How substances change
High on a mountain it is so cold that it never rains. It always snows. The flakes are blown about in the wind around the peak then settle on the carpet of snow that already exists there. In time more snow falls and the snow that fell previously is squashed beneath it. The squashed snow turns to a huge block of ice, which gradually begins to slide down the mountain. This block of moving ice is part of a glacier.

As the ice moves along it is strong enough to snap off rocks on the mountainside. The rocks are carried in the glacier ice. A few kilometres below the place where the glacier formed is the place where it ends. Here, the weather is too warm for the water to stay frozen so it melts. As the ice melts, the rocks it carried down the mountainside fall to the ground. The water gushes past them and forms a stream. Another kilometre away the stream enters a small lake.

The lake lies much lower down the mountain where the weather is even warmer. Here another change takes place in some of the water in the lake. As the Sun shines on the surface of the lake it warms the water. Heat is a form of energy and the water at the surface uses the energy to change once more. This time the water changes into a gas. Water in the form of a gas is called water vapour. It is invisible when it is present in the air. The water vapour mixes with the other air gases and moves away from the mountain in the wind.

It may seem strange that a substance can exist in different forms but most substances can. Every substance is made of particles. In the solid form the particles are locked together like pieces in a jigsaw. In the liquid form the particles still keep together but can slide around each other. In the gas form the particles move freely and can be close to each other or far apart.

All substances can change form like the water on the mountainside. The key to the way that substances change form is heat. As the glacier came into warmer conditions the heat in the air warmed the ice. It gave the particles in the ice more energy. When particles receive more energy they move faster. In a solid the particles are locked together but they can shake or vibrate. As they receive more heat they shake more strongly. Eventually they receive enough heat to shake themselves free. They are not completely free. They are still attracted to each other and keep close but they can slide around. This sliding movement allows the water to flow.

Heat is important in the second change that the mountain water made. The energy from the Sun's rays made the sliding particles at the water surface move more quickly. Some of them received so much energy that they moved completely away from the other particles and became truly free. These free-moving water particles formed the water vapour.

Next time you eat an ice-lolly in the sunshine and some of it dribbles down the stick, think about how the particles in the ice have changed. If some of the lolly runs onto your fingers the heat in your body will make it change into water vapour in the same way that the Sun's heat makes the surface water on a mountain lake change into a gas.

Can we see the particles of a substance?
Yes, but you need a very powerful microscope. You cannot see them with microscopes that use light such as school microscopes. You need a very powerful microscope called an electron microscope. When a solid is examined by a very powerful microscope the particles can be seen arranged in rows.

Who first had the idea that substances were made from particles?
A man called Democritus. He lived in Greece nearly two and a half thousand years ago. He thought about it this way. If you take an object and cut it in two you get two smaller pieces. If you take one of the halves and cut it in two, you get two smaller pieces still. If you keep cutting up the object in this way you will eventually come to something so small that it cannot be divided up into anything smaller. He called these tiny particles atoms.

When water turns into a solid the solid floats on the liquid. Why is this?
When the water freezes, the particles stop sliding over each other and hold themselves together firmly. The particles are not ball-shaped but more oval-shaped. The oval shapes are arranged in such a way that there is more space between them than when they form a liquid. This arrangement of the particles makes a block of ice lighter in weight than the same volume of liquid. As lightweight substances float on heavier substances, the ice floats on water.

Do other substances behave like ice when they freeze?
No. Their particles pack together more tightly. This makes a solid block of the substance heavier than a similar volume of liquid.

Are solids, liquids and gases states of matter?
Yes, they are. Matter is the name used to describe all the material from which the universe is made. There are many different materials or substances and most of them can exist in each of the three forms of matter. It is important to remember that when a material changes from one state of matter to the next it is still the same substance. For example, ice, liquid water and water vapour are all the same substance. All that has changed is the way that the particles behave.

There is a glass ball, which has purple rays that move about when you switch it on. Are the rays solid, liquid or gas?
They are none of the three states of matter. They belong to a fourth state of matter called plasma and the object you see them in is called a plasma ball. Plasma is made from particles, which are charged with electricity. They have positive and negative charges like the terminals on a battery. The plasma is present in the ball but you can only see it when you switch the electricity on. When this happens, a rod in the middle of the ball also becomes charged with electricity and the plasma carries it away as purple rays. The plasma in a plasma ball is made by taking a large amount of air out of the ball. In the rest of the universe, plasma is found in the centre of stars, like the Sun, where it is very hot.