Question

Explain the relationship between atmospheric pressure and the actual boiling point of a liquid.

Short answer

When atmospheric pressure decreases, the boiling point decreases and vice versa.

Step-by-step solution

- Define atmospheric pressure

Atmospheric pressure is the pressure exerted by the weight of the atmosphere above the liquid. It is usually measured in units like Pascal (Pa), atmospheres (atm), or millimeters of mercury (mmHg).

- Define boiling point

The boiling point of a liquid is the temperature at which its vapor pressure equals the surrounding atmospheric pressure, causing the liquid to turn into vapor.

- Describe the relationship

When atmospheric pressure decreases, the boiling point of the liquid decreases because less heat energy is required for the liquid's vapor pressure to equal the reduced atmospheric pressure. Conversely, when atmospheric pressure increases, the boiling point of the liquid increases because more heat energy is needed for the liquid's vapor pressure to match the higher atmospheric pressure.

- Examples

At high altitudes, where atmospheric pressure is lower, water boils at a temperature below 100°C. Conversely, in a pressure cooker, the increased pressure raises the boiling point of water above 100°C, allowing food to cook faster.

Key concepts

Atmospheric Pressure

Atmospheric pressure is the force exerted by the weight of the atmosphere onto the surface of the Earth. Every square inch of the planet is under the influence of this pressure, which is typically measured using instruments like barometers. Common units of measurement include Pascal (Pa), atmospheres (atm), and millimeters of mercury (mmHg). Atmospheric pressure decreases as you move to higher altitudes because there is less air pushing down from above.

Boiling Point

The boiling point of a liquid is the temperature at which it changes from a liquid to a gas. At this temperature, the vapor pressure of the liquid equals the surrounding atmospheric pressure. For example, the boiling point of water at sea level is 100°C (212°F). However, this value can change if the surrounding pressure changes. Lower atmospheric pressure means a lower boiling point, and higher atmospheric pressure means a higher boiling point.

Vapor Pressure

Vapor pressure is the pressure exerted by the vapor that forms above a liquid in a closed container. It increases with temperature because more molecules have enough energy to escape from the liquid phase to the gas phase. When the vapor pressure of a liquid equals the atmospheric pressure surrounding it, the liquid reaches its boiling point and starts to turn into vapor.

Altitude and Pressure

Altitude has a direct impact on atmospheric pressure. The higher you go above sea level, the lower the atmospheric pressure becomes. This happens because there is less air above you exerting downward force. As a result, water and other liquids boil at lower temperatures at higher altitudes. For example, in a mountainous area, water might boil at 90°C instead of the standard 100°C.

Pressure Cooker

A pressure cooker is a sealed pot that uses steam pressure to cook food quickly. When the lid of the pressure cooker is closed, it traps the steam inside, increasing the pressure within the pot. This elevated pressure raises the boiling point of water inside the cooker above 100°C, allowing food to cook faster. Because the boiling point is higher, the food can be cooked at higher temperatures, which shortens the cooking time and maintains the food’s nutrients and flavor.