Question

Assertion: Saltwater boils at a higher temperature than pure water.
Reason:The presence of salt increases the vapour pressure of water.
A: Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
B: Both Assertion and Reason are correct and Reason is not the correct explanation for Assertion.
C: Assertion is correct and Reason is incorrect.
D: Both Assertion and Reason are incorrect.

Answer 1

Hint: Boiling point elevation is a phenomenon in which boiling point of a solvent or a liquid becomes higher after adding another compound to it. This means that the final solution now possesses a higher boiling point in comparison to a pure solvent.


Complete solution:
Salt (or may be other solutes, such as sugar) get easily dissolved in water. But, to take the solute out of water and put it in the gas phase (say, air) needs an enormous amount of energy. At temperatures approximating the boiling point of water, these solutes remain in water.
Now total pressure in water as well as in air at the boundary are almost the same. Part of pressure in water comes from the solutes. So, the pressure owing to water alone gets reduced in comparison to that of the pure water at the same temperature. The vapour pressure, (i.e. pressure of water vapour staying in equilibrium with the liquid) gets reduced by the similar amount due to the solutes. Actually the pressure does not break up into the separate parts because of the salt and water.
So now we know that water boils once vapour pressure of water becomes as high as that of the pressure of the atmosphere. At this point, vapour bubbles in water can grow. Thus, salt in water increases the vapour pressure of water and saltwater possess a higher boiling point.
Therefore, from the above explanation we can say that the correct option is (A).


Note:The more solute or salt added to water, more is the rise in boiling point. The process depends upon the number of particles that are formed in solution. Another colligative property which works in the same manner is the freezing point depression i.e. If you add any salt to the water, the freezing point is lowered and its boiling point is increased. Elevation in boiling point of a solvent is inversely proportional to its freezing point.