Gas
A gas is a state of matter where particles have so much energy that they break free from each other completely. A gas has no fixed shape AND no fixed volume: it expands to fill whatever container it is in. The air around you, the helium in a balloon, the carbon dioxide in your fizzy drink and the natural gas in a cooker are all gases. They are mostly invisible, often odourless, and they fill space in ways that solids and liquids never can.
- ShapeNo fixed shapeFills container completely
- VolumeNo fixed volumeChanges with pressure and temperature
- ParticlesFar apart, fast movingFly freely in all directions
- CompressibleYes, easilyCan be squeezed to small volume
- Most common gas in airNitrogen (78 per cent)Plus 21 per cent oxygen
- Densest common gasSulfur hexafluoride6x denser than air
What makes a gas?
In a gas, the particles are very far apart compared to their own size. They move at very high speeds, fly through space in straight lines, and only change direction when they collide with each other or the walls of their container.
At room temperature, air molecules zip around at roughly 500 metres per second (about 1,800 km/h, faster than the speed of sound). They collide with each other billions of times per second, which is why they cannot travel far in a straight line before being knocked off course.
Three big ideas about gases
Three rules describe how a gas behaves, all of which can be tested with simple experiments:
- Heat a gas, it expands (or pressure rises): hotter particles move faster and push harder on the container.
- Squeeze a gas, the pressure rises (or volume falls): more crowded particles collide with container walls more often.
- Add more gas, the pressure rises (or volume increases): more particles means more collisions.
These rules can be combined into one equation called the ideal gas law (PV = nRT), but the basic ideas are easy to picture: heat, squeezing or adding more gas all crowd or speed up the particles, which pushes harder on the walls of any container.
Useful gases
- Oxygen (O2): needed for breathing and combustion. Hospitals supply pure oxygen to patients with breathing problems.
- Nitrogen (N2): makes up 78 per cent of the air. Used in industry to keep things from reacting with oxygen (such as packaging snack food).
- Carbon dioxide (CO2): in fizzy drinks, fire extinguishers and dry ice.
- Hydrogen (H2): the lightest gas, used to make ammonia, refine oil and as a clean fuel.
- Helium (He): very light, non-flammable. Used in balloons, party balloons and to cool MRI scanners.
- Methane (CH4): the main ingredient in natural gas, used for cooking and heating.
- Neon, argon, krypton, xenon: noble gases used in glowing tube lights, lasers and double-glazed windows.
- Chlorine (Cl2): used to make plastics, paper, medicines and to disinfect drinking water.
Pressure
Pressure is the force a gas exerts on the walls of its container, divided by the area of those walls. It is measured in pascals (Pa) or atmospheres.
- Atmospheric pressure at sea level: around 101,000 Pa, or 1 atmosphere.
- Inside a car tyre: around 3 atmospheres.
- Inside a deep-sea submarine compartment: very different from the outside; the metal hull has to resist the pressure of the ocean.
- Inside an aerosol can: around 4 to 7 atmospheres.
- Inside a deep-sea diving cylinder: 200 to 300 atmospheres.
Pressure goes up as temperature goes up. That is why aerosol cans say "do not heat or burn". A hot aerosol can have so much pressure inside that it explodes.
How gases change state
Cool a gas enough and its particles slow down enough for attractions to start holding them together. The gas condenses into a liquid. Cool that liquid further and it freezes into a solid. The reverse happens on heating.
Different gases have different condensation points:
- Water vapour condenses to liquid water at 100 C (under normal pressure)
- Ammonia condenses to liquid at -33 C
- Methane condenses at -161 C
- Oxygen condenses at -183 C
- Nitrogen condenses at -196 C
- Helium condenses at -269 C (the coldest of any element)
Deeper dive: why hot air balloons work
Hot air balloons are a beautiful demonstration of how gases behave with temperature.
The air inside a balloon envelope is heated by a propane burner. As the air warms, the air molecules inside move faster and push harder on each other. Some are pushed out of the bottom of the open envelope. This means the air left inside is less crowded: fewer molecules in the same volume, so the air inside is less dense than the cooler air outside.
By a simple law of physics (Archimedes principle), any object less dense than its surrounding fluid will float upward. So the balloon (envelope, basket, passengers and all) rises into the sky. To go higher, the pilot fires the burner to make the inside air even hotter and less dense. To descend, the pilot lets the air cool, or releases some of it through a vent at the top of the envelope.
The first hot air balloon flight was in November 1783 in Paris, when the Montgolfier brothers sent up a balloon carrying a sheep, a duck and a rooster. They landed safely, proving the principle. A few weeks later, two French aristocrats flew in a balloon over the city of Paris, becoming the first humans ever to make a free flight in the air.
The same physics powers modern hot air balloons that drift over the Cappadocia rock formations in Turkey, the African savanna and the English countryside. They are some of the slowest, oldest and most beautiful aircraft humans have ever invented, all thanks to the simple gas law that hot gases expand and rise.