How gases mix

A liquid is made of tiny particles that slide around each other. As they move by, they pull on each other. These pulls hold the particles together. In the centre of a liquid, the particles pull in all directions and are pulled by all the other particles around them. At the surface of a liquid, the particles are only pulled by particles at their side and below them. Nothing is pulling to hold them in place from above the surface.

All particles have energy. It gives them the power to move. In the middle of a liquid, the pulls of all the particles restrict some of their movements. At the liquid surface, where there are fewer particles pulling on others, some particles use their energy to break free from the surface and shoot into the air. When this happens, the particle changes from being a liquid particle to being a gas particle. There may be a large number of newly escaped particles above the surface of a liquid but they do not stay there long. A gas particle is not pulled by other particles so it can move freely.

There are so many particles in the air that the new gas particles do not travel far before they crash into them as well as into each other. When particles collide, they bounce off each other and move away in another direction. In almost less than a moment the particles will have had another collision and moved off again. The particles in the air hit and bounce off each other many times a second. If you could look at them with a microscope they would look a little like the balls that bounce around in a lottery show.

As the new gas particles escape from a liquid, they are knocked about by the other particles in the air. This knocking about makes the new gas particles move away from the liquid surface. In time they spread out and travel a great distance from the liquid. They become completely mixed up with the other gas particles in the air.

The air is a rich mixture of gases. The first gases in the air came from erupting volcanoes about four and a half billion years ago when the Earth first formed. The main gases at that time were water vapour, carbon dioxide and a gas called ammonia. In time, plants grew and made oxygen. This gas made the ammonia change into nitrogen. Today about four fifths of the air is made from nitrogen and about a fifth is made from oxygen but there are many other gases present in very small amounts.

Can any gases be seen mixing with the air?
Yes. There are a few gases which have a colour. For example, chlorine, the gas used to kill germs in tap water, is green. Another coloured gas is called bromine. It is brown. Bromine is a brown liquid at normal temperatures but if it is heated to 59°C the liquid boils and the bromine turns into a gas. As the particles of bromine gas are hit by other particles in the air, they are moved away from the liquid and spread out. The gas, which has just escaped from the liquid, is a deep brown colour due to the presence of many particles. Further above the liquid the gas is a paler brown due to there being fewer particles of bromine. They have been spread out by colliding with the other particles in the air.

When a gas mixes with the air, do the air particles mix with the gas?
Yes, they do. They move into the gas by colliding with its particles. This results in the gas particles spreading out into the air and the air particles spreading out into the gas.

How does air in a room become stale?
It becomes stale due to the people in the room. They release hot moist air into the room when they breathe out. They also take oxygen from the air when they breathe in and release carbon dioxide when they breathe out. The warmth, moisture and lack of oxygen makes the air stale, uncomfortable and unhealthy.

If gases in the air can be mixed, can they also be separated?
Yes, they can. This is done by cooling down the air so much that all the gases in it turn to liquids. Each liquid has a different boiling point from the other liquids. The boiling point of a liquid is the temperature at which the liquid turns into a gas. When the air has been made into a liquid, it is pumped into a tower, which is warm at the bottom and cool at the top. The gases separate out in different parts of the tower according to their boiling points. For example, nitrogen, with a low boiling point, separates out at the top of the tower where it is cooler and oxygen, which has a higher boiling point than nitrogen, separates out at the bottom of the tower where it is warmer.

Is there the same mixture of gases in all parts of the air?
No. Another name for the air is the atmosphere. This is a layer of gases around the Earth. It is about a thousand kilometres thick. The air we breathe is in the first few kilometres above the Earth's surface. It is made up of 78% nitrogen, 21% oxygen and 1% argon. Above this, is a layer of the atmosphere called the stratosphere. Here the concentration of oxygen is a little lower and there is a layer of ozone gas which helps to shield the Earth's surface from some harmful rays from the Sun. Above the stratosphere the composition of the atmosphere is 70% nitrogen, 15% oxygen and 15% helium. As the atmosphere reaches space the two main gases it contains are hydrogen and helium.