Question

Explain why a hot saturated solution of potassium nitrate forms crystals as it cools.

Answer 1

The solubility of ionic solutions is directly proportional to the temperature. At increasing temperatures, solubility increases due to an increase in the kinetic energy of molecules. And as the solution cools down, the solute particles will start to precipitate out.

Complete answer:
Potassium nitrate is an ionic salt having potassium cations and nitrate anions. The chemical formula of potassium nitrate is $\text{KN}{{\text{O}}_{3}}$. It occurs in nature as a mineral known as nitre. Potassium nitrate belongs to a class of nitrogen-containing compounds collectively referred to as saltpetre.
A saturated solution of potassium nitrate is a solution of salt in water containing the maximum amount of potassium nitrate that can be dissolved in a given solvent at a particular temperature.
The solubility of solids in liquids increases with an increase in temperature and thus hot solutions dissolve more of salt. So, a hot saturated solution of potassium nitrate has more salt dissolved as compared to the solution at room temperature. When this hot saturated of potassium nitrate cools, the solubility will start to decrease and thus the amount of salt that can remain dissolved in solution will also decrease. As a consequence of decreased solubility, potassium nitrate begins to precipitate out of the solution in the form of crystals.
Hence, due to a decrease in the solubility of potassium nitrate salt with a decrease in temperature, its hot saturated solution forms crystals as it cools down.

Additional information:
There are many uses of potassium nitrate such as in fertilizers, rocket propellants, and fireworks. It is one of the major constituents of gunpowder.

Note:
There are three types of saturation in solutions. As stated earlier a solution is said to be saturated when the maximum amount of solute is added that is capable of dissolving in a solvent at a given temperature. Adding more solute to the saturated solution will result in the settling down of particles.
The unsaturated solution has the amount of solute that completely dissolves in the solution. While a supersaturated solution contains more undissolved solute than the saturated solution.