving absolute temperature.

Laboratory Measurements

Celsius may be used for direct measurements, while values are often converted to Kelvin when required for calculations.

Everyday Measurements in the United States

Fahrenheit is commonly used for:

• weather
• ovens
• body temperature
• household temperature settings

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Temperature Differences

Because Celsius and Kelvin intervals are the same size:

A temperature increase from: 20^oC → 30^oC is an increase of: 10^oC

In Kelvin: 293.15 → 303.15 K

This is an increase of: 10 K

Therefore: ΔT = 10^oC = 10 K for a temperature difference.

This does not mean that 10^oC = 10 K as absolute temperatures. It means their interval sizes are equal.

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Temperature and States of Matter

Temperature affects the motion and behaviour of particles.

As a substance is heated, particles generally gain kinetic energy.

This can eventually contribute to changes of state such as:

Solid → Liquid → Gas

During cooling:

Gas → Liquid → Solid

However, during a phase change such as melting or boiling, energy can be transferred without immediately increasing temperature because energy is involved in changing the arrangement and interactions between particles.

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Worked Example: Choosing a Scale

Consider each situation.

Recording Outdoor Temperature

Best choice: ^oC for everyday use in most countries.

Calculating Gas Behaviour

Best choice: K because gas equations generally require absolute temperature.

Following an American Cooking Recipe

Likely choice: ^oF because many American recipes specify oven temperatures in Fahrenheit.

Measuring Laboratory Temperature

A scientist might initially record: 25^oC but convert this to: 298 K if the temperature is needed in a scientific equation.

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Common Misconceptions

Kelvin does not use the degree symbol.

Correct: 300 K

Not: 300^oK

Zero Celsius is not absolute zero.

0^oC = 273.15 K

Absolute zero is: 0 K = -273.15^oC

Temperature and heat are not the same thing.

Temperature is related to average particle kinetic energy, while heat refers to energy transfer caused by a temperature difference.

Particles are not necessarily completely motionless at absolute zero.

The simple particle model may suggest this, but quantum mechanics predicts residual motion associated with the lowest-energy state.

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Did You Know?

Scientists have cooled matter to temperatures extremely close to absolute zero.

At these temperatures, matter can display unusual quantum behaviour that is not normally visible at everyday temperatures.

However, reaching exactly: 0 K

is not achievable by a finite sequence of ordinary cooling processes.

Absolute zero therefore represents a fundamental boundary in thermodynamics.

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Key Terms

Temperature – A measure related to the average kinetic energy of particles.

Kinetic energy – Energy an object or particle has because of its motion.

Celsius (°C) – A commonly used temperature scale with water freezing at approximately ^oC and boiling at 100^oC at standard atmospheric pressure.

Kelvin (K) – The SI unit of thermodynamic temperature and an absolute temperature scale.

Fahrenheit (°F) – A temperature scale commonly used for everyday purposes in the United States.

Absolute zero – The lowest thermodynamic temperature, equal to 0K or -273.15^oC.

Heat – Energy transferred because of a temperature difference.

Thermometer – An instrument used to measure temperature.

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Key Takeaways

• Temperature is related to the average kinetic energy of particles.
• Higher temperature generally means particles have greater average kinetic energy.
• The three commonly encountered temperature scales are Celsius, Kelvin, and Fahrenheit.
• Celsius is widely used for everyday temperatures and laboratory measurements.
• Fahrenheit is commonly used for everyday temperatures in the United States.
• Kelvin is especially important for scientific calculations.
• Kelvin is an absolute temperature scale.
• Absolute zero is: 0 K = -273.15^oC
• Celsius and Kelvin have equal-sized intervals.
• Convert Celsius to Kelvin using: T_K = T_C + 273.15
• Convert Kelvin to Celsius using: T_C = T_K - 273.15
• Kelvin temperatures are written without a degree symbol.
• The appropriate temperature scale depends on the context and application.
• Temperature and heat are related concepts, but they are not the same thing.
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2. States of Matter

States of Matter

Everything around us is made of matter. Matter is anything that has mass and occupies space.

Matter can exist in different states, depending largely on the energy, arrangement, and movement of its particles.

The four states we will focus on are:

• Solid
• Liquid
• Gas
• Plasma

The particle model of matter helps explain why these states have different properties and what happens when matter changes from one state to another.

Learning Targets

• I can identify the properties of solids, liquids, gases, and plasma.
• I can describe how particle arrangement differs in each state of matter.
• I can explain how matter changes state when energy is added or removed.
• I can distinguish between melting, freezing, boiling, condensation, sublimation, and deposition.
• I can relate changes of state to the movement and energy of particles.

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The Particle Model of Matter

The particle model describes matter as being made of tiny particles such as atoms, molecules, or ions.

According to the model:

• All matter is made of particles.
• Particles are constantly moving or vibrating.
• There are spaces between particles.
• Particles attract one another.
• Heating generally increases particle kinetic energy.
• Cooling generally decreases particle kinetic energy.

The arrangement and movement of these particles help determine whether a substance behaves as a solid, liquid, or gas.

Explore the particle arrangements of the three familiar states here:

Solids

A solid has:

• A fixed shape
• A fixed volume
• Relatively high density
• Very low compressibility

Examples include:

• Ice
• Iron
• Wood
• Glass
• Salt
• Rock

In a solid, particles are packed closely together.

They are held in relatively fixed positions by forces of attraction between the particles.

However, the particles are not completely motionless.

They vibrate around their fixed positions.

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Why Solids Keep Their Shape

The particles in a solid cannot easily move past one another.

Because their relative positions remain approximately fixed, the solid maintains its own shape.

A block of ice placed inside a bowl does not take the shape of the bowl.

Its particles remain arranged in approximately the same positions.

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Why Solids Are Difficult to Compress

Particles in a solid are already packed very closely together.

There is relatively little empty space between them.

Therefore, it is difficult to push the particles significantly closer together.

This makes most solids:

very difficult to compress

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Liquids

A liquid has:

• A fixed volume
• No fixed shape
• The ability to flow
• Relatively low compressibility

Examples include:

• Water
• Cooking oil
• Mercury
• Ethanol

Liquid particles are still close together.

