The Gaseous State: Ideal and Real Gases and pV = nRT

1. What Is Gas Pressure and Where Does It Come From?

The Origin of Gas Pressure

Gas pressure is the force that gas particles exert on the walls of their container. But where does this force come from?

Imagine a sealed container filled with gas. Inside, there are millions of tiny gas molecules moving around very quickly in all directions - up, down, left, right, forwards, backwards. These molecules are constantly colliding (crashing) with the walls of the container.

Every time a gas molecule hits the wall, it pushes against it for a tiny moment before bouncing back. Each individual collision creates a very small force. But because there are so many molecules hitting the walls so many times every second, all these tiny forces add up to create the overall pressure we can measure.

Key point: Gas pressure is caused by gas molecules constantly colliding with the container walls.

How Does Changing Volume Affect Pressure?

Let's say you have a gas in a container with a movable piston (like a syringe). If you decrease the volume of the container by pushing the piston down, what happens?

• The gas molecules now have less space to move around in
• They become squashed closer together
• This means they hit the container walls more frequently (more often)
• More frequent collisions = higher pressure

Important relationship: Volume and pressure are inversely proportional at constant temperature. This means:

• If volume decreases → pressure increases
• If volume increases → pressure decreases

If you plot a graph of volume against 1/pressure, you get a straight line passing through the origin. This shows the inverse relationship clearly.

How Does Changing Temperature Affect Pressure?

Now imagine heating the gas while keeping the volume constant. What happens?

• The gas molecules gain more kinetic energy (energy of movement)
• They start moving faster
• Faster-moving molecules hit the walls more frequently
• They also hit the walls with more force
• Both of these effects increase the pressure

Important relationship: Temperature and pressure are directly proportional at constant volume. This means:

• If temperature increases → pressure increases
• If temperature decreases → pressure decreases

If you plot a graph of temperature (in Kelvin) against pressure, you get a straight line passing through the origin.

Note: Temperature must always be measured in Kelvin (K) for gas law calculations, not in degrees Celsius (°C). To convert: K = °C + 273