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 the pressure
C) More steam is available to cook the food at ${{100}^{0}}C$
D) More pressure is available to cook the food at ${{100}^{0}}C$

Answer 1

Hint: Food gets cooked by the heat energy transmitted by the liquid in the food which gets converted to steam at its boiling point. The higher the boiling point, the higher is the thermal energy of the steam.

Complete step-by-step answer:
It is observed that the boiling point of a liquid increases with the increase in its vapour pressure, because the vapour exerts pressure on the liquid molecules and does not allow them to convert into their vapour form at a lower temperature.
The process of cooking of food involves the conversion of the liquid or water in the food to steam upon boiling. The water vapour then proceeds to transmit its thermal energy to the contents of the food and cooks them.
What a pressure cooker does, essentially, is that it raises the vapour pressure inside the pressure cooker by not letting the steam escape from inside it. As boiling point of a liquid increases with increase in vapour pressure as explained above, the liquid in the food boils at a higher temperature. This higher temperature steam transfers greater thermal energy to the food and helps it to cook faster and better.
Hence, the correct option is A) The boiling point of water is raised by the increased pressure inside the cooker.

Note: This is also the reason why pressure cookers are required for cooking in hilly areas. In hilly areas, the atmospheric pressure is low and the boiling point of water comes down heavily (even up to${{70}^{0}}C$). Food does not get cooked well in steam of this temperature. The pressure cooker raises the pressure and helps in producing steam at a higher temperature and greater thermal energy.
An easy way to remember that the boiling point of a liquid increases with the vapor pressure, is to understand that the vapor applies pressure on the liquid molecules tending to bring them closer, however, for the liquid to turn into its vapour form, it must expand and the molecules must go away from each other. To do that, it would require more thermal energy and hence, a boiling point at a higher temperature.