Density and the Properties of Fluids
1. What Is a Fluid?
Learning outcomes
- I can define a fluid and explain how fluids differ from solids.
- I can describe the properties of liquids and gases as fluids.
- I can explain why fluids can flow and change shape.
- I can identify examples of fluids in everyday life.
- I can compare the behaviour of liquids and gases under different conditions.
What Is a Fluid?
A fluid is a substance that can:
flow and change shape.
The two main types of fluids are:
- liquids
- gases
Unlike a solid, a fluid does not maintain a fixed shape when a force is applied to it.
Instead, it can move and take the shape of its:
container.
Water, air, oil, gasoline, and steam are all examples of:
fluids.
Fluids Are Not Just Liquids
In everyday language, people sometimes use the word fluid to mean:
liquid.
In science, however, the term has a broader meaning.
Both:
liquids AND gases
are fluids because both can flow.
For example:
- water flows through a pipe
- air flows through a ventilation system
- oil flows through an engine
- natural gas flows through a pipeline
All are examples of:
fluid flow.
Solids, Liquids, and Gases
The three familiar states of matter behave differently.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Fixed shape | Yes | No | No |
| Fixed volume | Usually yes | Approximately yes | No |
| Can flow | No | Yes | Yes |
| Takes container shape | No | Yes | Yes |
| Easily compressed | No | No | Yes |
| Particle spacing | Very close | Close | Far apart |
Liquids and gases are grouped together as fluids because they can both:
flow.
Why Can Fluids Flow?
To understand fluid behaviour, we can use the:
particle model of matter.
Matter consists of particles such as atoms or molecules.
How these particles are arranged and how freely they move help determine whether a substance behaves as a:
solid, liquid, or gas.
Particles in a Solid
In a solid, particles are held in relatively fixed positions.
They can:
vibrate
but they do not normally move freely past one another.
This gives a solid:
- a definite shape
- a definite volume
- resistance to flowing
For example, a steel block does not normally change shape simply because it is placed in a differently shaped:
container.
Particles in a Liquid
Particles in a liquid are:
close together,
but they are not locked into fixed positions.
They can move and:
slide past one another.
This allows a liquid to:
- flow
- change shape
- take the shape of its container
However, because the particles remain close together, a liquid maintains an approximately:
fixed volume.
Particles in a Gas
Gas particles are much farther apart than particles in a liquid.
They move:
rapidly and randomly.
Because there is considerable space between the particles, a gas can:
- flow
- change shape
- expand
- be compressed
- fill its container
A gas therefore has neither a fixed shape nor a fixed:
volume.
Why Don't Fluids Have a Fixed Shape?
A solid can resist forces that try to continuously change its shape.
A fluid behaves differently.
When a force acts parallel to the surface of a fluid—a shear force—the fluid can continue to deform and:
flow.
This ability to continuously deform under shear is one of the more precise ways scientists define a:
fluid.
Liquids Have a Fixed Volume
Imagine pouring 500 mL of water from a bottle into a bowl.
The water changes:
shape.
However, assuming none is spilled or evaporates, its volume remains approximately:
500 mL.
Therefore:
liquid → variable shape, approximately fixed volume
Gases Have a Variable Volume
Now imagine releasing air into a larger container.
The gas particles spread throughout the available:
space.
The gas therefore changes both:
shape and volume.
Therefore:
gas → variable shape, variable volume
Liquids Form Surfaces
If you pour water into an open glass, the water occupies the lower part of the container and forms a:
free surface.
It does not normally expand to completely fill the glass.
This is a characteristic behaviour of:
liquids.
Gases Fill Their Containers
A gas behaves differently.
If air is introduced into an empty container, it spreads throughout the available:
volume.
Gas particles move in all directions and occupy the available space.
Therefore gases naturally:
fill their containers.
Compressibility
Compressibility describes how easily the volume of a substance can be reduced by applying:
pressure.
Liquids and gases behave very differently.
Liquids are difficult to compress.
Gases are relatively easy to compress.
This difference is extremely important in fluid science and engineering.
