Question

What happens when salt dissolved in water is heated?
A. There is an increase in boiling point.
B. There is no change in boiling point.
C. There is a decrease in boiling point.
D. All of the above.

Answer 1

Hint: When a non volatile solute is added in a solution, then the solute molecules form a layer on top of the surface, and blocks the vapours to escape, so more energy is required to break through.

Complete answer:
In order to answer our question, we need to learn about Raoult’s law and elevation in boiling point in presence of a non volatile solute. Let us see how this happens. Now, consider a liquid which acts as a solvent, and no solute is present on it. So it has no hindrance on top of the liquid surface and that is why when the liquid is given temperature in the form of heat energy it boils at a lower temperature. Now, consider that a non volatile solute like common salt is added to the solvent. The non volatile solute acts as an extra layer on top of the solvent molecules. So, when the original boiling point temperature of the solution is given, then the vapours might not get formed as the solute molecules give some resistance or hindrance. So, in order to overcome the hindrance, and to let the vapours escape, more energy is required. So, more energy means that the boiling point should increase.
Water has a boiling point of 1000 degree Celsius. Now, when some amount of non volatile solute like common salt is added to the solution, then the boiling point will increase.

Hence, we get option A as the correct answer.

Note:
The difference in boiling point $\Delta {{T}_{B}}$ is given by the formula $\Delta {{T}_{B}}=i\times {{K}_{b}}\times m$, where m is the molality, I is the Van’t hoff factor and ${{K}_{b}}$ is the boiling point elevation constant.