Question

In a pressure cooker, cooking is faster because the increase of vapor pressure:
A.) Increases specific heat
B.) Decreases specific heat
C.) Decreases the boiling point
D.) Increases the boiling point

Answer 1

Hint: A pressure cooker increases vapor pressure by not letting the steam escape. This increased vapor pressure pushes the molecules in the liquid to stay together and not expand to attain the vapor state. Thus, the liquid has to be heated to a higher temperature (given more thermal energy) to enable it to boil. The steam transfers heat to the food and cooks it.

Complete step by step answer:
A pressure cooker is air tight and does not let steam escape from inside the container. Thus, accumulation of steam inside the small space of the container increases the vapor pressure inside the container.
Now, this vapor pressure exerts force on the liquid (generally water) which is present along with the food to be cooked. Hence, the molecules in the liquid are forced to come closer together. This causes an opposition in the process of boiling, that is, the liquid turning into its vapour.

To turn into vapour, the molecules of the liquid need to overcome this opposition and move far apart. For this, they require more thermal energy and increased temperature. Hence, the boiling point of the liquid increases.

Hence, boiling point of water increases with increase in vapor pressure and decreases with decrease in vapor pressure.

The increase in boiling point means that water boils above ${100}^{0}C$ and the steam has more thermal energy. The steam transfers this thermal energy to the food that is being cooked. This heat is used up by the food to get cooked properly. Thus, in a pressure cooker since the boiling point increases, the steam has greater thermal energy which it can transfer to the food and hence the cooking is faster.

Therefore, the correct option is D) increases the boiling point.

Note: This question could have also been framed as ‘why are pressure cookers used to cook food in hilly areas?’ and the answer would essentially have been the same. Since, in hilly areas, the atmospheric pressure is low, the boiling point of water decreases. So, the food cooks slower as the steam has lower thermal energy to provide to the food.
A pressure cooker solves this problem by increasing the pressure, hence increasing the boiling point of water, producing steam with higher thermal energy and enabling faster and better cooking of food by this greater thermal energy provided by the steam.