Question

A pressure cooker reduces cooking time for food because: (a) the higher pressure inside the cooker crushes the food material (b) cooking involves chemical changes helped by a rise in temperature (c) heat is more evenly distributed in the cooking space (d) boiling point of water involved in cooking is increased

Short answer

The boiling point of water is increased inside the pressure cooker due to higher pressure, which speeds up the cooking time (answer d).

Step-by-step solution

Title - Understand Physical Principles of Pressure Cookers

Recognize that a pressure cooker works by increasing the pressure inside the sealed vessel. This increase in pressure raises the boiling point of water, allowing cooking to occur at higher temperatures which speeds up the cooking process.

Title - Evaluate the Options

Examine each option to identify which one aligns with the understanding of how pressure cookers work. (a) Increased pressure does not crush food, but rather affects the boiling point of water. (b) While cooking does involve chemical changes, the key factor is the temperature change due to increased pressure. (c) Even distribution of heat is also a characteristic of a pressure cooker, but it is not the primary reason for reduced cooking time. (d) The correct answer is that the boiling point of water increases with increased pressure inside the cooker, which is why food cooks faster.

Key concepts

Boiling Point Elevation

Understanding the boiling point elevation is fundamental when exploring the workings of a pressure cooker. Boiling point is defined as the temperature at which a liquid's vapor pressure equals the external pressure surrounding the liquid. At higher elevations where atmospheric pressure is lower, water boils at temperatures lower than 100°C. Conversely, by increasing the pressure, as in a pressure cooker, the boiling point of water is elevated.

This means that when you're cooking at increased pressure, water and, consequently, steam can reach temperatures above 100°C without boiling away. This is crucial because the higher temperature allows food to cook faster and more efficiently.

For instance, the boiling point of water at sea level (1 atmosphere of pressure) is 100°C. In a pressure cooker, where pressures often reach around 2 atmospheres, the boiling point of water can escalate to approximately 120°C. This phenomenon, known as boiling point elevation, is governed by the Clausius-Clapeyron relation, which describes how the boiling point increases with pressure.

Pressure Effects on Boiling Point

The pressure effects on the boiling point are a cornerstone of physical chemistry and are expertly exploited in the design of pressure cookers. As the pressure inside the cooker builds up, the boiling point of the water inside it rises. This is because the molecules of the liquid must overcome greater atmospheric pressure to transition into a gaseous state.

Relation to Colligative Properties

In physical chemistry, the boiling point elevation is a colligative property, meaning it depends on the number of particles in a solution and not on their nature. While this concept is more commonly applied to solutions where the addition of solute particles raises the boiling point, the principle extends to pure substances like water in a pressure cooker, where increased pressure has a similar, albeit distinct, effect.

In short, higher pressure leads to an increased boiling point because it requires more energy for water molecules to break free from the liquid and become gas.

Physical Principles of Pressure Cooking

The physical principles of pressure cooking leverage the established understanding of pressure and boiling point elevation to enable efficient cooking. A pressure cooker seals in steam, creating a high-pressure environment that raises the water's boiling point. It is this increase in temperature that speeds up the cooking process.

How Pressure Cookers Accelerate Cooking

When the boiling point of water is raised in a sealed environment, foods are subjected to higher temperatures and cook more quickly. Moreover, the steam's pressure also forces moisture into the food, which helps tenderize tough cuts of meat and allows flavors to penetrate more deeply.

The sealed environment also avoids loss of heat to the surroundings, making pressure cooking a more energy-efficient method. Typically, steam builds up to about 15 pounds per square inch (psi) above atmospheric pressure in the average home pressure cooker, speeding up cooking times significantly compared to conventional boiling or steaming methods.