Gases in the Universe Gases on Earth What makes a gas different from solids and liquids? Measuring gases Pressure, temperature and volume

Gases are probably the least familiar state to most people. Although we stand on solid ground, we actually move through a gas; the air. We breathe air, use it to pump up tyres, fly through it in planes and can even be blown over by it. Our own bodies even produce gases but, because we cannot see it, air and its component gases are not generally well understood.

We can sense the presence of gas by smell if they have an odour, by feel if they move, and by sight if they are coloured. A still, odourless and colourless gas, like nitrogen, oxygen or carbon dioxide, may as well not exist in many people's minds.

Gases in the Universe
Next to the all powerful plasma state, gases are the next most common state in the Universe. Gases are present in the vast gas nebula clouds spread throughout space and in the giant gas planets like Jupiter and Saturn. Most of the 1% of matter not in the stars as plasma is in the gas state.

Gases on Earth
Most of the Earth's gas is found in its atmosphere. Natural gas accumulates in pockets underneath the Earth's surface and some gas is dissolved in the oceans and fresh water. By mass, gases are the third most common state on Earth, behind liquids and solids.

What makes a gas different from solids and liquids?
Gases have no set volume, completely fill their container and take the shape of that container. It does not matter how much or how little gas is present, a gas will always fill its container.

If gases are not held in a container, dense heavy gases like carbon dioxide will flow under the influence of gravity like a fluid. Light gases like helium, on the other hand, will quickly escape from an open container and mix or diffuse into surrounding gases.

Substances which are gases under normal conditions (25°C and 1 atmosphere pressure) are usually made of light atoms or molecules weakly bonded together. The particles in a gas vibrate, spin and move about with great speed. Gas particles have enough motion to move about within their container, bouncing off the walls of the container and each other.

In a normal gas, each particle has 1000 times its own space to move in all directions. If gas particles were the same size as a marble with a diameter of 1cm, there would be, on average, 10m to the next marble in all directions. Gases under normal atmospheric pressure are 99.9% nothing and 0.1% particles.

Particles in a gas.

Measuring gases
With solids, the best way of measuring how much you have of a substance is by weighing it and getting its mass. For liquids, volume is commonly used. We buy butter by the kilogram and petrol by the litre, but what about measuring the quantity of a gas? Since gases always fill their container, volume is not enough. A pressure and temperature are required to specify the quantity of a gas.

What is wrong with weighing a gas? Nothing, if you weigh them in a sealed container with the weighing machine in a vacuum! Try to weigh a balloon full of helium gas, it would rather float away than sit on a scale. The problem is we are sitting in an ocean of invisible gas and air. Other gases will float in air like a rubber duck floats on water.

Pressure, temperature and volume
By measuring the volume, temperature and pressure of a gas we can truly state how much gas is present. If any one of the three properties changes, the other two will also change. If a gas is kept in a container with a set volume, such as a steel gas bottle, then, is heated, the pressure will also rise, hence the danger of exploding gas bottles in a fire.

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