Solubility: Solubility Product & Henry's Law

Solubility is a property of a substance that can be defined as the maximum solute that will be dissolved in a known quantity of a solvent at a specific temperature. A solute is a substance which is dissolved in a solvent and can be solid, liquid or gas.

• For sparingly soluble salts, the solubility product determines the solubility.
• The solubility product is the maximum product of the molar concentration of the ions which are produced due to the dissociation of ions.
• Higher the solubility product, the higher the solubility and vice versa.
• Solubility is based on the composition of solute and solvent (including both its pH and other dissolved substances), alongside temperature and pressure. 

The extent of the solubility of a substance in a specific solvent can further be measured as the solute’s concentration in a saturated solution, where no more solute can be dissolved.

Key Terms: Solutions, Colligative Properties, Ideal Solutions, Non-ideal solutions, Solute, Solubility Product, Azeotropic Distillation

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What is Solubility?

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Solubility can be defined as:

A solution can be described as a homogeneous mixture of one or more than one solutes in a given solvent.

• Sugar cubes in a cup of tea are one common example of a solution.
• The property helping sugar molecules to dissolve is referred to as solubility.
• Thus, solubility can be described as a property of a substance (i.e. solute) to dissolve in a certain solvent.

Solubility

Solubility Product

Solubility Product can be applicable to sparingly soluble salts.

• Solubility Product can be defined as the “maximum product of the molar concentration of ions (which has been raised to their suitable powers) that are yielded due to the dissociation of the given compound.”
• The solubility product, at a certain temperature, remains constant.
• The lesser the value of the solubility product, the lower the solubility.
• And the higher the value of the solubility product, the greater the solubility.

Depending on solubility, the factors affecting solubility change the state of the solute. Thus,

• Liquids In Liquids
• Solids In Liquids
• Gases In Liquids

Solutions Video Explanation

Henry’s Law

As per Henry’s law, the solubility of a gas in a liquid is in direct proportion to the pressure of the gas at a fixed temperature. Henry’s Law states that:

“The partial pressure of the gas in the vapour phase (p) is proportional to the mole fraction of the gas (x) in the solution.” It can be further written as:

⇒ p = K_Hx

Here,

• K_H = Henry’s Law constant.

Applications of Henry’s Law

Some of the important Henry’s law applications are:

• Manufacturing of carbonated drinks.
• Water diving

Raoult’s Law as a special case of Henry’s Law

As per Raoult’s law,

p = x_ip_i⁰

According to Henry’s law, it is soluble in water.

Thus,

⇒ p = K_Hx

Therefore, Raoult’s law is considered as a specific case of Henry’s law, wherein K_H equals p_i⁰.

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Solubility of Liquids in Liquids

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Water is known to be a universal solvent since it dissolves almost every solute, excluding just a few. The new bond formed between the solute and the solvent is known as Solubility. According to the quantity, solubility can be defined as the maximum concentration of a solute which is seen to dissolve, at a given temperature, in a known concentration of a solvent,

• Based on this, solutes can be categorised into three categories, namely: Soluble, Sparingly Soluble and Insoluble.
• A solute is said to be soluble if 0.1g or more is dissolved in a 100ml solvent.
• Similarly, a solute is sparingly soluble if 0.1g or less of it is dissolved in 100ml of solvent.
• A saturated solution is one where a given amount of solute is completely dissolved in the solvent at a given temperature.
• A supersaturated solution is one where solute starts precipitating after a certain concentration is dissolved at the given temperature.

Factors Affecting Solubility

The factors that affect solubility are:

Temperature

​A change in temperature can highly affect the solubility of a solution. Liquids or sparingly soluble solid solutes can be dissolved completely if the temperature is increased. In the case of gaseous solutes, an increase in temperature decreases the solubility as the gases escape at high temperatures.

Forces and Bonds

​The nature of intermolecular forces and bonds is variable with different molecules. Hence, there are high chances of solubility in like substances than in unlike substances.

Pressure

​Pressure plays a more vital role in gases than in solids and liquids. Hence the solubility of gases in liquids increases with an increase in the partial pressure of the gas.

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Solubility of Solids in Liquids

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In the case of solids, solubility depends on the nature of the solute as well as the solvent. For example, naphthalene does not dissolve in water as compared to salt or sugar.

This is mainly because of the results of experiments and observations which show that polar solutes dissolve in polar solvents and non-polar solutes dissolve in non-polar solvents. This makes the nature of the solvent a major factor affecting the solubility of the solution.

