Question

The boiling point of water in a pressure cooker is (1) below ${100}^{\circ }\mathrm{C}$ (2) Above ${100}^{\circ }\mathrm{C}$ : (3) ${100}^{\circ }\mathrm{C}$ (4) depends upon the size of pressure cooker

Short answer

Above 100°C.

Step-by-step solution

- Understand the Concept

Understand that boiling point is the temperature at which a liquid turns to vapor. Boiling point increases with an increase in pressure.

- Analyze the Pressure Cooker Environment

A pressure cooker increases the pressure inside it. This elevated pressure will raise the boiling point of water.

- Compare to Given Options

Now compare this information with the given choices: (1) below 100°C, (2) above 100°C, (3) 100°C, and (4) depends upon the size of pressure cooker.

- Select the Correct Answer

Since the pressure cooker increases the pressure, the correct answer is (2) above 100°C.

Key concepts

Pressure Cooker

A pressure cooker is a kitchen appliance that uses steam pressure to cook food more quickly than traditional methods. When you seal a pressure cooker, it traps steam generated from the boiling liquid inside it. This trapped steam increases the internal pressure. As a result, the boiling point of water inside the cooker increases.
For instance, under normal conditions, water boils at 100°C (212°F) at sea level. However, in a pressure cooker, the pressure can surpass regular atmospheric pressure, making the water boil at a temperature above 100°C. This higher boiling point accelerates cooking, making it a popular choice for preparing meals quickly.
Understanding how a pressure cooker works helps us see why the boiling point of water in it is higher than in an open pot on the stove. This elevated pressure is central to how it can cook food faster.

Temperature and Pressure Relationship

The relationship between temperature and pressure plays a crucial role in understanding how a pressure cooker works. According to the gas laws, specifically Gay-Lussac’s Law, there is a direct relationship between the temperature and pressure of a gas, provided the volume remains constant.
In the case of boiling water, when the pressure is increased, the boiling point also increases. This is because higher pressure requires more energy (higher temperature) for the water molecules to escape into the vapor phase.
In practical terms, as the pressure inside the pressure cooker increases, it forces the water molecules to stay in liquid form longer, even at temperatures above their normal boiling point of 100°C. This principle is why pressure cookers can achieve higher temperatures than open-air cooking methods.

Phase Change

A phase change involves the transformation of a substance from one state of matter to another, such as from liquid to gas. Boiling is a type of phase change where a liquid turns into vapor.
Under normal atmospheric pressure, water undergoes this phase change at 100°C. However, in a pressure cooker, the increased pressure means that water will not transition to the gas phase at its usual boiling point. Instead, it will require a higher temperature for the phase change to occur.
When the water reaches this elevated boiling point, it transforms into steam inside the pressure cooker. This steam builds up pressure, cooking the food more rapidly. Understanding this phase change is essential for grasping why water boils at a higher temperature in a pressure cooker and for making the right choice in the given exercise, which is that the boiling point of water in a pressure cooker is above 100°C (answer (2)).