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.

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 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:

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.