Why Are Liquids Difficult to Compress?
Liquid particles are already:
close together.
There is relatively little empty space between them.
Applying pressure therefore produces only a small change in:
volume.
For many everyday calculations, liquids can be treated as:
incompressible.
This is an approximation, because real liquids can be compressed slightly.
Why Are Gases Compressible?
Gas particles are separated by much larger:
distances.
There is plenty of space between them.
When pressure is applied, the particles can be forced:
closer together.
Therefore the gas volume can decrease substantially.
The Syringe Experiment
A simple comparison can be made using sealed syringes.
Imagine one syringe contains:
air.
Another contains:
water.
Seal the ends and push the plungers.
The air-filled syringe can be compressed noticeably because:
air is a gas.
The water-filled syringe changes volume very little because:
water is a liquid.
This demonstrates an important difference between the two types of fluids.
Pressure Can Change Gas Volume
When pressure on a gas increases, the gas particles can be pushed closer together.
The gas volume therefore:
decreases.
When pressure decreases, the gas can:
expand.
This relationship is important in:
- pumps
- compressors
- breathing
- pneumatic systems
- scuba diving
- weather
- engines
Temperature Can Change Fluid Behaviour
Temperature affects particle motion.
When a substance is heated, its particles generally gain:
kinetic energy.
They move more vigorously.
Both liquids and gases generally expand when heated, although gases usually show much larger volume changes under common conditions.
Heating a Gas
Suppose a gas is contained in a flexible balloon.
If the gas is heated, its particles move faster and collide more energetically with the:
balloon walls.
If the balloon can expand, its volume may:
increase.
If the gas is trapped in a rigid sealed container instead, heating can cause its:
pressure to increase.
Cooling a Gas
Cooling reduces the average kinetic energy of gas particles.
Depending on the conditions, the gas may:
- decrease in pressure
- decrease in volume
- eventually condense into a liquid
Gas behaviour therefore depends strongly on:
temperature and pressure.
Fluids Have Density
All fluids have:
density.
Density describes the amount of mass contained in a particular volume.
The equation is:
density = mass ÷ volume
or:
ρ = m / V
where:
ρ = density
m = mass
V = volume
Liquid and Gas Density
Liquids are usually much denser than gases because their particles are:
much closer together.
For example, at ordinary conditions:
liquid water is much denser than air.
Gas density can also change considerably when:
- pressure changes
- temperature changes
Liquid density generally changes much less under ordinary conditions.
Fluids Exert Pressure
Fluids can exert:
pressure.
Pressure is force acting over an area.
The basic equation is:
P = F / A
where:
P = pressure
F = force
A = area
Fluid pressure is important in:
- hydraulic systems
- atmospheric pressure
- blood circulation
- diving
- dams
- aircraft
- weather systems
Pressure in Liquids
The pressure in a liquid generally increases with:
depth.
A point deeper underwater has more liquid above it.
This is why structures such as dams must withstand greater water pressure near their:
bottom.
Atmospheric Pressure
Air is a gas and therefore a:
fluid.
The atmosphere surrounding Earth exerts pressure called:
atmospheric pressure.
We do not normally notice it because our bodies and surroundings exist within this pressure all the:
time.
Atmospheric pressure is extremely important in:
- weather
- aviation
- breathing
- fluid movement
- vacuum systems
Fluids Can Have Different Viscosities
Not all fluids flow equally easily.
Viscosity describes a fluid's resistance to:
flow.
A high-viscosity fluid flows more slowly under comparable conditions.
A low-viscosity fluid flows more easily.
Examples of Viscosity
Consider:
water
and:
honey.
Both are liquids.
Both are fluids.
However, honey usually flows much more slowly because it has a higher:
viscosity.
Other relatively viscous fluids include:
- syrup
- some oils
- shampoo
- molten materials
Gases Also Have Viscosity
Viscosity is not limited to liquids.
Gases also resist flow because their particles interact and transfer momentum.
Air therefore has:
viscosity.