• Let’s understand how a solid solute dissolves in a solvent.
• Once the solute is added to the solvent, solute particles collide with each other and some particles get separated out of the solution which is called crystallisation.
• The dissolving of solutes in the solvents is called dissolution.
• When the number of particles entering the solution becomes equal to the number of particles leaving the solution, a state of dynamic equilibrium is achieved.
• This results in the constant concentration of solute in the solution at a given pressure and temperature.
• A saturated solution is obtained when no more solute can be dissolved in the solvent at a given temperature and pressure and the solute is also said to have achieved maximum concentration.

Factors Affecting Solubility

Some factors affecting Solubility are:

Temperature

​According to Le Chatelier's principle, if the process of dissolution is endothermic in nature, then the solubility increases with the temperature. Similarly, if the process of dissolution is endothermic in nature, the solubility of the solid should decrease with the increase in temperature.

Pressure

Due to the high incompressibility of solids and liquids, solid solubility is hardly affected by pressure.

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Solubility of Gases in Liquids

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Gas solubility in liquids deals with the dissolving of gas in a solvent. 

• The gas solubility in liquids is affected by temperature, pressure and also the nature of the solute and solvent.
• There are several gases which are known to readily dissolve in water, whereas there are gases that don’t dissolve in water under normal conditions.
• Oxygen is known to be sparingly soluble in water. However, HCl or ammonia is seen to readily dissolve in water.

Factors Affecting Solubility

The factors that affect solubility are:

Effect of Pressure

• Gas solubility in liquids increases with an increase in pressure. 
• Consider a system of a gas solution in a solvent present in a closed container in a state of dynamic equilibrium.
• The solution can now be considered to be in equilibrium and hence, the rate of gaseous molecules which is seen to enter the solution is equivalent to the rate of gaseous molecules leaving it.

Now assume that the system’s pressure is increased by means of compressing the gas molecules in the solution.

• Due to the increase in pressure, the gas molecules will be concentrated in a smaller volume, causing an increase in the number of gas molecules per unit volume.
• As the number of gas molecules has increased, the rate with which the gas molecules will enter the solution will increase as well.

Finally, the result is that there is an increase in the number of gas molecules in the solution, till a new equilibrium point is achieved. Therefore, it can be said that the solubility of gases increases with the increase in pressure of a gas above the solution.

Solubility of Gases in Liquids

Henry’s Law represents a quantitative relation between pressure and gas solubility in a liquid. It further claims that: 

The general way of using Henry’s Law is that the mole fraction of the gas is directly proportional to the partial pressure of the gas.

⇒ P = K_Hx

Where,

• p = Partial pressure of the gas
• x = Mole fraction of the gas in a solution
• K_H = Henry’s law constant

Effect of Temperature

The gas molecules in a liquid are dissolved by dissolution, during which the evolution of heat is observed.

• According to Le Chatelier’s principle stating when the equilibrium of the system is disturbed, the system readjusts itself in such a way that the effect that has caused the changes in equilibrium is countered.
• Hence, as the process of dissolution is an exothermic process, the solubility should decrease with the increase in temperature validating Le Chatelier’s principle.

Solubility

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Things to Remember

• The maximum solute that will be dissolved in a known quantity of a solvent at a specific temperature is known as solubility.
• Solutes can be categorised into three categories, namely: Soluble, Sparingly Soluble and Insoluble.
• Factors that affect solubility include - temperature, forces and bonds and pressure.
• Henry’s Law states that the dissolvability of a gas in a liquid substance is directly in proportion to the limited pressure of the gas present above the surface of the solution.
• According to Le Chatelier’s principle, when the equilibrium of the system is disturbed, the system readjusts itself in such a way that the effect that has caused the changes in equilibrium is countered.

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Previous Year Questions

1. A 5% solution of cane sugar (MW = 342) isisotonic with 1%….[NEET 1998]
2. Pure water can be obtained from sea water by...[NEET 2001]
3. The vapour pressure of due to….[KCET 2012]
4. Number of moles of ions which 1 mole of ionic compound produces...[NEET 2009]
5. The boiling point of this solution will be….[NEET 2016]
6. An aqueous solution of a protein contains its….[NEET 2011]
7. The highest osmotic pressure is exhibited by 0.1 M solution of...[NEET 1994]
8. Find out the molecular weight of the substance...[NEET 1999]
9. 1 M and 2.5 litre NaOH solution mixed with another….[NEET 1980]
10. A 0.1 molal aqueous solution of a weak acid is...[NEET 2011]
11. For an ideal solution, the correct option is….[NEET 2019]
12. Which one of the following salt will have the same value of vant Hoff's...[NEET 1994]