Gas Pressure and Temperature
Gas Pressure and Temperature
Gas behaviour can be explained using the particle model. The motion of gas particles and their collisions with container walls produce pressure, and changing the temperature or volume of a gas changes that pressure.
What Causes Gas Pressure?
Gas particles are in constant random motion. They collide with the walls of their container, and each collision exerts a tiny force on the wall. Gas pressure is the result of the total force of billions of these collisions per second acting on the container surface.
pressure = force normal to a surface / area of that surface
p = F / A
Where:
- p = pressure (pascals, Pa)
- F = force (newtons, N)
- A = area (metres squared, m squared)
1 Pa = 1 N/m squared. Standard atmospheric pressure is approximately 101,325 Pa (about 101 kPa).
The Effect of Temperature on Gas Pressure
When you heat a gas in a sealed container (fixed volume):
1. The particles gain kinetic energy and move faster
2. They collide with the walls more frequently and with greater force
3. This increases the pressure
Temperature must be measured in kelvin (K) for gas calculations:
T (in K) = T (in degC) + 273
At absolute zero (0 K, or -273 degC), particles have the minimum possible kinetic energy and, in theory, would stop moving entirely. Pressure would be zero.
For a gas at constant volume, pressure is directly proportional to absolute temperature:
p / T = constant (for a fixed mass of gas at constant volume)
or equivalently:
p1 / T1 = p2 / T2
Example: A sealed container of gas is at 300 K and 100 kPa. If heated to 450 K, what is the new pressure?
p2 = p1 x T2 / T1 = 100 x 450 / 300 = 150 kPa
The Effect of Volume on Gas Pressure
When you compress a gas (reduce its volume) at constant temperature:
1. The same number of particles occupy a smaller space
2. Particles collide with the walls more frequently (they have less distance to travel between collisions)
3. This increases the pressure
For a gas at constant temperature, pressure and volume are inversely proportional:
pressure x volume = constant (for a fixed mass of gas at constant temperature)
p x V = constant
or equivalently:
p1 x V1 = p2 x V2
This is called Boyle's law.
Example: A gas has a volume of 0.5 m cubed at 200 kPa. It is compressed to 0.25 m cubed at constant temperature. What is the new pressure?
p2 = p1 x V1 / V2 = 200 x 0.5 / 0.25 = 400 kPa
Halving the volume doubles the pressure — exactly as you would expect from inverse proportionality.
Work Done on a Gas
When a gas is compressed, work is done on the gas. This transfers energy to the gas, increasing the internal energy (kinetic energy of the particles) and therefore the temperature of the gas.
This is why a bicycle pump gets warm when you pump it — you are doing work on the gas, and the temperature rises.
Conversely, when a gas expands, it does work on its surroundings and cools down (this is how refrigerators work).
Explaining Gas Behaviour with the Particle Model
Always use this structure in exam answers about gases:
1. State what happens to the particles (e.g. they move faster)
2. Link to collisions (e.g. more frequent / harder collisions with the walls)
3. Link to force and pressure (e.g. greater force per collision and more collisions per second, so pressure increases)
Common mistake: Do not say the particles "expand" or "get bigger". Particles are the same size — only their speed and the gaps between them change.
Exam Tips
- Always convert temperature to kelvin before using gas equations
- When explaining pressure changes, give the full chain: particles gain KE then move faster then hit walls more often and harder then force increases then pressure increases
- If a question says "constant temperature", use p x V = constant; if it says "constant volume", use p / T = constant
- At absolute zero (-273 degC or 0 K), particles have minimum energy — do not say they have "no energy" or "stop completely" (this is an idealisation)