This becomes important in areas such as:
- aerodynamics
- ventilation
- weather
- aircraft design
- gas pipelines
Fluids in Everyday Life
We interact with fluids constantly.
Examples include:
Water — drinking, washing, rivers and plumbing
Air — breathing, wind and ventilation
Blood — circulates through the body
Oil — lubrication and machinery
Fuel — transported through engines and pipelines
Milk — food and drink
Steam — heating and industrial systems
Natural gas — energy and manufacturing
Fluids are essential to both living systems and:
technology.
Blood Is a Fluid
Blood flows through:
blood vessels.
It transports:
- oxygen
- carbon dioxide
- nutrients
- hormones
- waste products
- heat
Blood is more complex than a simple liquid because it contains cells suspended in:
plasma.
Nevertheless, it behaves as a fluid and can be studied using principles of:
fluid mechanics.
Air Is a Fluid
Air may not look like a fluid because we cannot normally:
see it.
However, air can:
- flow
- exert pressure
- change shape
- change volume
- move around objects
Wind is simply:
moving air.
Therefore wind is an example of:
fluid flow.
Fluids and Aerodynamics
When an aircraft moves through air, the air flows around its:
wings and body.
Engineers study this flow to understand:
- lift
- drag
- pressure
- turbulence
Because air is a fluid, aircraft design is an application of:
fluid mechanics.
Fluids and Hydrodynamics
The study of moving liquids is often called:
hydrodynamics.
Examples include:
- water flowing through pipes
- rivers
- ocean currents
- blood circulation
- water around ships
- pumps
The broader study of liquids and gases is called:
fluid mechanics.
Laminar and Turbulent Flow
Fluids can move in different ways.
In laminar flow, the fluid moves in relatively smooth layers.
In turbulent flow, the motion contains irregular fluctuations, mixing, and swirling structures.
Turbulent flow commonly occurs when fluids move rapidly or around complicated:
obstacles.
Both liquids and gases can show laminar and turbulent flow.
Changing State
A substance can change between different states of matter.
solid → liquid = melting
liquid → solid = freezing
liquid → gas = vaporization
gas → liquid = condensation
When a substance changes from solid to liquid, it gains the ability to:
flow.
When it changes from liquid to gas, it becomes much more:
compressible.
Is Steam a Fluid?
Yes.
Steam is water in the:
gas state.
Because gases can flow and change shape, steam is a:
fluid.
Liquid water and water vapour are both fluids even though they have very different:
properties.
Is Ice a Fluid?
Under ordinary conditions, ice is a:
solid.
It maintains its shape and does not continuously deform like a liquid when a small shear force is applied.
Therefore ordinary ice is not classified as a:
fluid.
Over very long times and under large stresses, some solids can deform and flow slowly, but this does not change their normal classification as solids.
Is Toothpaste a Fluid?
Some substances do not behave like simple liquids.
Toothpaste can flow when sufficient force is applied, but it may remain nearly stationary when left alone.
Materials with more complicated flow behaviour are called:
non-Newtonian fluids.
Other examples can include:
- ketchup
- paint
- blood
- mixtures of cornstarch and water
Newtonian and Non-Newtonian Fluids
A Newtonian fluid has a viscosity that remains approximately constant at a given temperature and pressure as the rate of deformation changes.
Examples include, approximately:
- water
- air
- many simple oils
A non-Newtonian fluid changes its apparent viscosity depending on how it is:
stressed or moved.
Oobleck
A mixture of cornstarch and water is often called:
oobleck.
When moved slowly, it can flow.
When struck or squeezed rapidly, it can temporarily resist deformation much more strongly.
This demonstrates that some fluids have much more complicated behaviour than ordinary:
water or air.
Comparing Liquids and Gases
Liquids and gases are both fluids, but their behaviours differ significantly.
Liquids
- flow
- take the shape of their container
- maintain approximately fixed volume
- are difficult to compress
- have closely spaced particles
- can form a free surface
Gases
- flow
- take the shape of their container
- expand to fill the container
- are easily compressed compared with liquids
- have widely spaced particles
- do not form a stable free surface in the same way
Example: Water in a Bottle
Pour water into a bottle.
