How do substances change?
Imagine that you were stranded on a desert island. What would you do? Probably the first thing you would do is make yourself a shelter. The island has plenty of trees and bushes growing on it so there are plenty of building materials. You snap off some branches and tie their ends together with the stems of some climbing plants. In an hour or two you have made some square frames and put them together to look like a simple hut. Next, you gather some large fern leaves for the walls and roof. When you have finished the hut, you climb inside. It seems shady, windproof and comfortable. Now it is time to find some food.

Your first stop is a pile of coconut which have fallen from a tree. You use a rock to break them open then eat the white flesh inside. You make a spear and catch a fish but do not want to eat it raw. You remember how people in Stone Age times made fire and you make a small bow and loop a stick through the bowstring. You place the end of the stick in a hole in a small dry log and put some very dry grass around it. When you move the bow backwards and forwards the stick in the bowstring turns round rapidly. Its end in the wood gets so hot that the dry grass close to it catches fire. You pile on some small twigs and they burn too. You have brought an irreversible change to the island.

As you cook your fish two kinds of changes take place. At first a little steam rises from the fish. This is caused by two reversible reactions. First, as the fish gets warm, water evaporates from its surface to form water vapour. Then, as the water vapour rises and cools, it condenses to form a cloud of water droplets that most people call steam. As the fish gets hotter changes begin to take place in it. They make it change colour and become tender. These changes in the cooking of fish are irreversible.

In time, you are rescued and return home. One of the first things that strikes you on your return is the range of materials which you find in your home. There is only one material here that you recognise from your desert island home. This material is wood and is a natural material. All the other materials in your home are not natural materials. They have been made or manufactured by using irreversible changes. For example, many homes are made of brick. Brick is a material that is made by baking clay. All windows contain glass. This material is made by mixing sand with limestone and soda and heating it strongly in a furnace. Metals are used on door handles and hinges. They are released from certain rocks by heating the rocks strongly.

The most widely used material in homes is plastic. Plastics are made by heating oil, collecting some of the vapours and mixing them with other substances. By performing these processes, a range of plastics is made which have special properties for different uses. Plastics are used to make a wide variety of things in the home such as window frames, kitchen worktops, ceiling tiles, plug sockets, chairs and fibres for carpets.
When you compare the Stone Age life style on the desert island with the way most of us live today, you can see that our world is really shaped by using irreversible changes.

How can water change to vapour and back again?
Water can also change to ice and back again. All substances can exist in three different states. They are called the states of matter. These three states are the solid state, the liquid state and the gas or gaseous state. Substances can be changed from one state to another by being either heated or cooled. For example, when water is cooled it freezes and becomes a solid. When ice is warmed it melts and becomes a liquid. If the liquid water is warmed, it evaporates and becomes a gas called water vapour. If the water vapour is cooled it condenses and forms liquid water again.

What is a reversible change?
A reversible change does not produce any new substances. For example the freezing, melting, evaporating and condensing of water are all reversible changes because none of them produces a new substance. At the end of each change the substance is still water. It has just changed its state.

Is mixing a reversible change?
Yes, it is. In mixing, two or more substances are brought together. They do not produce a new substance. Each substance in a mixture can be separated from the other. For example, you can mix flour and rice then pass the mixture through a sieve. The small grains of flour pass through the holes in the sieve but the rice grains are too large and remain in the sieve.

If steam isn't the cloud you see over a boiling kettle, what is it?
Steam is a gas. It is formed when water boils. It has a temperature of 100°C and cannot be seen. If you look at a boiling kettle the steam is in the gap between the spout and the cloud of water droplets.

Is boiling a reversible reaction?
Yes, it is. A new substance is not produced when boiling occurs. The boiling substance simply changes from the liquid state to the gas or gaseous state. A substance, which has been changed into a gas by boiling, can be changed back into a liquid again by cooling it until it condenses.

What is an irreversible change?
This is a change in which a substance or mixture of substances changes into one or more new substances. They cannot be separated by simple processes such as sieving or filtering. For example, if you mix iron filings and sulphur you get a new substance, which is called iron sulphide. It does not have the magnetic properties of iron or the yellow colour of sulphur. It is a grey non-magnetic substance and cannot be broken up into pieces of iron and sulphur. It is a completely different substance.