What is a liquid?
Imagine you are an alien in a spaceship. You are approaching a planet. It appears as a huge blue ball in space. There are brown blotches on the planet but most of its surface is covered with a blue substance. You decide that, as there is so much of this blue substance, you should set your spaceship down on it. As you bring down your spaceship you look for a flat place to land. In fact, in the place you have chosen, the whole surface of the blue substance is flat. Only in the far distance can you see the surface moving up and down as waves pass by.

You bring the spaceship down very slowly and it lands softly on the blue substance. You decide to go outside and walk about. You take one step off your spaceship and your foot disappears into the blue substance. This is quickly followed by the rest of your body. In a moment your head is above the planet surface and you kick your legs in the blue substance to keep it there. You move to the steps of your spaceship and climb back on board. You kneel down and cup some of the blue substance in your hands. You are surprised to see that when you look at it closely it does not have a colour. It also has a flat surface and when you pour it into a bottle, it takes up the shape of the bottle but still keeps its flat surface. When you tip the bottle to the left or the right the surface of the substance stays flat. You pour the substance out of the bottle and into a bowl. It takes up the shape of the bowl but still keeps its flat surface. Finally you decide to pour it back onto the planet surface. It is then you discover that you can pour little pieces of it back and each piece makes a round drop. When each drop hits the planet's surface it makes a splash then disappears into it.

If an alien landed on Earth it could make just the same discoveries as you in this story. Most of the Earth is not covered by a solid. It is covered by a liquid. Just because most of our bodies are made from solids and we live on solid land we think that liquids are not important. This little story may have helped you realise that we really live in a liquid world and the most important liquid is water. In fact, water is so important that there could not be life on Earth without it. You have a watery liquid streaming through your body. It is your blood. If you get hot your skin produces a watery liquid called sweat to cool you down. You replace the water you lose in sweat by taking a drink. Sea water, blood, sweat and drinks are just four liquids in your world. If you look in a kitchen and bathroom you will find more. They all have the properties that an alien would see in sea water. They have a flat surface, can be poured, they take up the shape of their container and they form drops.

Why does a liquid behave in a different way from a solid?
All substances are made from very tiny particles that can only be seen by using powerful electron microscopes. The particles are much smaller than grains of sand or specks of dust that you can see. In a solid these particles are linked together and cannot move about. That is why solids have a fixed shape. A liquid is made of particles too but they are not linked together. This means that they can slide over each other. When they do, the liquid flows. As the particles in a liquid are not linked together, they cannot hold the liquid in a shape. This means that the liquid will take the shape of any container it is placed in. If the liquid is not put in a container, it spreads out to form a pool. The edges of the pool may be any shape.

What happens if a liquid is cooled down?
The particles stop sliding over each other and link together. They form a solid. It is important to remember that the solid is still the same substance as the liquid. For example, when water turns into a solid we call it ice but it is really just solid water.

You can lay a pin on the surface of water. How can this happen?
It might seem that the liquid is behaving like a solid when it holds up a pin but it is not. The tiny particles from which a liquid is made hold each other loosely at the liquid surface. The way the particles hold together is called surface tension. It is just strong enough to hold the weight of the pin.

Why are water drops round?
It is due to the power of surface tension. All the tiny particles in the surface of the drop pull on each other and this pulling makes the drop take up a round shape. When the drop lands on a solid surface it flattens out a little because the surface tension is not strong enough to keep it completely round. You can still see the round surface at the edge of the drop where the surface tension is still strong enough to hold the water particles together.

When you cool down a liquid and make it into a solid, can you turn it back into a liquid again?
Yes. The substance you have cooled down has not changed in any way, other than changing from liquids to solid. You can make it into a liquid again by simply warming the solid. The process of changing a solid into a liquid is called melting. The temperature at which melting takes place is called the melting point.

What is the temperature called at which a liquid turns into a solid?
This is called the freezing point. The melting point of a substance is the same as its freezing point. For example, the melting point of ice is 0°C and the freezing point of water is also 0°C. Melting is the reverse of freezing and freezing is the reverse of melting. Changes, which have a reverse change, such as melting and freezing, are known as reversible changes.

What is a water bed?
It is a bed which has a mattress full of water. The water is covered by a waterproof sheet. Some people claim that the water bed is very comfortable. It works because water, like all liquids, cannot be squashed. It is incompressible.

Why do car brakes have a liquid?
The liquid is called brake fluid. It is in pipes which connect the brake pedal to the brake pads around the wheels. When the pedal is pressed the liquid immediately pushes on the brake pads and makes the wheels stop. The liquid also makes a small push on the brake pedal into a large push on the brake pads. This means that the wheels can be stopped without having to use up too much energy pressing down on the pedal.

Are liquids compressed in any other machines?
Yes. They are used in a kind of jack called a hydraulic jack. These are used in garages to raise up cars so mechanics can work underneath them. Liquids are also used in the pistons you see on diggers and bulldozers. They are used to raise the platform on some kinds of fire engines.