What Is a Gas?
A gas is a state of matter with no fixed shape and no fixed volume. Gas particles spread out to completely fill any container they are placed in. If you open a bottle of perfume, the scent gradually spreads through the whole room — that is gas particles moving in all directions.
Air, oxygen, carbon dioxide, water vapour, and natural gas are common examples.
Particles in a Gas
The particles in a gas are:
- Very far apart — most of the volume of a gas is empty space.
- Moving very fast in random directions — gas particles travel in straight lines until they collide with each other or with the walls of the container.
- Almost no attraction to each other — the particles are so far apart that they barely interact.
- Colliding constantly — these collisions create gas pressure.
Because the particles are far apart and moving freely, gases are very different from solids and liquids.
At room temperature (20 °C), the nitrogen molecules in the air around you are moving at an average speed of about 500 metres per second — faster than most aircraft. Despite this, they change direction millions of times per second due to collisions, so they don't travel very far in a straight line.
Physical Properties of Gases
| Property | Description |
|---|---|
| Shape | None — fills the container completely |
| Volume | None — expands to fill all available space |
| Compressibility | Highly compressible — can be squeezed into a smaller volume |
| Density | Very low — particles are far apart |
| Flow | Flows and diffuses in all directions |
Gas Pressure
When gas particles hit the walls of their container, they exert a force. The total force over the area of the walls is the pressure of the gas. If you pump more air into a bicycle tyre, you are forcing more particles into the same space, increasing pressure.
Temperature and pressure: Heating a gas makes its particles move faster, colliding harder and more often — this increases pressure (if the volume is fixed) or causes the gas to expand (if pressure is fixed). This is why a balloon expands in a warm room.
You can feel gas pressure by pressing on an inflated balloon. The air molecules inside are constantly hitting the rubber walls from the inside, keeping the balloon firm.
Diffusion
Diffusion is the movement of gas particles from an area of high concentration to an area of low concentration. This is why a smell from the kitchen reaches you in another room. Lighter gas particles diffuse faster than heavier ones.
Real world: Car tyres and gas pressure
Car tyres are inflated with air. The gas particles inside collide with the rubber walls, creating pressure that supports the car's weight. If you let air out (reducing the number of particles), the pressure drops and the tyre goes flat. Pumping more air in increases pressure and makes the tyre firm again.
Real world: Perfume spreading through a room
When you spray perfume, the tiny droplets evaporate into gas particles that diffuse through the air from high concentration (near the bottle) to low concentration (across the room). This is why you can smell it from several metres away — gas particles spread in all directions.
Real world: Weather and the atmosphere
The atmosphere is a mixture of gases (mainly nitrogen and oxygen). Weather forecasters measure atmospheric gas pressure using barometers. Low pressure usually brings clouds and rain; high pressure brings clear skies. Changes in gas pressure and temperature drive wind and weather patterns.
Where do you see this in real life? Bicycle pumps, balloons, fizzy drinks (CO₂ dissolved under pressure), and aerosol cans all involve gas pressure. Notice how a sealed crisp packet puffs up on an aeroplane as outside pressure drops.
Practice Problems
Why can a gas be compressed (squashed into a smaller volume) much more easily than a solid or liquid?
Show answer
What causes gas pressure inside a sealed container?