1. Solutes and Solvents

Learning Targets

• I can define solute, solvent, and solution.
• I can identify the solute and solvent in a solution.
• I can distinguish between homogeneous and heterogeneous mixtures.
• I can explain how solutions differ from pure substances.
• I can identify examples of solutions in everyday life.

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Introduction

Many of the substances we use every day are not pure substances but mixtures. Seawater, soft drinks, vinegar, air, and even blood are all mixtures made by combining different substances. One of the most common types of mixture is a solution, in which one substance dissolves evenly in another.

Understanding solutes, solvents, and solutions is an important foundation for chemistry. These concepts help explain how medicines are prepared, how nutrients are transported in living organisms, how cleaning products work, and why many chemical reactions take place more easily in solution.

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What Is a Solution?

A solution is a homogeneous mixture formed when one substance dissolves completely in another.

A solution contains:

• A solute.
• A solvent.

The particles are evenly distributed throughout the mixture, so every sample of the solution has the same composition.

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Figure 1. A solution forms when a solute dissolves uniformly in a solvent.

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What Is a Solute?

A solute is the substance that is dissolved.

The solute is usually present in the smaller amount.

Examples:

• Sugar in sugar water.
• Salt in seawater.
• Carbon dioxide in soft drinks.
• Oxygen dissolved in water for fish.

A solute may be a:

• Solid.
• Liquid.
• Gas.

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What Is a Solvent?

A solvent is the substance that does the dissolving.

The solvent is usually present in the greater amount.

The most common solvent is water, which is often called the universal solvent because it dissolves many substances.

However, not everything dissolves in water.

Other common solvents include:

• Ethanol.
• Acetone.
• Hexane.

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Identifying Solutes and Solvents

Solution	Solute	Solvent
Salt water	Salt	Water
Sugar solution.	Sugar	Water
Vinegar	Acetic acid	Water
Soft drink	Carbon dioxide, sugar.	Water
Brass	Zinc	Copper

In most liquid solutions, the solvent is the substance present in the greatest amount.

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Figure 2. Many common solutions consist of one or more solutes dissolved in a solvent.

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Homogeneous Mixtures

A homogeneous mixture has a uniform composition throughout.

This means:

• The different substances are evenly mixed.
• The individual components cannot be seen.
• Every sample is the same.

Examples:

• Salt water.
• Air.
• Vinegar.
• Brass.
• Soft drinks.

Solutions are always homogeneous mixtures.

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Heterogeneous Mixtures

A heterogeneous mixture has a non-uniform composition.

The different substances remain visibly separate.

Examples:

• Sand and water.
• Oil and water.
• Soil.
• Granite.
• Salad.

Different parts of a heterogeneous mixture may have different compositions.

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Comparing Homogeneous and Heterogeneous Mixtures

Homogeneous Mixture	Heterogeneous Mixture
Uniform throughout	Non-uniform
One visible phase	Two or more visible phases
Components evenly mixed.	Components remain separate
Example: Salt water	Example: Sand and water

Understanding this difference helps classify mixtures correctly.

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Figure 3. Homogeneous mixtures are uniform throughout, while heterogeneous mixtures are not.

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Solutions vs Pure Substances

A pure substance contains only one type of particle.

Examples:

• Pure water (H₂O).
• Oxygen gas (O₂).
• Copper (Cu).

A solution contains two or more substances mixed together.

Although a solution appears to be one substance, it is actually a mixture.

Pure Substance	Solution
One type of particle	Two or more substances
Fixed composition	Variable composition
Cannot be separated by simple physical methods into identical particles.	Components can often be separated by physical methods such as evaporation or distillation

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Everyday Examples of Solutions

Solutions are found almost everywhere.

Examples include:

• Seawater.
• Tea and coffee.
• Soft drinks.
• Vinegar.
• Cleaning solutions.
• Medicines.
• Perfume.
• Air.

Even many metals are mixed to form alloys, which are solid solutions.

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Figure 4. Solutions are common in everyday life, from drinks to medicines and cleaning products.

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Why Solutions Are Important

Solutions play an important role in:

Biology

• Blood transports nutrients and oxygen in solution.
• Plant roots absorb mineral ions dissolved in water.

Medicine

• Many medicines are prepared as solutions.
• Saline solution is used in hospitals.

Industry

• Cleaning products.
• Paints.
• Food production.
• Chemical manufacturing.

Environment

• Oceans contain dissolved salts.
• Rivers carry dissolved minerals.

Solutions are essential in many natural and technological processes.

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Particle Model of a Solution

When a solute dissolves:

• Solute particles separate.
• Solvent particles surround the solute particles.
• The particles spread evenly throughout the solvent.

Because the particles are so small and evenly distributed, they cannot usually be seen.

This explains why solutions appear uniform.

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Figure 5. In a solution, solvent particles surround and evenly distribute solute particles.

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Worked Example

Question

Identify the solute and solvent in each mixture.

Mixture	Solute	Solvent
Salt water	?	?
Sugar solution	?	?
Carbonated water.	?	?

 

 

Solution

Mixture	Solute	Solvent
Salt water	Salt	Water
Sugar solution	Sugar	Water
Carbonated water.	Carbon dioxide	Water

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Real-World Connection

Sports drinks are carefully prepared solutions containing water, sugars, and dissolved mineral ions such as sodium and potassium. During exercise, these solutions help replace water and electrolytes lost through sweating while also providing energy. Because the ingredients are dissolved evenly, every sip has the same composition.

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Did You Know?

Although water is often called the universal solvent, it cannot dissolve every substance. For example, oil does not dissolve in water because oil molecules are non-polar, while water molecules are polar. Chemists often summarise this idea with the phrase "like dissolves like," meaning substances with similar chemical properties tend to dissolve in one another.

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Key Terms

Heterogeneous mixture – A mixture with a non-uniform composition in which different components remain distinguishable.

Homogeneous mixture – A mixture with a uniform composition throughout.

Pure substance – A material made of only one type of particle with a fixed composition.

Solution – A homogeneous mixture formed when a solute dissolves in a solvent.

Solute – The substance that is dissolved in a solution.

Solvent – The substance that dissolves the solute, usually present in the greatest amount.