However, unlike particles in a solid, they are not held in fixed positions.

They can move and slide past one another.

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Why Liquids Flow

Because liquid particles can move past neighbouring particles, a liquid can change shape.

For example, water poured from a bottle into a glass takes the shape of the glass.

However, the particles remain close together.

Therefore, the liquid keeps approximately the same:

volume

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Why Liquids Are Difficult to Compress

Like solids, liquids contain particles that are already close together.

There is relatively little empty space between them.

Therefore, most liquids are difficult to compress significantly.

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Gases

A gas has:

• No fixed shape
• No fixed volume
• Low density compared with most solids and liquids
• High compressibility
• The ability to flow and spread through a container

Examples include:

• Oxygen
• Nitrogen
• Carbon dioxide
• Helium
• Water vapour

Gas particles are much farther apart than particles in solids and liquids.

They move rapidly in many different directions.

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Why Gases Fill Their Containers

Gas particles move freely throughout the available space.

As a result, a gas spreads out until it fills its container.

If gas is transferred from a small container into a larger container, it expands to fill the larger space.

Therefore, gases have:

no fixed shape and no fixed volume

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Why Gases Can Be Compressed

There is a large amount of empty space between gas particles.

When pressure is applied, the particles can be pushed closer together.

Therefore, gases are much easier to compress than solids or liquids.

This is why gases can be stored under pressure in cylinders.

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Plasma

Plasma is a high-energy state of matter containing electrically charged particles.

At extremely high temperatures, atoms can gain enough energy for electrons to become separated from their nuclei.

The resulting mixture contains:

• Positive ions
• Free electrons
• Sometimes neutral particles

This ionized material is called plasma.

Because plasma contains charged particles, it behaves differently from an ordinary gas.

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Where Is Plasma Found?

Plasma may seem unusual on Earth, but it is extremely common in the universe.

Examples include:

• The Sun
• Stars
• Lightning
• Auroras
• Some types of electrical discharge
• Plasma globes
• Some fluorescent and neon-type lighting

Stars consist largely of plasma because their temperatures are extremely high.

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Comparing the States

Solid

Shape: Fixed
Volume: Fixed
Particles: Very close together
Movement: Vibrate around fixed positions
Compressibility: Very low

Liquid

Shape: Takes the shape of its container
Volume: Fixed
Particles: Close together
Movement: Move and slide past one another
Compressibility: Low

Gas

Shape: Fills its container
Volume: Not fixed
Particles: Far apart
Movement: Rapid, random motion
Compressibility: High

Plasma

Shape: Not fixed
Volume: Not fixed
Particles: Contains ions and free electrons
Movement: Rapid movement of energetic charged particles
Special property: Conducts electricity and responds strongly to electric and magnetic fields

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Temperature and Particle Energy

Temperature is related to the average kinetic energy of particles.

When a substance is heated, energy is transferred to it.

Its particles generally gain kinetic energy and move more vigorously.

When a substance is cooled, energy is transferred away from it.

Its particles generally lose kinetic energy and move less vigorously.

Changes in particle energy can eventually cause a substance to change state.

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Changes of State

Matter can change from one state to another.

The six important changes between solids, liquids, and gases are:

• Melting
• Freezing
• Boiling or vaporization
• Condensation
• Sublimation
• Deposition

Explore how each process moves matter between states and whether thermal energy is added or removed:

These are physical changes.

The chemical identity of the substance does not necessarily change.

Ice, liquid water, and water vapour are all still:

H₂O

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Melting

Melting is the change:

Solid → Liquid

Energy must be transferred into the substance.

As a solid is heated, its particles vibrate more vigorously.

Eventually, the particles have enough energy to overcome some of the forces holding them in fixed positions.

They can then move past one another.

The solid becomes a liquid.

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Example of Melting

Consider an ice cube.

Initially, the water molecules are held in the solid structure of ice.

As thermal energy is transferred into the ice, the molecules vibrate more strongly.

At the melting point, the structure begins to break down.

The particles become able to move past one another.

The ice becomes:

liquid water

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Freezing

Freezing is the change:

Liquid → Solid

Energy is transferred from the substance to the surroundings.

As the liquid cools, its particles lose kinetic energy.

Their movement becomes less vigorous.

Eventually, attractive forces organize or hold the particles in relatively fixed positions.

The liquid becomes a solid.

Freezing is the reverse of melting.

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Boiling

Boiling is the change:

Liquid → Gas

Energy is transferred into the liquid.

Particles gain enough energy to overcome the attractive forces holding them within the liquid.

Gas bubbles form throughout the liquid and rise to the surface.

At standard atmospheric pressure, pure water boils at approximately:

100°C

The boiling point can change when pressure changes.

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Boiling and Evaporation

Both boiling and evaporation involve:

Liquid → Gas

but they are not exactly the same process.

Boiling:

• Occurs throughout the liquid
• Occurs at the boiling point for a given pressure
• Produces bubbles within the liquid

Evaporation:

• Occurs at the surface
• Can occur below the boiling point
• Usually occurs more slowly

A puddle can therefore disappear without ever reaching its boiling point.

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Condensation

Condensation is the change:

Gas → Liquid

Energy is transferred from the gas to the surroundings.

Gas particles lose kinetic energy.

They move more slowly and attractive forces become more effective at keeping them close together.

The gas becomes a liquid.

Water droplets forming on the outside of a cold drink are a familiar example.

The water does not normally come through the glass.

Water vapour in the surrounding air cools and condenses on the cold surface.

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Sublimation

Sublimation is the direct change:

Solid → Gas

The substance does not pass through the liquid state.

Energy must be transferred into the substance.

A common example is:

dry ice

Dry ice is solid carbon dioxide.

At ordinary atmospheric pressure, it changes directly from solid carbon dioxide into carbon dioxide gas.

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Deposition

Deposition is the reverse of sublimation:

Gas → Solid

Energy is transferred from the substance to the surroundings.

Particles lose energy and become arranged in a solid structure without first becoming a liquid.

Formation of frost is an important example.

Water vapour in the air can change directly into solid ice on a sufficiently cold surface.

