Question

In a pressure cooker, the cooking is fast because
(A) The boiling point of water is raised by the increased pressure inside the cooker.
(B) The boiling point of water is lowered by pressure
(C) More steam is available to cook the food at ${100^ \circ }C$
(D) More pressure is available to cook the food at ${100^ \circ }C$

Answer 1

Hint To answer this question, we need to investigate the process of cooking inside a pressure cooker. Then we have to see the variation of the pressure, and its effect on the boiling point.

Complete step by step answer
As we know from our common experience that whatever is being cooked inside a pressure cooker, is placed after adding some amount of water in the cooker. Dry food is never cooked inside the pressure cooker. When the wet uncooked food is kept inside a pressure cooker, and it is supplied heat, then the water present inside the cooker starts receiving the heat, and therefore the temperature of the water starts increasing. As soon as the temperature reaches the boiling point of water $\left( {{{100}^ \circ }C} \right)$ , the water starts being converted to the steam. The steam starts occupying all the vacant volume available inside the cooker and a point of time is reached when all the space of the cooker gets filled.
As the steam gets confined within an available volume, the pressure of the steam starts increasing as a result of the further increase in the temperature. So, the pressure inside the cooker is increased beyond the atmospheric pressure. Now, the boiling point of the water increases with the increase in the pressure. So, the boiling point of the remaining water reaches beyond ${100^ \circ }C$ . This means that the remaining water will take more heat before being converted to steam. So, the heat that it receives is utilized in increasing the temperature of the water, and not as the latent heat. More heat supply results in the fast cooking.
Thus, as we have seen from above, the faster cooking in a pressure cooker is due to the increased pressure inside, which increases the boiling point of water.
Hence, the correct answer is option A.

Note
From its name, it is very much clear that the pressure cooker increases the pressure of the liquid placed in it. So, in urgency we may choose option D as the correct answer, that is, more pressure is available to cook the food at ${100^ \circ }C$ . But we must note the pressure is not directly related with the rate of cooking. Instead, the quantity of heat supplied to the liquid before it gets vapourized determines the rate. So the effect on the boiling point of the liquid needs to be investigated.