What is a gas?

Can you remember the last time you played in the snow? Perhaps you made a snowman or threw snowballs with your friends. Snow is made from crystals of solid water. After a few days the snow may have become slushy. The top part became wet and turned to liquid water. You may have expected streams of water to form as the snow slowly melted but it did not happen. The ground became soggy and waterlogged but large pools and puddles did not form. The reason there was less water than you expected was that some of the water had turned into a gas. This gas is known as water vapour.

You might wonder how something that was once a solid crystal can disappear and mix with the air. The key to the change is the way solids, liquids and gases are made. If you could see into a solid, with a very powerful microscope called an electron microscope, you would see that it was made from millions of tiny particles all holding each other firmly together. The way the particles hold closely together explains why a solid cannot be squashed and why it has a fixed shape.

If you looked in a liquid, you would still see particles but they would not be held firmly together. They can slide over each other but they still stay close together. The movement of the particles allows the liquid to flow. The closeness of the particles means that the liquid, like the solid, cannot be squashed.

If you looked very closely at a gas, it would be very different. Gases are made of particles, just like solids and liquids, but the particles do not stay together. In fact, they move around very quickly. Imagine every one in your class running about in the school hall. When each person reached the wall, they touched it and ran off in a different direction. It would not be long before some people ran into each other. When they parted they would run off in different directions again. The particles in a gas behave in exactly the same ways as your classmates running around the school hall.

As the gas particles are free to move in any direction, a gas does not have a fixed shape like a solid. As there can be wide or narrow spaces between the particles, a gas does not have a fixed size like a solid or a liquid. If a gas is released into a completely empty container from which all the air has been removed, it will spread out and fill the container whether the container is as small as a matchbox or as large as a classroom.

When the melting snow changed into water vapour, the particles in the water stopped sliding around each other, moved apart and entered the air. They were not alone in the air because the air is a mixture of gases. The particles of each gas are mixed with all the others and all are moving fast and bouncing off each other.

How can you get a completely empty space and what is it called?
A space from which all gases have been removed is called a vacuum. There are special pumps, called vacuum pumps, which remove the particles of gas or air from a container. The VDU of your computer and the television screen both have a vacuum. It helps the electricity to make the words and pictures on the screen. Air is sucked out of the space behind the screen when the VDU or television set is being made.

Does a vacuum cleaner have a vacuum?
Not really. The pump in a vacuum cleaner removes some of the air particles from inside the machine. This makes other air particles from outside rush in. The vacuum cleaner is designed so that the air rushes in up the nozzle that is held close to the carpet. As the air rushes in, it picks up dust and grit in the carpet and carries it inside the vacuum cleaner.

If you can take air out of a container can you put more in?
Yes. You do this when you pump up a bicycle tyre. When you push in the pump handle, a piston inside the pump pushes air into the tyre. When you pull out the pump handle, a valve in the tyre stops the air coming back into the pump. By moving the pump handle in and out, you can squash a large amount of air into the tyre.

Why does a bicycle tyre become harder when you put more air in?
The tyre becomes harder because there is a large number of air particles hitting the inside wall of the tyre and pushing it outwards.

Why does a balloon eventually go down?
A balloon is blown up by putting a large amount of air into it. The wall of the balloon is made of rubber. There are very tiny holes in the rubber. They are large enough to let air particles pass through them. Once the balloon is inflated, some particles escape through the holes. This is a very slow process so the balloon will look and feel the same for a few days. Eventually it will become smaller and easier to squash because it has lost a large number of air particles through the tiny holes in the rubber wall.

When a substance changes into a gas, does it change into another substance?
No. The substance stays the same. We say that it has changed its state. A substance can exist in three states. They are called the three states of matter. These states are the solid state, the liquid state and the gas or gaseous state. For example, water exists in a solid state we call ice, a liquid state we call water and as a gas we call water vapour.

Why is air a gas and not a liquid?
Air is actually a mixture of gases. They are formed from substances which all have very low boiling points so all occur naturally as gases at the temperature on the Earth. The main gas in the air is nitrogen. It has a boiling point of 195°C. Oxygen, the gas we need to breathe, has a boiling point of 182°C.

Can air be turned into a liquid?
Air can be turned into a liquid by making it very cold. This is done in a special machine that presses the air hard and then releases it. When a gas is pressed hard and released it becomes cold. A can of deodorant contains a gas that is strongly pressed. When you squirt a deodorant on your skin, you can feel the cold of the gas as it is released from the pressure inside the can. The air in the special machine is pressed and cooled many times until it is so cool the gases turn to liquids.