Question

Saturated solution of NaCl on heating :
a.) Becomes supersaturated
b.) Becomes unsaturated
c.) Remains saturated
d.) Vapourises

Answer 1

Hint: Sodium chloride is an ionic solid. Remember the dissolution of ionic solids in polar solvents. To answer this question, you need to consider Le Chatelier’s principle. Remember that according to Le Chatelier’s principle, for an exothermic process, heating decreases the dissolution and for an endothermic process, dissolution increases upon heating.

Complete Solution :
Before answering this, let us discuss the meaning of a saturated solution.
 - We know that when we mix one or more soluble solvent particles in a particular solvent, the mixture is known as solution.
 - Now if we keep adding the solute in the solvent then one point will come where we cannot dissolve the solute any further in the solution. This type of a solution is known as a saturated solution.

Now in the given question, we have a saturated solution of NaCl i.e. a saturated solution of common salt in water.
 We have to discuss the factors affecting the solubility of a solute in a solvent.
- Solubility of a substance can depend upon temperature, pressure and surface area depending upon whether the solute is a solid, liquid or gas.
- Solubility of a solid in liquid depends upon whether the dissolution is exothermic or endothermic. According to Le Chatelier’s principle, for an exothermic process, heating decreases the dissolution and for an endothermic process, dissolution increases upon heating.
 - Although for NaCl this change is not much appreciable due to the lower heat effect (heat is ionization is almost equal to heat of hydration). But still such a solution where NaCl is an ionic compound i.e. it dissociates into ions in a polar solvent like water, the sodium ion is attracted towards the oxygen atoms of water and the chloride ions towards the hydrogen atom.
This results in decrease of solubility of the saturated solution and makes it slightly undersaturated.
So, the correct answer is “Option B”.

Note: We have discussed above that the solution in which no more solute can be dissolved in the solvent is called a saturated solution. Carbonated water is an example of a saturated solution.