Gases and Absolute Temperature

Gases are made of tiny particles (molecules) moving quickly in all directions. Understanding how they move explains temperature and pressure.

Particle Model of a Gas

• Far apart with lots of empty space.
• Move randomly and collide with each other and the container walls.
• Pressure is the force from many particle collisions on a surface.

Temperature and Particle Motion

Temperature tells us how much average kinetic energy the particles have. Hotter gas → faster particles → harder, more frequent collisions.

Absolute zero is the lowest possible temperature, −273 °C. At this point, particles have the least possible kinetic energy. The Kelvin scale starts at absolute zero.

Kelvin–Celsius link:

$$T(\text{K}) = \theta(^{\circ}\text{C}) + 273$$

Use kelvin (K) for gas law calculations.

Pressure Changes

• Heating at constant volume increases pressure (faster particles hit walls harder). Example: a sealed spray can becomes more pressurised if heated.
• Changing volume at constant temperature: decreasing volume increases pressure, increasing volume decreases pressure.

For a fixed mass of gas at constant temperature:

$$pV = \text{constant}$$

This is Boyle’s law. A graph of p against V is a curve (as one goes up, the other goes down). A graph of p against 1/V is a straight line.