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Key Takeaways

• A solution is a homogeneous mixture made when a solute dissolves in a solvent.
• The solute is the substance being dissolved, while the solvent is the substance that does the dissolving.
• Homogeneous mixtures are uniform throughout, whereas heterogeneous mixtures have a non-uniform composition.
• Solutions differ from pure substances because they contain two or more substances mixed together.
• Many everyday materials, including seawater, soft drinks, vinegar, medicines, and air, are examples of solutions.
• The particle model explains why solutions appear uniform: the solute particles are evenly distributed among the solvent particles.

 

 

 

2. The Process of Dissolving

Introduction

When sugar disappears in a cup of tea or salt seems to vanish in water, it may look as though the substance has disappeared completely. In reality, the particles of the substance have simply spread throughout the liquid to form a solution. This process is called dissolving or dissolution.

The particle model of matter helps explain exactly what happens during dissolving. It shows how particles of the solute separate, interact with particles of the solvent, and become evenly distributed throughout the solution. Understanding this process also helps explain why some substances dissolve easily while others do not.

What Is Dissolving?

Dissolving is the process in which the particles of a solute become evenly distributed throughout a solvent to form a solution.

During dissolving:

• The solute breaks into tiny particles.
• Solvent particles surround the solute particles.
• The particles spread evenly throughout the solvent.

The solute has not disappeared—its particles are simply too small to see.

 

 

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Figure 1. Solute particles separate and become evenly distributed among solvent particles.

Dissolving and the Particle Model

According to the particle model:

• All matter is made of tiny particles.
• Particles are constantly moving.
• There are spaces between particles.
• Particles attract one another.

When a solute is added to a solvent:

1. Solvent particles collide with the surface of the solute.
2. They pull individual solute particles away.
3. The solvent surrounds these particles.
4. The solute particles spread throughout the solvent.

Eventually, the solution becomes uniform.

Particle Interactions During Dissolving

During dissolution:

• Attractive forces between solute particles are weakened.
• New attractions form between solute particles and solvent particles.

If the attraction between the solute and solvent is strong enough, the substance dissolves.

If these attractions are too weak, the substance remains undissolved.

 

 

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Figure 2. Solvent particles surround and separate solute particles during dissolving.

Dissolving Is Not a Chemical Reaction

Dissolving is a physical change, not a chemical reaction.

During dissolving:

• No new substance is formed.
• The chemical identity of the solute remains unchanged.
• The process can often be reversed.

For example:

Salt dissolved in water can be recovered by evaporating the water.

Comparing Dissolving and Chemical Reactions

Understanding this difference helps identify physical and chemical changes.

Why Do Some Substances Dissolve?

Not every substance dissolves in every solvent.

A substance dissolves when:

• Solute particles are strongly attracted to solvent particles.

A useful rule is:

"Like dissolves like."

This means:

• Polar substances usually dissolve in polar solvents.
• Non-polar substances usually dissolve in non-polar solvents.

For example:

• Salt dissolves well in water.
• Sugar dissolves well in water.
• Oil does not dissolve in water.

 

 

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Figure 3. Some substances dissolve easily, while others do not because of differences in particle interactions.

Factors That Affect the Rate of Dissolving

Several factors affect how quickly a substance dissolves.

Temperature

Increasing the temperature usually causes solids to dissolve faster because:

• Solvent particles move more quickly.
• More frequent collisions occur.

For gases, increasing temperature usually makes them less soluble.

Stirring

Stirring mixes the solvent.

Fresh solvent particles are continually brought into contact with the solute, increasing the rate of dissolving.

Particle Size

Crushing a solid into smaller pieces increases its surface area.

Greater surface area allows:

• More solvent particles to collide with the solute.
• Faster dissolving.

Powdered sugar dissolves faster than a sugar cube.

Nature of the Solute and Solvent

Some substances naturally dissolve more readily than others because of differences in their particle attractions.

For example:

• Sugar dissolves easily in water.
• Sand does not dissolve in water.

 

 

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Figure 4. Temperature, stirring, and particle size all affect the rate at which a substance dissolves.

Dissolving vs Melting

Students sometimes confuse dissolving with melting.

For example:

• Ice melting forms liquid water.
• Sugar dissolving forms a sugar solution.

These are different processes.

Everyday Examples of Dissolving

Examples include:

• Sugar dissolving in tea.
• Salt dissolving in cooking water.
• Instant coffee dissolving in hot water.
• Medicines dissolving in the stomach.
• Oxygen dissolving in rivers and lakes for aquatic life.

Dissolving is an essential process in both nature and everyday life.

 

 

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Figure 5. Dissolving occurs in many everyday situations and natural processes.

Worked Example

Question

A student wants sugar to dissolve more quickly in a cup of tea.

Suggest three ways to increase the rate of dissolving.

Solution

The student could:

• Stir the tea.
• Use hotter tea.
• Crush the sugar into smaller pieces.

Each of these methods increases the rate at which sugar dissolves.

Real-World Connection

Many medicines are designed to dissolve at different rates inside the body. Some tablets dissolve quickly to provide rapid pain relief, while others have special coatings that dissolve slowly over several hours, releasing the medicine gradually. Scientists carefully control particle size and tablet design to achieve the desired rate of dissolution.

Did You Know?

Tiny fish and other aquatic animals depend on oxygen dissolved in water for survival. Although oxygen is a gas, some of its particles dissolve naturally in rivers, lakes, and oceans. Cold water can hold more dissolved oxygen than warm water, which is one reason why many fish thrive in cooler environments.

Key Terms

Dissolving (dissolution) – The process in which a solute becomes evenly distributed throughout a solvent to form a solution.

Particle model – A model describing matter as tiny particles that are constantly moving.

Physical change – A change that does not produce a new substance.

Solution – A homogeneous mixture formed when a solute dissolves in a solvent.

Solubility – The ability of a substance to dissolve in a particular solvent.

Solute – The substance being dissolved.

Solvent – The substance that dissolves the solute.

Surface area – The amount of exposed surface of a material; increasing surface area generally increases the rate of dissolving.

Key Takeaways

• Dissolving is the process in which a solute becomes evenly distributed throughout a solvent to form a solution.
• The particle model explains dissolving as solvent particles separating and surrounding solute particles.
• Dissolving is a physical change, not a chemical reaction, because no new substance is formed.
• A substance dissolves when the attraction between the solute and solvent particles is strong enough.
• The rate of dissolving is affected by temperature, stirring, particle size (surface area), and the nature of the solute and solvent.
• Dissolving is important in everyday life, from preparing food and medicines to supporting life in aquatic ecosystems.