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A Useful Change-of-State Pattern

When energy is transferred into matter:

Solid → Liquid → Gas

These processes include:

Melting

and:

Boiling/Vaporization

Energy is also required for:

Solid → Gas

which is sublimation.

When energy is transferred from matter:

Gas → Liquid → Solid

These processes include:

Condensation

and:

Freezing

Energy is also transferred away during:

Gas → Solid

which is deposition.

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Particle Energy During Melting

A common misconception is that the temperature must continuously increase whenever energy is added.

During melting, this is not necessarily true.

At the melting point, energy transferred into the substance is used primarily to overcome attractive interactions and change the arrangement of the particles.

As a result, the temperature can remain approximately constant while the substance changes state.

Once melting is complete, further heating can increase the temperature of the liquid.

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Particle Energy During Boiling

A similar process occurs during boiling.

At the boiling point, energy continues to enter the substance.

However, much of this energy is used to separate particles and overcome attractive interactions rather than increase their average kinetic energy.

Therefore, the temperature remains approximately constant during the phase change for a pure substance at constant pressure.

After all the liquid has become gas, further heating can increase the gas temperature.

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Heating Curves

A heating curve shows how temperature changes as energy is transferred into a substance.

Typical sections include:

Solid warming

Temperature increases.

Melting

Temperature remains approximately constant.

Liquid warming

Temperature increases.

Boiling

Temperature remains approximately constant.

Gas warming

Temperature increases.

The flat sections correspond to changes of state.

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Cooling Curves

The process can also occur in reverse.

As energy is transferred away:

Gas cools

↓

Condensation

↓

Liquid cools

↓

Freezing

↓

Solid cools

During condensation and freezing, temperature can remain approximately constant while the state changes.

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Worked Example — Identifying a State Change

A sample of ice is heated.

Its temperature increases until it reaches its melting point.

Heating continues, but the temperature remains constant for several minutes.

What is happening?

The substance is:

melting

The added energy is being used primarily to change the particle arrangement rather than increase temperature.

Once all the ice has melted, the temperature of the liquid water can begin increasing again.

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Worked Example — Particle Explanation

A gas is cooled until it becomes a liquid.

What happens to its particles?

As energy is transferred away:

• Average particle kinetic energy decreases.
• Particles move more slowly.
• Attractive forces become more significant.
• Particles remain closer together.

The gas therefore:

condenses into a liquid

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Worked Example — Dry Ice

A piece of dry ice becomes smaller while sitting at room temperature.

No liquid carbon dioxide is observed.

What change is occurring?

Solid → Gas

Therefore, the process is:

sublimation

The carbon dioxide particles gain energy and move from the solid state directly into the gas state.

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Changes of State and Conservation of Matter

When a substance changes state, the particles themselves do not simply disappear.

For example:

Ice → Water → Water vapour

The water molecules remain water molecules.

Their:

• Arrangement
• Separation
• Movement
• Energy

change.

The amount of matter is conserved in a closed system.

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State Changes Are Physical Changes

Changes of state are usually classified as physical changes.

During a physical change:

• The form or state may change.
• Particle arrangement may change.
• Particle movement may change.
• Chemical identity remains the same.

Melting ice does not produce a new chemical substance.

It is still water.

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Plasma and Energy

If enough energy is supplied to a gas, the particles can become extremely energetic.

Collisions may remove electrons from atoms.

This process is called:

ionization

The gas then becomes plasma.

Removing energy can allow electrons and ions to recombine, returning the material toward an ordinary gaseous state.

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Real-World Changes of State

Changes of state occur constantly around us.

Examples include:

• Ice melting in a drink
• Water freezing in a freezer
• Water boiling in a kettle
• Sweat evaporating from skin
• Water vapour condensing on a bathroom mirror
• Frost forming on cold surfaces
• Dry ice sublimating
• Clouds forming through condensation

These everyday observations can all be explained using particle motion and energy.

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Common Mistakes

Mistake: Thinking Solid Particles Do Not Move

Particles in solids still move.

They vibrate around relatively fixed positions.

Mistake: Thinking Liquid Particles Are Far Apart

Liquid particles are generally still quite close together.

They simply have greater freedom to move past one another than particles in a solid.

Mistake: Thinking Gas Particles Expand

Individual particles do not normally become larger when a gas expands.

Instead:

the average distance between particles increases

Mistake: Confusing Boiling and Evaporation

Boiling occurs throughout a liquid at its boiling point for the given pressure.

Evaporation occurs at the surface and can happen below the boiling point.

Mistake: Confusing Sublimation and Melting

Sublimation is:

Solid → Gas

Melting is:

Solid → Liquid

Mistake: Confusing Condensation and Deposition

Condensation is:

Gas → Liquid

Deposition is:

Gas → Solid

Mistake: Thinking Temperature Must Rise Whenever Energy Is Added

During a phase change such as melting or boiling, energy can be transferred into a substance while its temperature remains approximately constant.

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Key Terms

Matter – anything that has mass and occupies space.

Solid – state with fixed shape and fixed volume.

Liquid – state with fixed volume but no fixed shape.

Gas – state with neither fixed shape nor fixed volume.

Plasma – high-energy state containing charged particles.

Particle model – model describing matter in terms of tiny moving particles.

Melting – solid changing into liquid.

Freezing – liquid changing into solid.

Boiling – rapid vaporization occurring throughout a liquid.

Evaporation – vaporization occurring at the surface of a liquid.

Condensation – gas changing into liquid.

Sublimation – solid changing directly into gas.

Deposition – gas changing directly into solid.

Ionization – process in which atoms or molecules become charged, often through gaining or losing electrons.

