Equilibrium Involving Dissolution of Solid or Gases in Liquids

Dissolution of Solids in Liquids

Everyday experience tells us that only a fixed amount of a solid can dissolve in a given amount of liquid at a fixed temperature. If you keep adding sugar to a glass of water at room temperature, eventually no more sugar will dissolve, no matter how much you stir. The excess sugar simply sits at the bottom. This is a saturated solution — a solution that contains the maximum amount of solute that can be dissolved at that temperature.

The concentration of the solute in a saturated solution is not arbitrary; it is a fixed value for a given temperature. If you heat the water, you can dissolve more sugar. But if you then cool that hot, concentrated syrup back to room temperature, the extra sugar will crystallise out. This shows that the saturation concentration depends strongly on temperature.

What is happening at the molecular level inside a saturated solution? A dynamic equilibrium is established between the solid solute and the dissolved solute. For sugar, we can write:

At equilibrium, the rate at which sugar molecules leave the solid and enter the solution (dissolution) is exactly equal to the rate at which sugar molecules from the solution return to the solid surface and join the crystal lattice (crystallisation). The two opposing processes occur continuously, but because their rates are equal, there is no net change in the amount of solid or the concentration of the solution.

Dissolution of Gases in Liquids

The behaviour of gases dissolving in liquids is different from that of solids. When you open a bottle of soda water, you see bubbles of carbon dioxide gas fizzing out rapidly. This happens because the bottle was sealed under high pressure, forcing a large amount of CO₂ to dissolve in the liquid. When the bottle is opened, the pressure above the liquid drops to atmospheric pressure. The system is no longer at equilibrium, so the dissolved CO₂ escapes until a new equilibrium is reached.

The equilibrium for a gas dissolving in a liquid is written as:

This equilibrium is governed by Henry's law, which states:

The mass of a gas dissolved in a given mass of a solvent at any temperature is proportional to the pressure of the gas above the solvent.

In other words, the concentration of the dissolved gas is directly proportional to the partial pressure of that gas above the liquid. The amount of gas that dissolves also decreases with an increase in temperature.

When the soda water bottle is sealed under high pressure, the solubility of CO₂ is high. As soon as the bottle is opened, the pressure drops to the partial pressure of CO₂ in the atmosphere (which is very low). To reach the new equilibrium condition required by this lower pressure, some of the dissolved CO₂ must escape from the solution. This is why an open bottle of soda water eventually goes "flat" — it loses its dissolved gas.

Generalised Observations for Physical Equilibria

The four types of physical equilibria discussed — solid-liquid, liquid-vapour, dissolution of solids in liquids, and dissolution of gases in liquids — share a common theme: at a given temperature and pressure, the system reaches a state where the concentrations (or pressures) of the phases become constant. The specific conclusions are:

(i) Solid-Liquid Equilibrium: For a pure substance, at 1 atm (1.013 bar) pressure, there is only one temperature — the melting point — at which the solid and liquid phases can coexist. If the system is isolated (no heat exchange with the surroundings), the masses of the two phases remain constant. …