3. Solubility

Learning outcomes

• I can define solubility.
• I can describe what it means for a substance to be soluble or insoluble.
• I can interpret solubility data and tables.
• I can compare the solubility of different substances.
• I can predict whether a substance will dissolve under given conditions.

 

What Is Solubility?

When one substance is mixed with another, it may dissolve.

For example, when sugar is added to water and stirred, the sugar crystals gradually disappear and form a solution.

The ability of a substance to dissolve in a particular solvent is described by its solubility.

Solubility is the maximum amount of a solute that can dissolve in a given amount of solvent at a particular temperature.

A common way of expressing solubility is:

grams of solute per 100 g of water​

For example, if a substance has a solubility of:

36 g per 100 g of water

this means that a maximum of approximately 36 g can dissolve in 100 g of water under the stated conditions.

 

 

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Solute, Solvent, and Solution

Three important terms are used when discussing solubility.

Solute

The solute is the substance being dissolved.

For example, in salt water:

salt = solute​

 

Solvent

The solvent is the substance that dissolves the solute.

In salt water:

water = solvent​

 

Solution

A solution is the mixture formed when the solute dissolves in the solvent.

Therefore:

solute+solvent→solution​

For example:

salt+water→salt solution

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What Does Dissolving Mean?

When a substance dissolves, its particles become distributed throughout the solvent.

The solute has not disappeared.

Its particles are simply too small and too evenly distributed to see individually.

For example, when salt dissolves in water, the salt is still present.

We know this because the water tastes salty, and the salt can be recovered by evaporating the water.

Dissolving is therefore different from a substance simply vanishing.

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Soluble Substances

A substance is described as soluble if it can dissolve in a particular solvent.

For example:

• Sodium chloride is soluble in water.
• Sugar is soluble in water.
• Copper sulfate is soluble in water.

However, solubility always depends on the solvent.

A substance that dissolves well in one solvent may not dissolve well in another.

Therefore, instead of simply saying:

"This substance is soluble."

it is more accurate to say:

"This substance is soluble in water."

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Insoluble Substances

A substance is described as insoluble if very little of it dissolves in a particular solvent.

For example, sand is considered insoluble in water.

If sand is mixed with water, the particles remain visible and eventually settle.

 

 

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We can therefore compare:

Salt + water

→ solution

Sand + water

→ mixture with undissolved solid

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Soluble Does Not Mean Unlimited Solubility

Even a soluble substance cannot necessarily dissolve in unlimited amounts.

Suppose you gradually add sugar to a glass of water.

At first, the sugar dissolves.

If you continue adding more and more sugar, eventually some sugar will remain undissolved at the bottom.

The solution has reached the maximum amount of dissolved solute possible under those conditions.

It is now called a saturated solution.

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Saturated Solutions

A saturated solution contains the maximum amount of dissolved solute possible at a particular temperature.

If additional solute is added, it will not dissolve.

For example:

Suppose the solubility of substance X is:

40 g per 100 g of water

at 20^oC.

If we add 30 g to 100 g of water:

30 < 40

all of it can dissolve.

If we add 40 g:

40 = 40

the solution can become saturated.

If we add 50 g:

50 > 40

only 40 g can dissolve.

The remaining:

50 − 40 = 10 g

will remain undissolved.

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Unsaturated Solutions

An unsaturated solution contains less dissolved solute than the maximum possible amount.

For example, suppose the solubility is:

50 g per 100 g of water

but the solution currently contains only:

20 g

of dissolved solute.

More solute can still dissolve.

Therefore, the solution is:

unsaturated​

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Solubility Is a Quantitative Property

Solubility tells us how much of a substance can dissolve.

For example:

Substance	Solubility at 20°C (g per 100 g water)
Substance A	15
Substance B	36
Substance C	75
Substance D	5

Substance C has the greatest solubility.

Substance D has the lowest solubility.

We can therefore write:

C > B > A > D​

in order of decreasing solubility.

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Interpreting Solubility Tables

Consider the following data:

Temperature (°C)	Solubility of Substance X (g per 100 g water)
0	15
20	25
40	40
60	60
80	85

At:

20^oC

the solubility is:

25 g per 100 g water

This means a maximum of 25 g of substance X can dissolve in 100 g of water at 20^oC.

At:

60^oC

the solubility is:

60 g per 100 g water

Therefore, considerably more substance X can dissolve at the higher temperature.

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Temperature and Solubility

For many solid substances, solubility in water increases as temperature increases.

This means that hot water can often dissolve more solute than cold water.

 

 

 

For example:

20^oC → 30 g

 

40^oC → 50 g 

 

60^oC → 75 g

The increasing values show that the substance becomes more soluble as temperature increases.

However, this is not true for every substance, so actual solubility data should be used whenever possible.

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Reading a Solubility Curve

Solubility data can also be displayed on a graph called a solubility curve.

The graph usually has:

Temperature on the x-axis.

Solubility on the y-axis.

A point on the curve tells us the maximum amount of solute that can dissolve at that temperature.

For example, if the curve shows:

50 g per 100 g water

at:

40^oC

then a maximum of 50 g can dissolve in 100 g of water at that temperature.

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Comparing Solubility Curves

If several substances are shown on the same graph, we can compare their solubilities.

At a particular temperature, the substance with the higher curve generally has the greater solubility.

For example, at 40^oC:

A = 70 g per 100 g water B = 45 g per 100 g water

Therefore:

Substance A is more soluble than Substance B at 40^oC​

Always compare substances at the same temperature.

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Predicting Whether a Substance Will Dissolve

We can use solubility data to predict what will happen when a solute is added to water.

Suppose the solubility of potassium nitrate at a certain temperature is:

60 g per 100 g water

 

Adding 40 g

40 < 60

All 40 g can dissolve.

The solution is unsaturated.

Adding 60 g

60 = 60

The maximum amount can dissolve.

The solution is saturated.

Adding 75 g

75 > 60

Only 60 g can dissolve.

The amount remaining is:

75 − 60 = 15 g

Therefore:

15 g remains undissolved​

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Example: Using Solubility Data

The solubility of substance Y is:

32 g per 100 g water at 20^oC

A student adds 45 g of Y to 100 g of water.

How much can dissolve?

32 g​

How much remains undissolved?

45 − 32 = 13

Therefore:

13 g remains undissolved​

The resulting solution is saturated.

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What If the Amount of Water Changes?

