Final answer:
In a pressure cooker, the vapor pressure of a liquid initially increases as the temperature rises and becomes constant once the liquid boils.
Step-by-step explanation:
The vapor pressure of a liquid in a pressure cooker is affected by the high-pressure environment created inside the cooker. When a liquid is boiled in a pressure cooker, the pressure inside the cooker increases, which causes an increase in the boiling point of the liquid. This happens because the vapor pressure needed to boil the liquid must equal the increased pressure inside the cooker. Therefore, the vapor pressure of the liquid increases initially as the temperature rises. Once the boiling point is reached, the vapor pressure becomes constant as the liquid boils and the temperature remains steady.
Therefore, the vapor pressure initially increases, then becomes constant as boiling commences. This is due to the fact that as the temperature increases, so does the average kinetic energy of the molecules, leading more molecules to have sufficient energy to escape from the liquid. However, when the rate at which the molecules escape the liquid to become gas equals the rate at which the gas molecules re-enter the liquid, a state of equilibrium is reached, and the vapor pressure stabilizes even as boiling continues. This elucidates why a pressure cooker can cook food faster because the higher pressure increases the boiling point and hence the temperature at which cooking occurs.