Solution

Solubility: The maximum amount of a substance that can be dissolved in a specified amount of solvent at a specified temperature is called solubility of that substance. Solubility depends on the nature of solute and solvent. It also depends on temperature and pressure.

Solubility of a solid in a liquid:

Every solid does not dissolve in a given liquid. Generally, a solute dissolves in a solvent if the intermolecular interactions are similar in the two. Polar solute dissolves in polar solvent and non polar solute dissolves in non polar solvent.

Dissolution and Crystallization: When a solid solute is added to the solvent some solute dissolves and its concentration increases in the solution. This process is called dissolution. Some solute particles in solution collide with the solid solute particles and get separated out of solution. This process is called crystallization. A stage is reached when the two processes take place at the same rate. Under such conditions, number of solute particles going into solution will be equal to the solute particles separating out and a state of dynamic equilibrium is achieved.

Solute + Solvent ⇄ Solution …………. (1)

At the state of dynamic equilibrium the concentration of solute in solution remains constant under given conditions, i.e. temperature and pressure.

Saturated Solution: A solution in which no more solute can be dissolved at the given temperature and pressure is called a saturated solution.

Unsaturated Solution: A solution in which more solute can be dissolved at given temperature and pressure is called unsaturated solution.

Effect of temperature: In a nearly saturated solution, if the dissolution process is endothermic (Δsol >0), the solubility should increase with increase in temperature. If the dissolution process is exothermic (Δsol < 0), the solubility should decrease with increase in temperature.

Effect of pressure: Solids and liquids are highly incompressible. Hence, pressure does not have any significant effect on solubility of solids in liquids.

Solubility of Gases in Liquids

Solubility of gases in liquids is greatly affected by pressure and temperature. The solubility of gases in liquid increases with increase of pressure.

Henry’s Law: At constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of liquid or solution. Henry’s Law is also stated as, “the partial pressure of the gas in vapor phase (p) is proportional to the mole fraction of the gas (x) in the solution. It is expressed as follows:

`p=K_H\X` …………….. (2)

Here, `K_H` is the Henry’s law constant.

Solubility of gases in liquid increases with decrease in temperature. This is the reason that aquatic animals feel more comfortable in cold water than in warm water.

Following are some applications of Henry’s Law:

The bottle of soft drinks is sealed under high pressure to increase the solubility of CO₂.
A scuba diver has to cope with high concentrations of dissolved gases while breathing air at high pressure underwater. Increased pressure increases the solubility of atmospheric gases in blood. When the diver comes towards the surface the pressure decreases gradually. This releases the dissolved gases and results in formation of bubbles of nitrogen in the blood. The bubbles block capillaries and create a medical condition known as bends. The bends are painful and dangerous to life. To avoid bends, and to avoid the toxic effects of high concentration of nitrogen in the blood, the tanks used by scuba divers are filled with air diluted with helium (11.7% He, 56.2% N and 32.1% O₂).
At high altitudes the partial pressure of oxygen is less than that at the ground level. This results in low concentrations of oxygen in the blood and tissues. Low level of oxygen in the blood causes climbers to become weak and unable to think clearly. These are the symptoms of a condition known as anoxia.

Example: If N₂ gas is bubbled through water at 293 K, how many millimoles of N₂ gas would dissolve in 1 litre of water? Assume that N₂ exerts a partial pressure of 0.987 bar. Given that Henry’s law constant for N₂ at 293 K is 76.48 kbar.

Answer: The mole fraction of the gas in a solution is calculated by using Henry’s Law.

`p=K_H\X`

Or, `X(N)=(p(N))/(K_H)`

Or, `X(N)=(0.987)/(76.48)=1.29xx10^(-5)`

We know that 1 litre of water contains 55.5 mol of water. If n represents the number of moles of N₂ in solution

`X(N)=(n\text(mol))/(n\text(mol)+55.5text(mol))`

Or, `(n\text(mol))/(n\text(mol)+55.5text(mol))=1.29xx10^(-5)`

Since n < < 55.5 so its value in denominator is neglected. Then we have,

`(n\text(mol))/( 55.5text(mol))=1.29xx10^(-5)`

Or, `n\text(mol)=1.29xx10^(-5)xx55.5text(mol)=7.16xx10^(-4)` mol

`=0.716` mmol