Solubility is often given per 100 g of water, but an experiment may use a different amount.

Suppose the solubility is:

40 g per 100 g water

How much can dissolve in 200 g of water?

Since:

200 = 2×100

we can dissolve:

2×40 = 80

Therefore:

80 g​

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Another Example

Suppose the solubility is:

60 g per 100 g water

How much can dissolve in 50 g of water?

Since 50 g is half of 100 g:

60×\( \frac{50}{100} \) ​ =30

Therefore:

30 g​

can dissolve.

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A General Solubility Calculation

If solubility is given in grams per 100 g of water:

maximum mass dissolved=100solubility×mass of water​​

For example:

Solubility:

80 g per 100 g water

Water used:

150 g

Then:

maximum mass = \( \frac{80}{100} \)×150​ =120 g

Therefore:

120 g can dissolve​

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Cooling a Saturated Solution

Suppose a substance has the following solubilities:

At 80^oC:

80 g per 100 g water

At 20^oC:

30 g per 100 g water

We dissolve 80 g of the substance in 100 g of water at 80^oC.

The solution is then cooled to 20^oC.

At the lower temperature, only 30 g can remain dissolved.

Therefore:

80 − 30 = 50 g

of solid can come out of the solution.

This solid may form crystals.

 

 

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Solubility and Crystallisation

Changes in solubility can be used to separate and purify substances.

A common process is:

dissolve → heat → cool → crystallise

If a substance is much more soluble in hot water than cold water, a concentrated hot solution can be prepared.

As the solution cools, some of the dissolved substance can no longer remain in solution.

It forms solid crystals.

This process is called crystallisation.

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Dissolving vs. Melting

Dissolving and melting are different processes.

Dissolving

A solute mixes with a solvent to form a solution.

For example:

salt + water → salt solution

 

Melting

A solid changes into a liquid because of heating.

For example:

ice → liquid water

When sugar disappears into water, it has dissolved, not melted.

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Dissolving Faster Does Not Mean Greater Solubility

Another important distinction is between rate of dissolving and solubility.

Stirring can make a substance dissolve faster.

Crushing a solid into smaller pieces can also make it dissolve faster.

However, these changes do not necessarily increase the maximum amount that can dissolve.

Therefore:

Rate of dissolving asks:

How quickly does it dissolve?​

Solubility asks:

How much can dissolve?​

These are different properties.

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Predicting Solubility

When deciding whether a substance will dissolve, consider:

The Solute

Different substances have different solubilities.

The Solvent

A substance may dissolve in one solvent but not another.

Temperature

Temperature can affect how much solute can dissolve.

Amount of Solvent

More solvent can usually dissolve a greater total amount of solute.

To make an accurate prediction, we therefore need to know the substance, solvent, temperature, and quantities involved.

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Did You Know?

Gases can also dissolve in liquids.

Carbonated drinks contain dissolved carbon dioxide gas.

Unlike many solid solutes, gases generally become less soluble in water as temperature increases.

This is one reason a warm carbonated drink tends to lose its dissolved carbon dioxide more quickly than a cold one.

Solubility therefore depends not only on temperature, but also on the type of substance being dissolved.

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Key Vocabulary

Solubility – The maximum amount of a solute that can dissolve in a given amount of solvent at a particular temperature.

Solute – The substance being dissolved.

Solvent – The substance that dissolves the solute.

Solution – A mixture formed when a solute dissolves in a solvent.

Soluble – Able to dissolve in a particular solvent.

Insoluble – Unable to dissolve significantly in a particular solvent.

Saturated solution – A solution containing the maximum amount of dissolved solute possible under the given conditions.

Unsaturated solution – A solution that can still dissolve additional solute.

Solubility curve – A graph showing how the solubility of a substance changes with temperature.

Crystallisation – The formation of solid crystals from a solution.

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Key Takeaways

• Solubility describes the maximum amount of a solute that can dissolve under particular conditions.
• Solubility is often expressed as grams of solute per 100 g of water.
• The solute dissolves in the solvent to form a solution.
• A soluble substance can dissolve in a particular solvent, while an insoluble substance dissolves very little.
• A saturated solution contains the maximum amount of dissolved solute possible at that temperature.
• Solubility tables and curves can be used to compare different substances.
• For many solids, solubility in water increases as temperature increases, although there are exceptions.
• Solubility data can be used to predict whether all of an added substance will dissolve.
• If more solute is added than can dissolve, some will remain undissolved.
• Cooling a saturated solution can cause dissolved solute to form crystals.
• Stirring may increase the rate of dissolving, but it does not necessarily increase solubility.
• Accurate predictions about dissolving depend on the solute, solvent, temperature, and quantities involved.

4. Saturated and Unsaturated Solutions

Saturated and Unsaturated Solutions

A solution forms when a solute dissolves in a solvent.

For example, when sugar dissolves in water:

• sugar is the solute
• water is the solvent
• sugar solution is the solution

However, a solvent cannot dissolve an unlimited amount of solute. At a particular temperature, there is a maximum amount that can dissolve.

This leads to two important types of solution:

• unsaturated solutions
• saturated solutions

 

 

Unsaturated Solutions

An unsaturated solution contains less than the maximum amount of dissolved solute that the solvent can hold at a particular temperature.

This means:

more solute can still dissolve

For example, suppose a spoonful of sugar is added to water and completely disappears.

If another spoonful can also dissolve, the solution is still unsaturated.

An unsaturated solution has not yet reached its solubility limit.

Saturated Solutions

A saturated solution contains the maximum amount of dissolved solute that can remain dissolved in a particular amount of solvent at a particular temperature.

Once the solution reaches this point:

additional solute will not dissolve overall

Instead, extra solid solute may remain at the bottom of the container.

 

 

Comparing Saturated and Unsaturated Solutions

The important difference is not simply how much solute is present.

The key question is:

Can more solute dissolve under the current conditions?

How Saturation Occurs

Imagine adding sugar to a beaker of water one spoonful at a time.

At first:

• sugar particles separate
• solvent particles surround them
• the sugar dissolves

As more sugar is added:

• the concentration of dissolved sugar increases
• fewer additional sugar particles can remain dissolved

Eventually, the solution reaches its maximum dissolved concentration.

The solution is now saturated.

A Step-by-Step Example

Suppose we slowly add salt to water.

First addition:

All the salt dissolves.

The solution is unsaturated.

Second addition:

The salt still dissolves.

