Pressure in Gases and the Absolute Temperature Scale

Objectives

Lesson outcomes

Explain gas pressure in terms of particles colliding with the walls of a container.
Describe how gas pressure changes when temperature or volume changes.
Convert between degrees Celsius and kelvin.
Use the relationship between pressure and volume for a fixed mass of gas at constant temperature.

Syllabus

CIE 0625 syllabus points

6 linked

2.1.2.2 Describe the relationship between the motion of particles and temperature, including the idea that there is a lowest possible temperature (−273 °C), known as absolute zero, where the particles have least kinetic energy
2.1.2.3 Describe the pressure and the changes in pressure of a gas in terms of the motion of its particles and their collisions with a surface
2.1.2.7 Describe the pressure and the changes in pressure of a gas in terms of the forces exerted by particles colliding with surfaces, creating a force per unit area
2.1.3.1 Describe qualitatively, in terms of particles, the effect on the pressure of a fixed mass of gas of: (a) a change of temperature at constant volume (b) a change of volume at constant temperature
2.1.3.2 Convert temperatures between kelvin and degrees Celsius; recall and use the equation T (in K) = θ (in °C) + 273
2.1.3.3 Recall and use the equation pV = constant for a fixed mass of gas at constant temperature, including a graphical representation of this relationship

Definitions

Required definitions

Absolute zero
the lowest possible temperature, -273 °C, where particles have the least possible kinetic energy.

Lesson Notes

Student guidance and lesson notes

Overview

This lesson moves from the general particle model to gases in more detail. The key idea is that gas pressure is not a separate phenomenon: it comes directly from moving particles colliding with the walls of the container.

What You Need to Know

Gas particles move randomly and collide with the walls of the container.
These collisions exert forces on the walls, and pressure is the force per unit area.
If the temperature rises at constant volume, the particles move faster and collide harder and more often, so the pressure rises.
If the volume decreases at constant temperature, the particles hit the walls more often, so the pressure rises.
Use absolute zero as the fixed reference point for the kelvin scale.
Temperature in kelvin is found from: T = theta + 273
For a fixed mass of gas at constant temperature, pressure and volume are linked by pV = constant.

How to Work Through It

Start with a particle sketch of a gas in a sealed container and identify where the pressure comes from.
Compare what happens when the gas is heated and when it is compressed.
Practise converting temperatures between Celsius and kelvin until the absolute scale feels routine.
Finish with simple pressure-volume questions and a graph shape for p against V.

Check Your Understanding

Why does heating a gas at constant volume increase its pressure?
What happens to pressure when the volume of a gas is reduced at constant temperature?
What is 25 °C in kelvin?
Why must gas-law temperature be in kelvin rather than Celsius?

Common Mistakes

Saying pressure comes from particles pushing continuously rather than from collisions.
Using Celsius directly in gas-law work instead of converting to kelvin.
Forgetting that pV = constant only applies when the mass of gas and the temperature stay the same.

Next Steps

Practise explaining pressure changes in words before relying on equations.
Keep the link between temperature and particle energy clear because it will be used again in heating and expansion.