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Check Your Understanding

1. Describe the arrangement and movement of particles in a solid.

2. Why can liquids flow while solids normally maintain their shape?

3. Why are gases much easier to compress than liquids?

4. Give two properties of plasma that distinguish it from an ordinary gas.

5. Name the change from solid to liquid.

6. Name the change from liquid to solid.

7. Name the change from gas to liquid.

8. What is sublimation?

9. What is deposition?

10. Explain what happens to particle motion when a substance is heated.

11. Explain why temperature can remain constant while a substance is melting.

12. Explain the difference between boiling and evaporation.

13. Use the particle model to explain condensation.

14. Dry ice changes directly into carbon dioxide gas. Name and explain this process.

15. Explain why a change of state is normally considered a physical rather than chemical change.

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Key Takeaways

• Matter commonly exists as solids, liquids, gases, and plasma.
• The properties of each state can be explained using the particle model.
• Solid particles are close together and vibrate around relatively fixed positions.
• Liquid particles remain close together but can move past one another.
• Gas particles are widely separated and move rapidly in many directions.
• Plasma contains energetic charged particles such as ions and free electrons.
• Adding or removing energy can cause matter to change state.
• Melting changes a solid into a liquid.
• Freezing changes a liquid into a solid.
• Boiling and evaporation change liquids into gases.
• Condensation changes a gas into a liquid.
• Sublimation changes a solid directly into a gas.
• Deposition changes a gas directly into a solid.
• Heating generally increases particle kinetic energy.
• Cooling generally decreases particle kinetic energy.
• During a phase change, temperature can remain approximately constant even while energy is transferred.
• Changes of state involve changes in particle arrangement, separation, movement, and energy.
• Changes of state are physical changes because the chemical identity of the substance remains the same.

Did You Know?

Although solids, liquids, and gases are the states of matter we encounter most often in everyday life, plasma is thought to make up most of the visible matter in the universe. Stars, including our Sun, consist largely of plasma. So the state of matter that seems most unusual to us on Earth is actually extremely common on a cosmic scale.

3. Particle Model

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Particle Model

The particle model of matter is a scientific model used to explain how solids, liquids, and gases behave.

Although individual particles are far too small to see directly in everyday situations, imagining matter as collections of tiny moving particles helps us explain:

• States of matter
• Temperature
• Density
• Compression
• Diffusion
• Expansion
• Changes of state
• Many everyday observations

Learning Targets

• I can describe the main ideas of the particle model of matter.
• I can explain how particle motion changes with temperature.
• I can compare the spacing, motion, and forces between particles in solids, liquids, and gases.
• I can use the particle model to explain physical properties such as density, compressibility, and diffusion.
• I can apply the particle model to explain everyday observations involving matter.

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Main Ideas of the Particle Model

The particle model is based on several important ideas.

All matter is made of particles.

These particles may be atoms, molecules, or ions depending on the substance.

Particles are constantly moving.

Even particles in a solid are not completely stationary.

There are spaces between particles.

The amount of space differs between states of matter.

Particles attract one another.

The importance of these attractive forces depends on particle separation and energy.

Temperature affects particle motion.

Heating generally increases the average kinetic energy of particles.

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Why Do We Use a Particle Model?

Scientific models help us explain things that may be difficult or impossible to observe directly.

We cannot normally watch individual water molecules moving around inside a glass.

However, the particle model allows us to explain observations such as:

• Why gases can be compressed
• Why liquids flow
• Why solids keep their shape
• Why smells spread through a room
• Why materials expand when heated
• Why liquids evaporate
• Why substances have different densities

The model connects what happens at the particle level with what we observe at the macroscopic level.

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Particles in Solids

In a solid, particles are generally:

• Very close together
• Held in relatively fixed positions
• Strongly influenced by neighbouring particles
• Vibrating around equilibrium positions

The particles do not normally move freely through the solid.

This particle arrangement helps explain why solids usually have:

a fixed shape and a fixed volume

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Motion in Solids

It is incorrect to say:

"Particles in a solid do not move."

They do move.

They continually vibrate around relatively fixed positions.

If the solid is heated, the particles generally vibrate more vigorously.

This increased vibration helps explain thermal expansion.

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Particles in Liquids

In a liquid, particles are generally:

• Close together
• Randomly arranged
• Able to move around one another
• Still influenced significantly by attractive forces

Because the particles can move past one another, liquids can:

flow

and:

take the shape of their container

However, because the particles remain close together, liquids normally maintain approximately the same volume.

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Particles in Gases

Gas particles behave very differently.

They are generally:

• Far apart
• Moving rapidly
• Moving in many different directions
• Colliding with one another
• Colliding with the walls of their container

At ordinary gas densities, attractive forces have much less effect on their motion than in liquids and solids.

Because gas particles move freely, a gas spreads throughout the available space.

Solid

Spacing: Very close together

Arrangement: Relatively ordered or fixed

Motion: Vibrate around fixed positions

Attractive forces: Strong enough to maintain structure

Liquid

Spacing: Close together

Arrangement: Irregular and changing

Motion: Move and slide around one another

Attractive forces: Significant, but particles can rearrange

Gas

Spacing: Far apart

Arrangement: Random

Motion: Rapid and free

Attractive forces: Usually much less important between collisions

---

Temperature and Particle Motion

Temperature is related to the average kinetic energy of particles.

When a substance is heated:

Average particle kinetic energy increases.

The particles generally move or vibrate more vigorously.

When a substance is cooled:

Average particle kinetic energy decreases.

Particle motion becomes less vigorous.

---

Heating a Solid

When a solid is heated, its particles vibrate more vigorously around their equilibrium positions.

The particles often become slightly farther apart on average.

As a result, the solid may:

expand

This is called thermal expansion.

---

Everyday Example — Railway Tracks

Metal railway tracks become warmer during hot weather.

The metal particles vibrate more vigorously and their average separation increases slightly.

The entire rail therefore becomes slightly longer.

Engineers must allow for thermal expansion when designing:

• Railway tracks
• Bridges
• Pipelines
• Buildings

---

Heating a Liquid

When a liquid is heated, its particles generally move faster.

Their average separation may increase slightly.

Therefore, many liquids expand when heated.

If enough energy is supplied, particles may eventually overcome attractive interactions and escape into the gas state.

---

Heating a Gas

When a gas is heated, its particles gain kinetic energy.

They move faster.

If the gas is inside a rigid container, the faster particles collide with the walls:

• More frequently
• With greater momentum changes

This can increase the:

gas pressure

If the container can expand, the gas may instead increase in volume.

---

Density and the Particle Model

Density describes how much mass is contained in a given volume.

