# Equilibrium Involving Dissolution of Solids or Gases

## Equilibrium

Everything in the universe attempts to achieve stability by lowering its energy. Various chemical and physical processes help to attain this state. All the reactions continue until they reach a point of specific minimum energy that is in accordance with their surroundings. At this stage, the system is at a standstill with no visible change, and it continues to be in the same state unless disturbed. This state is known as equilibrium. In the equilibrium state, the 'rate of forward reaction' equals 'the rate of backward reaction'.

## The Solubility of Gases and Solids in Liquids

A substance's solubility refers to the maximum amount of substance that can be dissolved in any given solvent in a specific quantity. The solubility of a substance in a solvent depends upon:

1. Nature of solute.

2. Nature of the solvent.

3. Temperature.

4. Pressure.

### The Solubility of Solids in Liquids

The solubility of solute (solid) in liquids differs with temperature, nature of liquid and solid, and to a lesser degree on the system's pressure.

When a solute is added to a solvent, solid particles dissolve in the solvent, thus increasing its concentration in the solution. This process is called dissolution.

Some solid particles collide with other solid particulates in the solution and are precipitated out. This process is known as crystallization.

The dissolution process continues till the solution achieves a maximum concentration level, beyond which the dissolution of the solute stops. The solution where no excess solute particles can dissolve at the same pressure and temperature is known as a saturated solution.

### Equilibrium Involving Dissolution of Solids

Once a solution reaches the saturated stage, a type of equilibrium gets established. The equilibrium reached is between the process of crystallization and dissolution. At this stage, the number of solid particles entering the solution equals the solid particles separating out. This is the state of dynamic equilibrium. In a saturated solution, if the temperature and pressure remain constant, then the concentration of solute particles in the solution will stay the same.

Sugar (solution) = Sugar (solid)

Rate of dissolution of sugar = Rate of crystallization of sugar.

The maximum quantity of solute particles that can dissolve in a solvent (solution) at a specific temperature is known as its solubility. The solubility of solid (solute) in liquids depends on the following factors:

• Nature of solvent and solute.

• Effect of temperature.

### The Solubility of Gases in Liquids

Gases dissolve in liquid to form a homogeneous solution. The solubility of the gas in a liquid (solvent) depends on:

1. Nature of the solute (gas).

2. Nature of the solvents.

3. The temperature of the solution.

4. Pressure.

Gases like oxygen, helium, hydrogen, nitrogen, etc., dissolve in lesser quantity, whereas gases such as ammonia and hydrogen chloride are highly soluble. The solubility of gases in the liquid is influenced by:

• Effect of Temperature

With an increase in temperature, the solubility of gas decreases.

• Effect of Pressure

With an increase in pressure, the solubility of gas increases.

## Henry’s Law

Henry's law can derive the relation by which the 'quantity of gas' gets dissolved in the liquid. According to the law, the mass of a gas dissolved in the given amount of solvent at any temperature is proportional to the gas's pressure above the solvent. With a rise in temperature, the solubility of gases decreases.

(mass of gas) α (Pressure of gas)

m α p

mm = kHp

Here kH is called proportionality constant. It is also named Henry law constant and is dependent on the temperature. One such example of it is a cold drink bottle. The soft drink bottle is sealed at pressure (of the gas) that is higher than the atmospheric pressure. Here, the solubility of the gas in the solvent is high. When the bottle is opened, it gets exposed to the surrounding atmosphere (it has low pressure than a soft drink). As a result of opening the bottle, some amount of gas CO2 escapes the bottle. This happens to establish a dynamic equilibrium as per the low pressure existing in the atmosphere. If the soft drink is further exposed to the atmosphere, all the dissolved CO2 will escape due to low pressure and the corresponding high temperature. Thus, any cold drink kept at room temperature will lose more gas than the drink taken out of the refrigerator.

### Did You Know?

Consuming soft drinks causes belching as your stomach stretches due to the accumulation of CO2. As a result of belching, you may feel heartburn and a sour taste.

Q1. What is the Difference Between Dynamic Equilibrium and Static Equilibrium?

Ans. Static equilibrium or mechanical equilibrium implies that the reaction has completely stopped. As a result, there is no movement between the products and the reactants. Thus, the reaction is complete with forward, and the reverse reaction rates are 0.

In dynamic equilibrium, the reactions are reversible; it can proceed in either direction. However, in static equilibrium, the reactions are irreversible that can proceed in only one direction. It is to be underlined that both the static and dynamic balance depicts the system at a steady-state. Here the force action is zero on the system.

 Dynamic Equilibrium Static Equilibrium Reversible. Irreversible The reaction is occurring still. Reaction has stopped. Rate of forwarding reaction = Rate of the reverse reaction. Both reaction rates are Zero. Occurs in a closed system. Can happen in an open or closed system.

Q2. What is the Effect of Pressure on the Solubility of Gases in Liquids?

Ans. With an increase in pressure, the solubility of a gas in liquids increases.