Heating a Gas in a Closed Container

Introduction

Aerosol cans carry the warning "Do not expose to temperatures above 50 °C" and "Do not burn, even after use". Car tyres read a higher pressure after a long, fast journey than first thing in the morning. Both facts have the same explanation: when a gas trapped in a fixed space is heated, its pressure goes up. By combining what you know about temperature and about particle collisions, you can explain exactly why.

Core explanation

Temperature and particle speed. The temperature of a gas is a measure of the average kinetic energy of its particles. When a gas is heated, energy is transferred to its particles and they move faster on average. When it cools, they slow down.

A closed, rigid container. Imagine a fixed amount of gas sealed inside a strong steel container. No particles can get in or out, and the volume cannot change. Only the temperature is changed.

Two effects of faster particles. When the gas is heated:

1. The particles move faster, so they cross the container more quickly and hit the walls more often — more collisions every second. 2. Each particle is moving faster when it strikes, so each collision is harder and exerts a bigger force on the wall.

Both effects increase the total force on each unit of area of the wall, so the pressure rises . Cooling the gas does the opposite: particles slow down, collide less often and less hard, and the pressure falls.

How big is the effect? For a fixed amount of gas at constant volume, the pressure is proportional to the absolute temperature (measured in kelvin, where 0 °C is about 273 K). Warming a gas from 27 °C (300 K) to 127 °C (400 K) raises its pressure by one-third. This relationship is called the pressure law .

What if the container is not rigid? If the container can stretch, such as a balloon, heating makes the gas push harder and the container expands until the pressures balance again. The pressure changes much less, but the volume increases. In a rigid container there is no room to expand, so all of the effect appears as a rise in pressure.

Why this matters for safety. Every container has a maximum pressure it can withstand. If a sealed container of gas is heated strongly, its pressure can rise beyond this limit and the container can burst violently. This is why sealed containers and aerosol cans must be kept away from heat and flames, and why gas cylinders are stored in cool, ventilated places and fitted with pressure-relief valves.

Step-by-step reasoning

To explain the effect of heating a sealed gas:

1. Heating transfers energy to the particles, so they move faster. 2. Faster particles hit the container walls more frequently. 3. Faster particles also hit the walls with more force. 4. More force on each unit of area means a higher pressure; the volume is fixed, so the pressure must rise.

Visual explanation

In the simulation, lock the box size and raise the temperature slider. The particles speed up, the collision flashes on the walls become more frequent and brighter, and the pressure gauge rises steadily. Lower the temperature and the gauge falls as the particles slow down.

Real-world analogy

Picture a room full of people bouncing balls against the walls. If everyone starts throwing faster, the balls reach the walls more often and each one hits harder. The walls take a far greater battering, even though the number of balls has not changed.

Real-world example

Tyre pressures should be checked when tyres are cold. After driving, friction and flexing warm the air inside, raising its pressure by roughly 10%. Checking a hot tyre and letting air out to reach the recommended value would leave it under-inflated once it cools.

Why?

Why does a pressure cooker's valve start to hiss as it heats up? The gas and steam inside are trapped in a fixed volume. As the temperature rises, the particles move faster and hit the walls harder and more often, so the pressure climbs until the valve opens to keep it at a safe level.

Common misconception

"Heating a gas makes the particles bigger, so they press on the walls." The particles do not change size. The pressure rises because they move faster, hitting the walls more often and with greater force.

Worked example

Question: A sealed steel container of gas is at 100 kPa and 300 K. It is heated to 450 K. What is the new pressure? Explain the change using particles.

Reasoning: At constant volume, pressure is proportional to absolute temperature. 450 ÷ 300 = 1.5, so the pressure is multiplied by 1.5: 100 × 1.5 = 150 kPa.

Answer: 150 kPa. The particles move faster, so they collide with the walls more often and harder, increasing the force on each unit of area.

Quick check

1. Why must aerosol cans not be heated? Answer: The gas pressure inside rises as the particles speed up, and the can could burst.

Exam focus

For full marks, give both reasons: particles collide with the walls more frequently and with more force . State that the volume is constant. If a calculation is set, always convert temperatures to kelvin (add 273) before using proportion.

Advanced insight

Extrapolating the pressure–temperature graph for any gas to zero pressure gives the same temperature, about −273 °C. This is absolute zero , the temperature at which particles would have the minimum possible energy. It is the starting point of the kelvin scale and cannot actually be reached.

Summary

Heating a fixed amount of gas in a sealed, rigid container makes its particles move faster. They hit the walls more often and with more force, so the pressure rises; cooling lowers it. At constant volume, pressure is proportional to temperature in kelvin. Because pressure can exceed a container's strength, sealed containers must be kept away from heat.

Practice questions

1. Give two reasons why the pressure of a sealed gas rises when it is heated. Answer: The particles collide with the walls more frequently, and each collision exerts a greater force. 2. A sealed container of gas is placed in a freezer. What happens to its pressure? Explain. Answer: It falls, because the particles slow down and hit the walls less often and less hard. 3. A gas at 200 kPa and 300 K is heated in a rigid container to 600 K. Calculate the new pressure. Answer: 600 ÷ 300 = 2, so the pressure doubles to 400 kPa. 4. Explain why tyre pressures should be measured before a journey rather than after. Answer: Driving warms the air in the tyre, raising its pressure; a reading taken when hot would be higher than the true cold value.