Materials tend to have greater density when their particles contribute more mass within the same volume.

Particle spacing is therefore one factor that can affect density.

In many substances:

Solid → particles close together → relatively high density

Gas → particles far apart → very low density

Liquids generally have densities much greater than their gases because their particles are much closer together.

---

Comparing Liquid Water and Water Vapour

Liquid water contains molecules that are close together.

Water vapour contains water molecules that are much farther apart.

A given volume of liquid water therefore contains many more water molecules than the same volume of water vapour under ordinary conditions.

Therefore:

Liquid water is much denser than water vapour.

The molecules themselves have not become heavier.

Their spacing has changed.

---

An Important Exception — Water

We must be careful not to oversimplify the particle model.

Most substances are denser as solids than as liquids.

Water is an important exception.

When water freezes, its molecules form an open crystal structure that keeps them farther apart on average than in liquid water.

Therefore:

Ice is less dense than liquid water.

This is why ice floats.

---

Compressibility

Compressibility describes how easily the volume of a substance can be reduced by applying pressure.

The particle model explains why gases are highly compressible.

Gas particles are widely separated.

There is a large amount of space between them.

When pressure is applied:

particles can be pushed closer together

---

Why Liquids Are Difficult to Compress

Liquid particles are already close together.

There is relatively little space available for them to be pushed into.

Therefore:

Liquids have low compressibility.

This property makes liquids useful in hydraulic systems.

---

Why Solids Are Difficult to Compress

Particles in solids are also already very close together.

Trying to compress a solid requires pushing particles closer against strong interactions between them.

Therefore, most solids have:

very low compressibility

---

Everyday Example — A Syringe

Imagine sealing the end of a syringe containing air and pushing the plunger.

The air can be compressed because gas particles have large spaces between them.

Now imagine filling the syringe completely with water.

It becomes much more difficult to push the plunger.

The difference can be explained by particle spacing.

---

Diffusion

Diffusion is the net movement of particles from a region of higher concentration toward a region of lower concentration due to random particle motion.

Particles are constantly moving.

Because of this motion, particles gradually spread out and mix.

Watch how randomly moving particles spread from an initial concentration difference:

Diffusion continues even after concentrations become uniform because particles never stop moving.

At that stage there is no longer a net movement from one region to another.

---

Diffusion in Gases

Diffusion occurs relatively quickly in gases because gas particles:

• Move rapidly
• Are widely separated
• Move freely throughout the available space

Suppose perfume is sprayed at one side of a room.

After some time, people elsewhere in the room can smell it.

Perfume molecules mix with air molecules and spread through the room.

---

Diffusion in Liquids

Diffusion also occurs in liquids.

For example, place a drop of food colouring into still water.

Even without stirring, the colour gradually spreads.

The particles are moving randomly and gradually become distributed throughout the water.

Diffusion is generally slower in liquids than in gases because particles are more closely packed and their movement is more constrained.

---

Temperature and Diffusion

Increasing temperature generally makes diffusion occur more rapidly.

Why?

Higher temperature means:

greater average kinetic energy

Therefore, particles move more rapidly.

Faster particle motion generally causes substances to mix more quickly through diffusion.

---

Everyday Example — Tea

Imagine placing a tea bag into hot water.

Particles from the tea move into the surrounding water and spread.

The process generally occurs more rapidly in hot water than in cold water.

The particle model explains this in terms of increased particle kinetic energy at higher temperature.

---

Gas Pressure and Particle Collisions

The particle model also explains gas pressure.

Gas particles continually collide with the walls of their container.

Each collision exerts a tiny force on the wall.

The combined effect of enormous numbers of collisions produces:

pressure

Changing particle speed, number of particles, or container volume can therefore change gas pressure.

---

Everyday Example — A Balloon

Inside an inflated balloon, gas particles move rapidly and collide with the inner surface.

These collisions produce outward pressure.

The stretched rubber pushes inward.

The balloon reaches a size where these effects balance.

Heating the gas can make the particles move faster and can cause the balloon to expand if the material allows it.

---

Brownian Motion

One important piece of evidence supporting the particle model is Brownian motion.

Small visible particles suspended in a fluid can sometimes be observed moving in an irregular, unpredictable pattern.

The visible particles are being struck unevenly by much smaller, rapidly moving molecules.

Their irregular motion provides evidence that particles in fluids are continually moving.

---

Dissolving and the Particle Model

The particle model can also help explain dissolving.

Suppose salt is added to water.

The salt appears to disappear.

However, its particles have not vanished.

Salt particles separate and become distributed among the water particles.

The particles are still present even though they may no longer be visible.

---

Everyday Example — Smelling Food

Suppose someone begins cooking in a kitchen.

After a while, you may smell the food from another room.

The smell is caused by molecules that enter the air.

These molecules:

• Move randomly
• Collide with air molecules
• Spread through diffusion

Eventually, some reach your nose.

The particle model explains how matter can travel through apparently empty air.

---

Everyday Example — Wet Clothes Drying

Wet clothes eventually dry even if the water never boils.

Some water molecules at the surface have enough kinetic energy to escape from the liquid.

They enter the air as water vapour.

This process is:

evaporation

Warm temperatures, moving air, and low humidity can increase the rate of evaporation.

---

Everyday Example — Condensation on a Cold Drink

Water droplets often appear on the outside of a cold glass.

The water has not normally leaked through the glass.

Water vapour particles in the surrounding air encounter the cold surface.

They lose energy.

Their movement becomes less vigorous, and attractive forces allow them to remain closer together.

The gas becomes liquid water.

This process is:

condensation

---

Using the Particle Model to Explain Observations

A strong particle-model explanation usually connects three levels.

Observation

What can you see or measure?

Example:

A gas expands when heated.

Particle Behaviour

What are the particles doing?

Particles gain kinetic energy and move faster.

Explanation

How does that produce the observation?

Faster particles can cause the gas to occupy more space when the pressure is able to remain approximately constant.

This approach produces much stronger scientific explanations than simply stating that "heat makes things expand."

---

Worked Example — Why Can Gas Be Compressed?

Observation: Air in a sealed syringe can be compressed.