The water takes the shape of the:
lower part of the bottle.
It maintains approximately the same volume.
Therefore water demonstrates typical:
liquid behaviour.
Example: Air in a Bottle
A bottle that appears empty actually contains:
air.
The air occupies the available space throughout the bottle.
If the air is transferred into a larger container, it spreads out and fills the new:
volume.
This demonstrates typical:
gas behaviour.
Example: A Bicycle Pump
A bicycle pump contains:
air.
When the handle is pushed, the air is compressed into a smaller volume.
Its pressure:
increases.
This works because gases are:
compressible.
Example: Hydraulic Brakes
Hydraulic braking systems use:
liquid.
Because liquids are difficult to compress, pressure applied in one part of the system can be transmitted through the:
fluid.
This makes liquids useful in many:
hydraulic systems.
Example: Pneumatic Systems
Pneumatic systems use compressed:
gas, usually air.
Compressed air can store energy and produce movement.
Pneumatic systems are used in:
- tools
- factory equipment
- vehicle systems
- control systems
The compressibility of gases is essential to how these systems:
operate.
Comparing Hydraulic and Pneumatic Systems
Hydraulic systems
use:
liquids.
Liquids are difficult to compress.
Pneumatic systems
use:
gases.
Gases can be compressed significantly.
Therefore the different properties of liquids and gases determine how they can be used in:
technology.
Worked Example 1
A substance flows and takes the shape of its container.
Can it be a fluid?
Yes.
Both liquids and gases can behave this way.
More information would be needed to determine whether it is specifically a liquid or:
gas.
Worked Example 2
A substance has a fixed volume but changes shape when moved to another container.
What state is it most likely in?
Liquid.
Liquids have approximately fixed volume but no fixed:
shape.
Worked Example 3
A substance expands until it fills its entire container.
What state is it most likely in?
Gas.
Gases have neither fixed shape nor fixed:
volume.
Worked Example 4
Why can water flow?
Water molecules are close together but are not fixed permanently in position.
They can:
move past one another.
Therefore water can change shape and flow.
Worked Example 5
Why can air be compressed more easily than water?
Air is a gas.
Its particles have relatively large spaces:
between them.
Water particles are already much closer together.
Worked Example 6
A sealed syringe containing air can be pushed inward.
Why?
The gas particles can be forced:
closer together.
The gas therefore occupies a smaller volume.
Worked Example 7
Why does water not fill the entire volume of an open bottle?
Water is a liquid and has an approximately fixed:
volume.
It takes the shape of the lower part of the container and forms a:
free surface.
Worked Example 8
Why is blood considered a fluid?
Blood can:
flow and continuously change shape.
It therefore behaves as a fluid even though it contains suspended cells and has more complicated flow properties than water.
Worked Example 9
Why is air considered a fluid?
Air can:
- flow
- change shape
- exert pressure
- fill a container
Therefore air meets the definition of a:
fluid.
Worked Example 10
Water and air are placed under increased pressure.
Which will normally show the larger decrease in volume?
Air.
Gases are much more compressible than liquids because their particles have much greater:
spacing.
Common Mistake: Fluid Means Liquid
In everyday language this is common.
In physics:
liquids and gases are both fluids.
Common Mistake: Gases Have No Mass
Gases are made of particles and therefore have:
mass.
Air also has:
density.
A container filled with compressed air has slightly more mass than the same container after some of that air is released.
Common Mistake: Gases Have No Pressure
Gas particles collide with surfaces.
These collisions exert:
forces.
Force distributed over an area creates:
pressure.
Common Mistake: Liquids Cannot Be Compressed at All
Liquids can be compressed slightly.
However, compared with gases, they are:
very difficult to compress.
Treating liquids as incompressible is often a useful approximation.
Common Mistake: All Fluids Flow at the Same Rate
Different fluids have different:
viscosities.
Honey, water, and air all flow, but they do not respond identically under the same conditions.
Common Mistake: A Fluid Must Be Visible
Air is usually invisible, but it is still:
matter.