The solution is still unsaturated.

More salt is added:

The concentration increases.

Eventually:

Some newly added salt remains at the bottom.

The solution has reached saturation.

This sequence can be represented as:

solvent → unsaturated solution → saturated solution → saturated solution + excess solid

What Happens When More Solute Is Added?

If more solute is added to an unsaturated solution, the added solute can dissolve.

Therefore:

more solute added → concentration increases

This continues until the saturation point is reached.

If more solute is added to a saturated solution, the solution cannot hold more dissolved solute under those conditions.

Therefore:

extra solute remains undissolved

The amount of dissolved solute stays approximately constant.

Saturated Does Not Mean "Full of Solid"

A saturated solution may appear completely clear.

It does not have to contain a visible pile of solid.

A solution is saturated when it contains the maximum amount of dissolved solute possible under those conditions.

Visible undissolved solute is strong evidence that a solution is saturated, provided enough time has been allowed for dissolving.

Particle View of an Unsaturated Solution

At the particle level, an unsaturated solution contains:

• solvent particles
• dissolved solute particles spread throughout the solvent
• room for additional solute particles to dissolve

The dissolved particles are distributed throughout the liquid.

There may be no solid solute remaining at the bottom.

Particle View of a Saturated Solution

In a saturated solution:

• many solute particles are already dissolved
• the maximum stable amount is dissolved
• additional solute cannot increase the dissolved concentration

If extra solute is present, some particles remain together as solid material at the bottom.

At the particle level, particles may continuously:

• leave the solid and dissolve
• return from the solution to the solid

In a saturated solution at equilibrium, these processes occur at equal average rates.

Dynamic Equilibrium

A saturated solution containing undissolved solute is not completely inactive.

Particles continue moving.

Some solute particles leave the solid and enter the solution.

At the same time, some dissolved particles return to the solid.

Eventually:

rate of dissolving = rate of crystallizing

This is called dynamic equilibrium.

The dissolved concentration remains constant even though individual particles continue to move between the solid and dissolved states.

Interpreting Particle Diagrams

Particle diagrams can be used to distinguish saturated and unsaturated solutions.

When examining a diagram, look for:

• how many solute particles are dispersed in the solvent
• whether undissolved solute remains at the bottom
• whether additional solute appears able to dissolve

An unsaturated particle diagram normally shows dissolved solute particles dispersed throughout the liquid with no evidence that the maximum has been reached.

A saturated diagram may show dissolved particles throughout the liquid plus undissolved solid particles.

Example: Interpreting a Particle Diagram

Suppose Diagram A shows:

• water particles
• several dissolved solute particles
• no solid at the bottom

Another amount of solute is added and dissolves.

Conclusion:

Diagram A represents an unsaturated solution.

Now suppose Diagram B shows:

• many dissolved solute particles
• a pile of undissolved solute at the bottom

Additional solute remains at the bottom.

Conclusion:

Diagram B represents a saturated solution.

Predicting What Happens as Solute Is Added

Suppose solute is added gradually to a fixed amount of solvent.

At first:

solute added → solute dissolves → concentration increases

As more is added:

more dissolves → concentration continues increasing

At saturation:

maximum dissolved concentration is reached

After saturation:

additional solute remains undissolved

Therefore, the concentration of dissolved solute stops increasing once saturation is reached, as long as temperature and solvent quantity remain unchanged.

Solubility

Solubility is the maximum amount of a solute that can dissolve in a given amount of solvent under particular conditions.

A saturated solution has reached this maximum.

An unsaturated solution is below this maximum.

Therefore:

unsaturated = below the solubility limit

saturated = at the solubility limit

Temperature Matters

Saturation depends on temperature.

For many solid solutes, more can dissolve in a solvent at higher temperatures.

For example, sugar generally dissolves in greater amounts in hot water than in cold water.

Therefore, a solution that is saturated at one temperature may become unsaturated if the temperature increases.

If more solute can now dissolve:

saturated solution + heating → potentially unsaturated solution

However, the exact effect of temperature depends on the particular solute and solvent.

Example: Heating a Saturated Solution

Suppose a saturated sugar solution contains undissolved sugar at 20°C.

The solution is heated.

If sugar becomes more soluble at the higher temperature:

• some of the remaining solid dissolves
• the amount of dissolved sugar increases
• the solution may temporarily become unsaturated until more sugar dissolves

Eventually, a new saturation point may be reached at the higher temperature.

Cooling a Saturated Solution

Suppose a hot saturated solution is cooled.

If the solubility decreases as temperature falls:

• the solution may temporarily contain more dissolved solute than is stable
• some solute may crystallize out

Crystals form because the cooler solvent can no longer keep as much solute dissolved.

Adding More Solvent

Another way to change saturation is to add more solvent.

Suppose a saturated salt solution contains undissolved salt.

If more water is added:

• the amount of solvent increases
• more salt may dissolve
• the solution may become unsaturated

The new solvent provides additional capacity for dissolving solute.

Saturated vs Concentrated

These terms do not mean exactly the same thing.

A concentrated solution contains a relatively large amount of dissolved solute.

A saturated solution contains the maximum amount that can dissolve under the current conditions.

A solution can be:

• concentrated but still unsaturated
• dilute but saturated if the solute has very low solubility

Saturation describes whether the solubility limit has been reached.

Concentration describes how much solute is actually present.

Dissolving Rate vs Solubility

It is also important to distinguish between:

how fast something dissolves

and

how much can dissolve

Stirring may make a solute dissolve faster.

Crushing a solid may make it dissolve faster.

These changes affect the rate of dissolving.

They do not necessarily increase the final amount that can dissolve.

That maximum amount is the solubility.

Worked Example: Adding Sugar

A student adds 10 g of sugar to water.

All of it dissolves.

Another 5 g is added.

This also dissolves.

What can we conclude?

The solution was unsaturated before the second addition because more sugar was able to dissolve.

Worked Example: Undissolved Salt

A student adds salt to water until some salt remains at the bottom after stirring for several minutes.

What does this indicate?

The solution has reached its solubility limit.

Therefore, the liquid portion is saturated with salt under those conditions.

Worked Example: Adding More Solute

A saturated solution contains 25 g of dissolved solute.

Another 5 g of the same solute is added at the same temperature.

What is most likely to happen?

The extra solute will remain undissolved.

The amount of dissolved solute remains approximately 25 g.

Worked Example: Adding More Water

A saturated solution has undissolved crystals at the bottom.