Particle explanation:

Gas particles are far apart with large spaces between them.

When the plunger is pushed inward, the particles are forced into a smaller volume.

The individual particles do not become smaller.

Instead:

the average distance between them decreases

---

Worked Example — Why Does Perfume Spread?

Observation: Perfume sprayed in one part of a room can eventually be smelled elsewhere.

Particle explanation:

Perfume molecules enter the gas phase.

They move randomly and collide with air molecules.

Over time, they spread from the region where perfume molecules are highly concentrated into regions where they are less concentrated.

This process is:

diffusion

---

Worked Example — Why Does Heating Increase Gas Pressure?

Consider a gas trapped inside a rigid sealed container.

Heating increases the average kinetic energy of the gas particles.

Therefore, particles move faster.

They collide with the container walls more frequently and with larger momentum changes.

The result is:

greater pressure

provided the volume remains fixed.

---

The Particle Model Has Limitations

The particle model is useful, but it is still a model.

Simple diagrams often show particles as small solid circles.

Real atoms and molecules are much more complicated.

Particle diagrams may also:

• Not be drawn to scale
• Exaggerate particle sizes
• Exaggerate or reduce particle spacing
• Ignore molecular structure
• Simplify attractive forces
• Represent three-dimensional systems in two dimensions

A model does not need to show every detail to be scientifically useful.

---

Common Mistakes

Mistake: Saying Particles in Solids Do Not Move

Solid particles vibrate around relatively fixed positions.

Mistake: Saying Particles Get Bigger When Heated

Usually, the particles themselves do not significantly increase in size.

Their motion and average separation can change.

Mistake: Saying Gas Has No Particles

Gas is matter and contains particles.

The particles are simply much farther apart than in liquids and solids.

Mistake: Drawing Liquid Particles Far Apart

Liquid particles are generally still close together.

The main difference is that they can move past one another.

Mistake: Saying There Is Air Between Gas Particles

Air itself is made of particles.

In the simple particle model, the space between gas particles is mostly empty space.

Mistake: Saying Diffusion Happens Because Particles "Want to Spread Out"

Particles do not make decisions.

Diffusion results from their continuous random motion.

Mistake: Saying Gas Particles Slow Down When Compressed

Compression does not automatically mean particles move more slowly. Their speed depends mainly on temperature, while compression reduces their average separation.

---

Key Terms

Particle model – scientific model describing matter as collections of tiny moving particles.

Particle – general term for microscopic units such as atoms, molecules, or ions.

Kinetic energy – energy associated with motion.

Temperature – quantity related to the average kinetic energy of particles.

Density – mass per unit volume.

Compressibility – ability of a substance to decrease in volume when pressure is applied.

Diffusion – net spreading of particles from higher to lower concentration due to random motion.

Brownian motion – irregular movement of visible microscopic particles caused by collisions with smaller fluid particles.

Thermal expansion – increase in dimensions of a material associated with heating.

Gas pressure – effect produced by gas-particle collisions with surfaces.

---

Check Your Understanding

1. State three main ideas of the particle model.

2. Describe the arrangement and motion of particles in a solid.

3. How does particle movement in a liquid differ from particle movement in a solid?

4. Why can gases fill their containers?

5. What happens to average particle kinetic energy when temperature increases?

6. Use the particle model to explain why most solids expand when heated.

7. Why are gases much easier to compress than liquids?

8. Explain why liquid water is much denser than water vapour.

9. What is diffusion?

10. Why does diffusion generally occur more quickly at higher temperatures?

11. Use the particle model to explain why perfume can be smelled across a room.

12. Explain how gas particles produce pressure.

13. Why does heating a gas in a rigid container usually increase its pressure?

14. Explain why water droplets form on the outside of a cold glass.

15. Describe one limitation of the simple particle model.

---

Key Takeaways

• All matter is made of particles.
• Particles are continually moving or vibrating.
• There are spaces between particles.
• Attractive interactions occur between particles.
• Temperature is related to average particle kinetic energy.
• Heating generally increases particle motion.
• Cooling generally decreases particle motion.
• Solid particles remain close together and vibrate around relatively fixed positions.
• Liquid particles remain close together but can move past one another.
• Gas particles are widely separated and move rapidly in many directions.
• Particle spacing helps explain differences in density and compressibility.
• Gases are easily compressed because there is substantial space between their particles.
• Diffusion results from continuous random particle motion.
• Diffusion generally occurs more rapidly at higher temperatures.
• Gas pressure results from particle collisions with surfaces.
• Brownian motion provides evidence for continuous molecular motion.
• The particle model can explain expansion, evaporation, condensation, dissolving, diffusion, and many other everyday observations.
• Scientific particle diagrams are simplified models and should not be interpreted as exact pictures of atoms and molecules.

Did You Know?

In 1905, Albert Einstein developed a mathematical explanation of Brownian motion showing how the irregular movement of visible microscopic particles could result from collisions with molecules too small to see. Experimental tests of these predictions helped provide strong evidence that atoms and molecules were physically real—not merely convenient ideas used in chemical calculations.

4. Thermal Expansion

What Is Thermal Expansion?

Thermal expansion is the increase in the dimensions of a material when its temperature increases.

When most materials are heated, they expand. When they are cooled, they contract.

This means that an object's:

• length can increase
• width can increase
• height can increase
• volume can increase

The opposite generally happens when temperature decreases.

Heating → expansion

Cooling → contraction

An important point is that thermal expansion normally changes the dimensions of an object, not the amount of matter in it. A metal rod does not gain mass simply because it becomes warmer.

---

Explaining Thermal Expansion Using Particles

Thermal expansion can be explained using the particle model of matter.

Particles in solids, liquids, and gases are always moving.

When a material is heated:

• energy is transferred to the material
• the particles gain kinetic energy
• particle motion becomes more energetic
• the average separation between particles generally increases
• the material occupies more space

The individual atoms or molecules do not simply become larger. Instead, the average spacing between them changes.

When the material cools, the opposite occurs:

• particles lose kinetic energy
• particle motion becomes less energetic
• their average separation generally decreases
• the material contracts

A Simple Example

Imagine a metal rod.