It has mass, occupies space, exerts pressure, and flows.
Therefore it is a:
fluid.
Check Your Understanding
- Define a fluid.
- Name the two main states of matter classified as fluids.
- Why is a liquid considered a fluid?
- Why is a gas considered a fluid?
- Why is an ordinary solid not considered a fluid?
- Compare the particle arrangement in solids and liquids.
- Compare the particle arrangement in liquids and gases.
- Why can liquid particles move past one another?
- Why can gas particles move freely?
- Which states have a fixed shape?
- Which states have approximately fixed volume?
- Which states can flow?
- What happens to the shape of water when it is poured into a new container?
- What happens to its volume?
- What happens when a gas is placed in a larger container?
- Why does a gas fill its container?
- Define compressibility.
- Which is more compressible: a liquid or a gas?
- Explain why gases are compressible.
- Explain why liquids are difficult to compress.
- Describe a syringe experiment that compares liquid and gas compressibility.
- What happens to a gas when its pressure is increased?
- How can heating affect a gas?
- What can happen when a gas is cooled?
- Define density.
- Write the density equation.
- Why are liquids generally denser than gases?
- Define pressure.
- Write the pressure equation.
- Why does a fluid exert pressure?
- How does pressure change with depth in a liquid?
- What is atmospheric pressure?
- Define viscosity.
- Which has greater viscosity under ordinary conditions: water or honey?
- Do gases have viscosity?
- Give five examples of everyday fluids.
- Explain why blood is considered a fluid.
- Explain why air is considered a fluid.
- What is laminar flow?
- What is turbulent flow?
- What happens to fluid behaviour when a substance melts?
- Is steam a fluid? Explain.
- Is ice normally classified as a fluid? Explain.
- What is a non-Newtonian fluid?
- Give two examples of non-Newtonian fluids.
- Compare the shape and volume of liquids and gases.
- Explain why hydraulic systems usually use liquids.
- Explain why pneumatic systems use gases.
- Compare how water and air respond when pressure increases.
- Explain, using the particle model, why liquids and gases are both fluids but behave differently.
Key Terms
Fluid: Substance that can flow and continuously change shape when subjected to shear.
Liquid: State of matter with approximately fixed volume but no fixed shape.
Gas: State of matter with neither fixed shape nor fixed volume.
Flow: Continuous movement and deformation of a fluid.
Particle model: Model explaining matter in terms of moving particles.
Compressibility: Measure of how easily a substance's volume can be reduced by pressure.
Density: Mass per unit volume.
Pressure: Force acting per unit area.
Viscosity: Resistance of a fluid to flow.
Laminar flow: Smooth fluid motion in which neighboring layers move in an orderly manner.
Turbulent flow: Fluid motion containing irregular fluctuations and mixing.
Hydraulics: Use of liquids to transmit forces and energy.
Pneumatics: Use of compressed gases to transmit forces and energy.
Non-Newtonian fluid: Fluid whose apparent viscosity changes depending on how it is stressed or deformed.
Fluid mechanics: Study of fluids and the forces acting on them.
Key Takeaways
- A fluid is a substance that can flow and continuously change shape.
- Both liquids and gases are fluids.
- Solids normally maintain their shape because their particles cannot freely move past one another.
- Liquid particles remain close together but can move past each other.
- Gas particles are widely separated and move rapidly in all directions.
- Liquids have approximately fixed volume but no fixed shape.
- Gases have neither fixed shape nor fixed volume.
- Liquids take the shape of their containers but do not normally fill the entire available volume.
- Gases expand to fill their containers.
- Gases are much more compressible than liquids because there is much more space between their particles.
- Both liquids and gases have mass, density, pressure, and viscosity.
- Fluid behaviour can change with temperature and pressure.
- Fluids are essential in biological systems, weather, transportation, engineering, and everyday life.
- Hydraulic systems make use of relatively incompressible liquids, while pneumatic systems make use of compressible gases.
- Understanding how liquids and gases behave provides the foundation for studying pressure, buoyancy, hydraulics, aerodynamics, and fluid flow.