More water is added.

What happens?

Some of the crystals may dissolve because there is now more solvent available.

The solution can become unsaturated until enough solute dissolves to reach the new saturation limit.

Worked Example: Heating

A saturated solution of a solid is heated, and the solid becomes more soluble at higher temperatures.

Predict what happens.

The solution can dissolve additional solute.

Any undissolved solid may begin to dissolve.

Analyzing a Sequence

Imagine four beakers containing the same amount of water.

Beaker A:

5 g solute added; all dissolves.

Beaker B:

10 g solute added; all dissolves.

Beaker C:

15 g solute added; all dissolves.

Beaker D:

20 g solute added; 3 g remains at the bottom.

What can we conclude?

Beakers A, B, and C are below the saturation limit.

Beaker D contains a saturated solution plus excess undissolved solute.

At least 17 g of the solute dissolved in Beaker D.

A Useful Decision Test

To determine whether a solution is saturated, ask:

Can more of this solute dissolve at the same temperature?

If yes:

unsaturated

If no:

saturated

If added solute remains undissolved after sufficient mixing and time:

the solution is saturated

Common Misconceptions

A saturated solution contains no solvent.

Incorrect. It is still a solution containing both solvent and dissolved solute.

Saturated means the container is completely full.

Incorrect. Saturation refers to how much solute has dissolved, not how full the container is.

Any solution with undissolved solid is automatically saturated.

Not always. The solid may simply need more time to dissolve, or it may be insoluble. The system must be considered under appropriate conditions.

Adding more solute always increases concentration.

Incorrect. Once saturation is reached, additional solute remains undissolved.

Stirring increases solubility.

Stirring usually increases the rate of dissolving, but it does not normally change the maximum amount that can dissolve at a fixed temperature.

A concentrated solution must be saturated.

Incorrect. It may contain a lot of solute while still being able to dissolve more.

Saturated solutions contain particles that have stopped moving.

Incorrect. Particles continue moving, and a saturated solution can exist in dynamic equilibrium.

Did You Know?

A saturated solution can appear unchanged even though particles are constantly moving between dissolved and solid states.

At equilibrium:

dissolving continues

and

crystallization continues

but because the two processes occur at equal average rates, there is no overall change in the amount of dissolved solute.

Key Terms

Solution – A homogeneous mixture formed when a solute dissolves in a solvent.

Solute – The substance being dissolved.

Solvent – The substance that dissolves the solute.

Unsaturated solution – A solution that can still dissolve more solute under the current conditions.

Saturated solution – A solution containing the maximum stable amount of dissolved solute under the current conditions.

Solubility – The maximum amount of solute that can dissolve in a given amount of solvent under specified conditions.

Undissolved solute – Solute that remains as solid because it has not entered the solution.

Dynamic equilibrium – A condition in which opposing processes occur at equal average rates.

Concentration – The amount of solute present in a given amount of solution.

Crystallization – The formation of solid crystals from dissolved particles.

Key Takeaways

• An unsaturated solution can dissolve more solute.
• A saturated solution contains the maximum amount of dissolved solute possible under the current conditions.
• Saturation occurs when the solution reaches its solubility limit.
• Adding solute to an unsaturated solution increases the amount dissolved.
• Once saturation is reached, additional solute remains undissolved.
• Particle diagrams can show dissolved solute particles and excess solid solute.
• Saturated solutions can exist in dynamic equilibrium.
• Saturation depends on factors such as temperature and the amount of solvent.
• Adding more solvent can allow additional solute to dissolve.
• Heating often increases the solubility of solid solutes, although this depends on the substance.
• Concentrated and saturated do not mean the same thing.
• Stirring affects dissolving rate more than the final solubility.
• A useful test is: Can more solute dissolve under the current conditions?

5. Factors Affecting Solubility

Factors Affecting Solubility

Solubility is the maximum amount of a solute that can dissolve in a given amount of solvent under specific conditions.

Solubility is not always constant. Changing environmental conditions can change how much of a substance can remain dissolved.

Two particularly important factors are:

• temperature
• pressure

Temperature is especially important for both solids and gases, while pressure has its greatest effect on gases dissolved in liquids.

 

 

 

5

Solubility of Solids and Temperature

For many solid solutes, solubility increases as temperature increases.

This means that a larger amount of solid can often dissolve in hot solvent than in cold solvent.

For example, many salts and sugars become more soluble as water temperature increases.

A general trend for many solids is:

temperature increases → solubility increases

However, this is a general trend rather than a universal rule. Some solids show little change, and a few become less soluble as temperature increases.

---

Example: Dissolving a Solid

Suppose 35 g of a particular solid can dissolve in 100 g of water at 20°C.

At 60°C, perhaps 70 g can dissolve in the same amount of water.

Heating has therefore increased the solubility.

At 20°C:

Maximum dissolved = 35 g

At 60°C:

Maximum dissolved = 70 g

The warmer solution can hold more dissolved solute.

---

Why Temperature Can Affect Solid Solubility

Dissolving involves interactions between:

• solute particles
• solvent particles

Changing temperature changes the energy of these particles and can change the balance of interactions involved in dissolving.

For many common solids dissolved in water, higher temperatures favor greater solubility.

Because different substances have different particle interactions, however, they do not all respond to temperature in exactly the same way.

---

Heating a Saturated Solution

Suppose a solution is saturated at 20°C.

If it is heated and the solute becomes more soluble:

• the solubility limit increases
• additional solute can dissolve
• undissolved solid may begin to dissolve

Therefore, a solution that was saturated at a lower temperature may become unsaturated after heating.

More solute can then dissolve until the new saturation point is reached.

 

 

 

4

Cooling a Saturated Solution

The opposite can happen when a hot saturated solution cools.

If solubility decreases as temperature falls:

• the solvent can no longer hold as much dissolved solute
• excess dissolved solute leaves the solution
• crystals may form

This process is called crystallization.

Therefore:

cooling → lower solubility → excess solute crystallizes

This principle can be used to produce crystals in the laboratory.

---

Growing Crystals

A common crystal-growing method involves:

1. Heating a solvent.
2. Dissolving a large amount of solid solute.
3. Producing a concentrated or saturated hot solution.
4. Allowing the solution to cool slowly.
5. Allowing excess dissolved solute to form crystals.

Slow cooling can allow well-formed crystals to develop.

