Question

When $NaOH$ is dissolved in water, heat is evolved. If the temperature is raised, the solubility of $NaOH$ :
A. Increases
B. Decreases
C. Remains same
D. Cannot be predicted

Answer 1

Hint: sodium hydroxide that is, $NaOH$ dissolves in water and releases heat. This means that the reaction is exothermic. Dissolving $NaOH$ also increases the concentration of $NaOH$ in pure water. At saturation point, the process becomes endothermic.

Complete step by step answer:
The process of dissolution mentioned in the question is constrained by Le Chatelier’s principle. This principle is observed in reactions which are in equilibrium. It states that in case there is a change in any of the conditions which are imposed on a reaction, the reaction will either move forwards or backwards to compensate for the change.
In this reaction we can see that the reaction is an exothermic reaction. Usually an increase in the temperature for an exothermic reaction will lead to the reaction going backwards, that is, an increase in temperature does not favour exothermic reactions but favours endothermic reactions. But the reaction mentions solubility which is defined as the dissolution of a substance in a saturated solution.
Dissolving $NaOH$ in a saturated solution is an endothermic reaction. Therefore, the increase in temperature leads to an increase in the process of dissolving $NaOH$ in water.

So, the correct answer is Option A .

Note: Solubility of a solution is the dissolution of solute particles in a saturated solution.
The process of dissolution changes from exothermic to endothermic.
This happens because on dissolution of solute in an already saturated solution, the solution cannot take anymore solute molecules and thus becomes endothermic to inhibit further dissolution.