At a lower temperature, its particles vibrate around fixed positions.

When the rod is heated, the particles vibrate more energetically. Their average separation increases slightly.

Each individual increase is extremely small, but there are enormous numbers of particles along the rod.

The result is a measurable increase in the rod's length.

---

Thermal Expansion in Solids

In a solid, particles are held in relatively fixed positions by forces between them.

They cannot freely move around the material, but they vibrate.

Heating increases the energy of these vibrations. The average separation of the particles increases, causing the solid to expand.

Solids can expand in all three dimensions.

For example, a metal block may become:

• slightly longer
• slightly wider
• slightly taller

Therefore, its overall volume increases.

For long objects such as railway tracks, bridges, pipes, and electrical cables, even a very small percentage expansion can become important.

---

Linear Expansion

When dealing with long, thin objects such as rods, rails, or wires, we are often most interested in the change in length.

The amount of expansion depends on three main factors:

• the original length of the object
• the change in temperature
• the material from which the object is made

This can be represented by:

$\Delta L = \alpha L_0 \Delta T$

where:

• $\Delta L$= change in length
• $L_0$= original length
• $\Delta T$= change in temperature
• $\alpha$= coefficient of linear thermal expansion

The coefficient

$\alpha$

tells us how strongly a particular material expands when its temperature changes.

You do not always need to calculate thermal expansion to understand the relationship:

larger original length → more expansion

larger temperature increase → more expansion

larger expansion coefficient → more expansion

---

Worked Example: Comparing Two Metal Rods

Two identical aluminium rods are heated.

Rod A increases in temperature by

$20^\circ\text{C}$

.

Rod B increases in temperature by

$60^\circ\text{C}$

.

Which rod expands more?

Both rods have the same:

• starting length
• material
• coefficient of expansion

The only difference is the temperature change.

Rod B experiences three times the temperature increase, so it will undergo approximately three times as much linear expansion, provided the temperature range is one over which the linear approximation is suitable.

---

Thermal Expansion in Liquids

Liquids also expand when heated.

Unlike solids, liquid particles are not fixed in particular positions. They can move past one another.

As a liquid becomes warmer, its particles generally become more widely separated, increasing the liquid's volume.

This principle is used in liquid-in-glass thermometers.

As the temperature increases:

• the liquid in the bulb expands
• its volume increases
• the liquid rises through the narrow tube

When the temperature decreases, the liquid contracts and the level falls.

Different liquids expand by different amounts for the same temperature change. This is one reason the choice of liquid matters when designing a thermometer.

---

Thermal Expansion in Gases

Gases can show very large changes in volume when their temperature changes, particularly when they are free to expand at approximately constant pressure.

Gas particles are already much farther apart than particles in solids or liquids.

When a gas is heated:

• its particles gain kinetic energy
• they move faster
• collisions with the walls of the container become more energetic and frequent
• if the container can expand, the gas can occupy a greater volume

A familiar example is a balloon.

If a flexible balloon containing gas is warmed, its volume may increase. If it is cooled, its volume may decrease.

However, if a gas is trapped inside a rigid sealed container, it cannot simply expand outward. Instead, heating it can cause its pressure to increase.

This distinction is important.

---

Comparing Solids, Liquids, and Gases

In general, the effects of heating can be summarized as:

These are general trends. The exact amount of expansion depends on the particular substance and the conditions.

---

Different Materials Expand by Different Amounts

Not all materials expand equally.

Suppose equal-length pieces of aluminium, steel, and another material experience the same temperature increase. Their changes in length will generally not be identical.

This happens because different materials have:

• different atomic structures
• different types and strengths of bonding
• different responses to increased particle energy

The coefficient of thermal expansion describes this behaviour quantitatively.

A material with a larger coefficient expands more for the same original length and temperature change.

---

Bimetallic Strips

The fact that different materials expand by different amounts can be useful.

A bimetallic strip consists of two different metals bonded together.

When heated:

• one metal expands more than the other
• the two metals cannot separate
• the strip bends

When cooled, it bends back toward its original shape.

Bimetallic strips have been used in devices such as:

• thermostats
• temperature-sensitive switches
• circuit breakers
• older mechanical temperature-control systems

The device works precisely because the two materials have different thermal expansion coefficients.

---

Thermal Expansion in Bridges

A bridge may be hundreds or even thousands of metres long.

Although each metre expands only slightly when heated, the total change in length across a long structure can become significant.

Engineers therefore design bridges so that parts of the structure can move safely as temperature changes.

One solution is an expansion joint.

Expansion joints allow sections of a structure to move relative to one another as the materials expand and contract.

Without appropriate engineering allowances, repeated temperature changes could produce large stresses that contribute to:

• bending
• buckling
• cracking
• damage to connections

Expansion joints are used in many structures, including bridges and some sections of buildings and piping systems.

---

Thermal Expansion and Railway Tracks

Railway rails become warmer in hot weather and cooler in cold weather.

As their temperature rises, steel tends to expand.

If thermal expansion is strongly restrained, large internal stresses can develop.

Historically, jointed tracks could include small gaps between rail sections. Modern railways also use continuous welded rail, where thermal effects are managed through careful engineering of the rails, fastenings, installation temperature, and track structure rather than simply leaving frequent gaps.

Special rail expansion devices may still be used in locations such as some bridge transitions where substantial relative movement must be accommodated.

---

Thermal Expansion of Power Lines

Overhead electrical wires also expand and contract.

On a hot day, the metal conductors expand and become longer. This can cause the wires to sag farther downward.

During colder weather, they contract and become shorter, increasing their tension and reducing the sag.

Engineers must therefore consider the range of temperatures that power lines will experience throughout the year.

If wires were installed too tightly, contraction during very cold conditions could produce excessive tension.

If they were allowed to sag too much, hot conditions could cause clearance problems.

---

Heating a Metal Lid

Thermal expansion can even be useful in the kitchen.

Suppose a metal lid is stuck tightly on a glass jar.

Running warm water over the lid can make it easier to open.

Why?

The metal lid expands when heated. Its diameter increases slightly, which can loosen its grip on the jar.

This is a simple everyday application of thermal expansion.