 

 

 

5

Solubility Curves

A solubility curve is a graph showing how the solubility of a substance changes with temperature.

Usually:

• temperature is shown on the horizontal axis
• solubility is shown on the vertical axis

Solubility may be expressed as:

grams of solute per 100 g of water

Each line represents a different substance.

 

 

 

4

Reading a Solubility Curve

Suppose a graph shows that the solubility of substance X is:

30 g per 100 g water at 20°C

and:

70 g per 100 g water at 60°C

This means:

At 20°C, a maximum of 30 g can dissolve in 100 g water.

At 60°C, a maximum of 70 g can dissolve in 100 g water.

Therefore:

70 − 30 = 40 g

An additional 40 g can dissolve at 60°C.

---

Interpreting the Shape of a Solubility Curve

The direction and steepness of a solubility curve provide useful information.

A line that rises steeply means:

solubility increases greatly with temperature

A line that rises gradually means:

temperature has a smaller effect

A nearly horizontal line means:

temperature has little effect on solubility

A downward-sloping line means:

solubility decreases as temperature increases

---

Points on a Solubility Graph

For a typical solubility curve:

On the curve

The solution is saturated.

It contains the maximum stable amount of dissolved solute at that temperature.

Below the curve

The solution is unsaturated.

More solute could dissolve.

Above the curve

The amount shown is greater than the normal solubility at that temperature.

Depending on the circumstances, this could represent excess undissolved solute or a temporarily supersaturated solution.

---

Worked Example: Reading a Solubility Graph

Suppose the solubility of substance A is:

40 g per 100 g water at 30°C.

A solution contains 25 g of A in 100 g water.

Because:

25 g < 40 g

the solution is unsaturated.

An additional:

40 − 25 = 15 g

could dissolve before the solution becomes saturated.

---

Worked Example: Cooling a Solution

Suppose a substance has a solubility of:

80 g per 100 g water at 70°C

and:

30 g per 100 g water at 20°C.

A saturated solution containing 80 g dissolved solute is cooled from 70°C to 20°C.

At 20°C, only 30 g can remain dissolved.

Therefore:

80 − 30 = 50 g

50 g of solute crystallizes out.

---

Comparing Different Solutes

Solubility curves can also be used to compare substances.

Suppose at 40°C:

Substance A = 60 g per 100 g water

Substance B = 35 g per 100 g water

Substance C = 15 g per 100 g water

At 40°C:

A is the most soluble

and:

C is the least soluble

It is important to compare substances at the same temperature.

---

Solubility of Gases

Gases often show the opposite temperature trend to many solids.

For many gases dissolved in liquids:

temperature increases → gas solubility decreases

Therefore, gases are generally more soluble in colder liquids.

This is why cold water can usually hold more dissolved oxygen than warmer water.

 

 

 

5

Why Gases Escape from Warm Liquids

Gas particles dissolved in a liquid can escape from the liquid surface.

When temperature increases:

• particles have greater kinetic energy
• dissolved gas particles can escape more readily
• less gas remains dissolved

Therefore:

warming a liquid often decreases gas solubility

---

Example: Warm Soft Drinks

Carbon dioxide is dissolved in carbonated drinks.

A cold carbonated drink generally keeps carbon dioxide dissolved more effectively than a warm one.

As the drink warms:

• carbon dioxide becomes less soluble
• more CO₂ escapes
• the drink loses carbonation more quickly

This is one reason carbonated drinks are usually best kept cold.

---

Temperature and Aquatic Life

Water contains dissolved oxygen that aquatic organisms use for respiration.

Cold water can generally contain more dissolved oxygen than warm water.

If water temperature rises significantly:

• oxygen solubility decreases
• dissolved oxygen levels may fall
• aquatic organisms may experience greater stress

Temperature therefore affects not only chemistry but also aquatic ecosystems.

---

Pressure and Gas Solubility

Pressure has a particularly important effect on gases dissolved in liquids.

For gases:

pressure increases → gas solubility increases

Increasing the pressure of a gas above a liquid causes more gas particles to remain dissolved.

Decreasing the pressure allows more dissolved gas to escape.

This relationship is described quantitatively by Henry's law under appropriate conditions.

 

 

 

5

Particle Explanation of Pressure

Imagine a gas above a liquid.

Gas particles constantly collide with the liquid surface.

At higher pressure:

• more gas particles are present per unit volume above the liquid
• collisions with the liquid surface occur more frequently
• more gas can be maintained in the dissolved state

Therefore, higher gas pressure generally increases gas solubility.

---

Carbonated Drinks and Pressure

Carbonated drinks provide a familiar example.

During manufacturing, carbon dioxide is placed above and dissolved into the drink under high pressure.

Inside a sealed bottle:

high CO₂ pressure → more CO₂ remains dissolved

When the bottle is opened:

pressure decreases

The carbon dioxide becomes less soluble.

Gas escapes from the liquid as bubbles.

This produces the familiar fizz.

---

Why a Warm Carbonated Drink Fizzes More

Opening a warm carbonated drink can produce especially rapid bubbling.

Two changes favor the escape of CO₂:

Higher temperature

reduces the solubility of the gas.

Lower pressure after opening

also reduces gas solubility.

Therefore:

warming + pressure decrease → more CO₂ escapes

---

Solids and Pressure

Pressure usually has very little effect on the solubility of solids and liquids because their particles are already relatively close together and their volumes change very little under ordinary pressure changes.

Therefore, in introductory chemistry:

pressure is mainly considered when discussing gases

---

Comparing Solids and Gases

Condition	Many Solids in Liquids	Gases in Liquids
Temperature increases.	Solubility usually increases	Solubility usually decreases
Temperature decreases	Solubility usually decreases.	Solubility usually increases
Pressure increases	Little effect	Solubility increases
Pressure decreases	Little effect	Solubility decreases

These are useful general trends, although individual substances can behave differently.

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A Visual Comparison of Typical Trends

For many solids, a typical trend might look like this:

Typical solid solubility trend

 

The important feature is the upward trend, not the particular values.

For gases, the typical temperature trend is reversed:

Typical gas solubility trend

Here the important feature is the downward trend.

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Predicting Changes in Solubility

When asked to predict what happens, first identify whether the solute is a solid or gas.

Then identify which condition changes.

For many solids:

increase temperature → increase solubility

For gases:

increase temperature → decrease solubility

For gases:

increase pressure → increase solubility

This provides a useful starting point for predictions.

---

Worked Example: Heating Salt Water

A saturated solution contains a solid whose solubility increases with temperature.

The solution is heated.

Prediction:

The solubility increases.

Therefore, more solid can dissolve.

If undissolved solid is present, some may dissolve.

---

Worked Example: Cooling a Hot Solution

A hot saturated solution contains 90 g of dissolved solute.

After cooling, the solubility is only 55 g under the new conditions.

Amount that crystallizes:

90 − 55 = 35 g

Therefore:

35 g crystallizes from the solution.

---

Worked Example: Heating a Gas Solution

Water contains dissolved oxygen.

The water temperature increases.

For gases:

temperature increases → solubility decreases

Therefore, less oxygen can remain dissolved.

Some dissolved oxygen may escape.

---

Worked Example: Increasing Pressure

Carbon dioxide gas is in contact with water.

The pressure of CO₂ above the water increases.

Prediction:

CO₂ solubility increases.

More carbon dioxide can dissolve in the water.

---

Worked Example: Opening a Bottle

A sealed carbonated drink is opened.

Pressure above the liquid suddenly decreases.

Prediction:

The solubility of carbon dioxide decreases.

CO₂ leaves the solution and forms bubbles.

---

Worked Example: Interpreting a Curve

Suppose a solubility graph shows:

At 20°C: 25 g/100 g water

At 40°C: 40 g/100 g water

At 60°C: 70 g/100 g water

What happens to solubility as temperature increases?

The solubility increases.

Between 20°C and 60°C:

70 − 25 = 45 g

The solubility increases by:

45 g per 100 g water

---

Worked Example: Crystallization from a Graph

A saturated solution contains 75 g of solute in 100 g water at 80°C.

The solution is cooled to 30°C.

The solubility at 30°C is 35 g per 100 g water.

Amount remaining dissolved:

35 g

Amount crystallized:

75 − 35 = 40 g

Therefore:

40 g of crystals form.

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Solubility vs Rate of Dissolving

A very important distinction is the difference between:

solubility

and:

rate of dissolving

Solubility tells us:

how much can dissolve

Rate of dissolving tells us:

how quickly it dissolves

---

Stirring

Stirring usually increases the rate of dissolving.

It brings fresh solvent particles into contact with the solute.

However, stirring does not normally change the maximum amount that can dissolve at equilibrium.

Therefore:

stirring → faster dissolving

but not necessarily:

stirring → greater solubility

---

Particle Size

Crushing a solid into smaller pieces increases its surface area.

This allows more contact between solute and solvent.

Therefore:

smaller particles → faster dissolving

Again, this affects the rate rather than necessarily changing the final solubility.

---

Temperature Can Affect Both

Temperature can be slightly confusing because it may affect:

• the rate of dissolving
• the solubility itself

A solid may dissolve faster in hot water because particles move more rapidly.

At the same time, for many solids, a larger total amount may also be able to dissolve.

These are related but different observations.

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A Useful Analysis Strategy

When analyzing a solubility question, ask:

1. What type of solute is involved?

Solid or gas?

2. What condition changes?

Temperature or pressure?

3. Which direction does the condition change?

Increase or decrease?

4. What happens to solubility?

Increase or decrease?

5. What happens to the solution?

More dissolves, gas escapes, or crystals form?

For example:

Gas + temperature increases → solubility decreases → gas escapes

or:

Many solids + cooling → solubility decreases → crystals may form

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Common Misconceptions

All solids become more soluble when heated.

Incorrect. Many do, but the exact relationship depends on the substance.

Heating always increases solubility.

Incorrect. Heating generally decreases the solubility of gases in liquids.

Pressure greatly changes the solubility of solids.

Under ordinary conditions, pressure has little effect on solid solubility.

Stirring increases solubility.

Stirring usually increases the rate of dissolving, not the final equilibrium solubility.

Crushing a solute increases its solubility.

Crushing increases surface area and usually makes dissolving faster, but it does not normally change the equilibrium solubility.

A point below a solubility curve represents a saturated solution.

Incorrect. A point below the curve normally represents an unsaturated solution.

A point on the curve represents an unsaturated solution.

Incorrect. A point on the curve represents saturation.

Cooling a saturated solution always causes all of the solute to crystallize.

Incorrect. Only the amount that exceeds the new solubility limit crystallizes.

Did You Know?

Fish can be affected by changes in water temperature partly because warmer water generally holds less dissolved oxygen.

At the same time, many aquatic animals have higher metabolic demands at warmer temperatures.

This can create a difficult combination:

less oxygen available + greater oxygen demand

Solubility therefore has important consequences not only in chemistry laboratories but also in natural ecosystems.

Key Terms

Solubility – The maximum amount of solute that can dissolve in a given amount of solvent under specified conditions.

Solute – The substance being dissolved.

Solvent – The substance that dissolves the solute.

Saturated solution – A solution containing the maximum stable amount of dissolved solute under the current conditions.

Unsaturated solution – A solution that can still dissolve additional solute.

Solubility curve – A graph showing how solubility changes with temperature.

Crystallization – Formation of solid crystals from dissolved solute.

Supersaturated solution – A solution temporarily containing more dissolved solute than would normally be stable at that temperature.

Pressure – Force acting per unit area.

Henry's law – The relationship describing how the solubility of a gas in a liquid depends on the gas pressure above the liquid under appropriate conditions.

Rate of dissolving – How quickly a solute dissolves.

Key Takeaways

• Solubility is the maximum amount of solute that can dissolve under specified conditions.
• Temperature can change solubility.
• For many solids, increasing temperature increases solubility.
• Cooling a saturated solution may cause crystals to form.
• Gases generally become less soluble as temperature increases.
• Cold liquids can generally hold more dissolved gas than warm liquids.
• Increasing gas pressure generally increases the solubility of a gas in a liquid.
• Decreasing pressure causes dissolved gases to escape more easily.
• Pressure has relatively little effect on the solubility of solids under ordinary conditions.
• Solubility curves show how solubility changes with temperature.
• A point on a solubility curve represents a saturated solution.
• A point below the curve represents an unsaturated solution.
• The steepness of a curve shows how strongly solubility changes with temperature.
• Solubility and rate of dissolving are different concepts.
• Stirring and crushing usually affect how fast a solid dissolves rather than how much can ultimately dissolve.
• A useful general comparison is: many solids become more soluble when heated, while gases become less soluble when heated.