---

Hot-Air Balloons

Thermal effects in gases are also important in hot-air balloons.

When air inside the balloon is heated, it expands. Because the balloon is open at the bottom and remains close to atmospheric pressure, some air leaves the balloon.

The warmer air remaining inside is therefore less dense than the cooler surrounding air.

The resulting buoyant force can lift the balloon.

Thermal expansion is therefore connected to changes in density as well as volume.

---

Predicting Changes in Dimensions

You can often predict thermal expansion without performing a calculation.

Situation A

A metal rod is heated.

Prediction:

Length increases.

Situation B

The same rod is cooled below its original temperature.

Prediction:

Length decreases.

Situation C

Two rods of the same material experience the same temperature increase, but one is twice as long.

Prediction:

The longer rod undergoes a greater absolute change in length.

Situation D

Two equal-length rods made from different materials experience the same temperature increase.

Prediction:

The material with the larger coefficient of thermal expansion expands more.

---

Worked Example: Bridge Expansion

Suppose a steel bridge section is:

$L_0=50.0\text{ m}$

and its coefficient of linear thermal expansion is approximately:

$\alpha=12\times10^{-6}/^\circ\text{C}$

The temperature increases by:

$\Delta T=30^\circ\text{C}$

Use:

$\Delta L=\alpha L_0\Delta T$

Substitute:

$\Delta L=(12\times10^{-6})(50.0)(30)$$\Delta L=0.018\text{ m}$

Therefore:

$\Delta L=18\text{ mm}$

The bridge section becomes approximately 18 mm longer.

That may sound small, but structures contain long sections and experience repeated temperature changes. Engineers must allow for these movements rather than assuming a structure remains exactly the same size.

---

Worked Example: Cooling

A metal pipe is 10.0 m long.

Its temperature decreases significantly overnight.

Without calculating an exact value, predict what happens.

The temperature decreases, so particle vibrations become less energetic and the average particle separation decreases.

Therefore:

the pipe contracts and becomes slightly shorter.

If the pipe is prevented from contracting, thermal stresses may develop.

---

Thermal Expansion Can Be Useful

Thermal expansion is not always a problem.

It can deliberately be used in engineering and technology.

Examples include:

• liquid thermometers
• bimetallic thermostats
• heat-sensitive switches
• fitting metal components together
• loosening metal lids
• shrink-fitting components

For example, a metal ring can be heated so that its diameter increases. It can then be fitted over another object. When it cools, it contracts and forms a tight fit.

---

Thermal Expansion Can Cause Problems

Thermal expansion becomes particularly important when a material is not free to change dimensions.

Imagine a long metal beam trapped rigidly between two immovable walls.

When heated, the beam tries to expand.

If it cannot expand freely, large forces and stresses can develop.

This can contribute to:

• deformation
• buckling
• cracking
• structural damage

For this reason, engineers do not simply ask:

“How much will this material expand?”

They also ask:

“What will happen if the material is prevented from expanding?”

---

Common Mistakes

“The particles themselves get bigger.”

Usually incorrect.

Thermal expansion mainly occurs because the average separation between particles changes, not because individual atoms suddenly become much larger.

“Heating increases mass.”

Heating an ordinary object does not add more matter to it. Its dimensions can increase while its mass remains essentially unchanged for ordinary classroom purposes.

“Everything expands by the same amount.”

Incorrect.

Different materials have different coefficients of thermal expansion.

“Only solids undergo thermal expansion.”

Incorrect.

Solids, liquids, and gases can all expand when heated.

“Gases always increase in volume when heated.”

Not necessarily.

A gas in a flexible or movable container can expand. A gas trapped in a rigid sealed container cannot significantly increase the container's volume, so its pressure generally rises instead.

“Modern railway tracks simply need gaps everywhere.”

This is an oversimplification.

Modern continuous welded rail is specifically engineered to manage thermal stresses. Expansion devices are used where required by the design and structural conditions.

---

Check Your Understanding

1. What normally happens to the dimensions of a solid when it is heated?

2. Explain thermal expansion using the particle model.

3. Do the individual particles become larger when a material expands?

4. Why can thermal expansion be especially important in very long structures?

5. Why are expansion joints used in some bridges?

6. Why do overhead power lines sag more during hot weather?

7. Two equal-length metal rods are heated through the same temperature change. Rod A expands more than Rod B. What does this tell you about the materials?

8. A 2 m aluminium rod and a 10 m aluminium rod undergo the same temperature increase. Which experiences the greater change in length?

9. Why does a bimetallic strip bend when heated?

10. Predict what happens to a metal pipe when its temperature decreases overnight.

11. Why can heating the metal lid of a glass jar make the lid easier to remove?

12. A gas is heated inside a rigid sealed container. Why might its pressure increase rather than its volume increasing significantly?

---

Key Terms

• Thermal expansion – increase in a material's dimensions caused by an increase in temperature.
• Thermal contraction – decrease in dimensions caused by a decrease in temperature.
• Coefficient of thermal expansion – measure of how much a material expands for a given temperature change.
• Linear expansion – change in the length of an object due to temperature change.
• Expansion joint – structure designed to accommodate movement caused by expansion and contraction.
• Bimetallic strip – two different metals bonded together that bend when their different rates of expansion cause unequal length changes.
• Kinetic energy – energy associated with particle motion.
• Thermal stress – stress produced when thermal expansion or contraction is restricted.

---

Key Takeaways

• Most materials expand when heated and contract when cooled.
• Thermal expansion occurs because heating changes particle motion and their average separation.
• Solids, liquids, and gases can all undergo thermal expansion.
• Gases generally show much larger volume changes than solids when they are free to expand.
• Different materials expand by different amounts because they have different structures, bonding, and thermal expansion coefficients.
• Greater temperature changes generally produce greater dimensional changes.
• Longer objects generally experience larger absolute changes in length for the same material and temperature change.
• Thermal expansion must be considered when designing bridges, railway systems, pipes, power lines, and other structures.
• Differences in thermal expansion can also be useful, as in bimetallic strips and thermostats.
• The particles themselves should not be pictured as simply becoming larger; it is mainly the spacing between particles that changes.

5. Temperature and Heat