Heating
Do you have a favourite mug? Perhaps it has your name on it. Mugs are made from pottery and pottery is made from clay. It may seem strange, but there is a connection between a pile of soft powdery clay and a hard lump of granite. The particles of clay were once part of a piece of granite. All rocks are broken down by weathering and when granite breaks down, the smallest particles it makes are clay particles. They are carried away by streams and collect at the mouths of large rivers. Today clay is dug out of the ground at the sites of ancient rivers. The clay in your mug came from such a place.
A pile of dry clay powder cannot be shaped. The particles simply tumble over each other and form a cone when the clay is poured. If water is added to the clay, it holds the particles together and the piece of clay can be pushed into a shape.

In the making of a clay object there is a reversible change and an irreversible change. The reversible change occurs first.

Once the piece of clay has been made into a shape, such as a mug, it is allowed to dry. The reason for this is that if the wet clay were heated strongly straight away, the water in the clay would quickly turn to steam. Water expands enormously when it changes to steam so it would push its way out rapidly from inside the clay and possibly make the piece of clay explode.

The drying process is the reversible process in the making of a mug. In the drying process the water in the mixture of clay and water separates. It does this by evaporation. No new substance is made. If water was added to the clay again, a new clay and water mixture would be made.

After the clay has dried, it is ready for taking part in an irreversible change. It is placed in a special oven called a kiln. Inside the kiln the temperature is gradually raised higher and higher. It passes 100°C, the temperature of boiling water, and keeps on rising.

Inside the clay, the particles are stacked one on top of another like a troupe of acrobats at a circus. Just like the acrobats they only touch each other at certain places. The acrobats touch each other where their arms and legs meet. The particles touch each other where projections on their sides meet.

When the temperature reaches over 800°C, changes happen at the places where the clay particles touch. At these sites an irreversible change takes place which changes the clay into a glassy substance. This sets hard and holds the particles firmly together. When the heater in the kiln is switched off and the kiln is allowed to cool, this glassy substance does not change. It is a new substance made by the clay and gives the clay object both its strength and its brittleness.

Making pottery is an ancient skill. People learned how to do it eleven thousand years ago. We know this because the pottery still exists from that time. This helps us to understand that baking clay to make pottery is a truly irreversible change and that the material will last a very long time. Who could be using your mug in 13,000 AD?

How can water change from a liquid to a gas?
All substances are made from particles. They are very much smaller than clay particles and you need very powerful microscopes called electron microscopes to see them. In a liquid substance, like water, the particles slide round each other. This allows the water to flow and to take up the shape of any container it is poured into. In a gas such as water vapour the particles are completely separate and can move freely. Heat is a form of energy. When a wet object, like a wet clay object, is left to dry, the water takes up heat from its surroundings. The heat makes the particles move faster. Those nearest the surface move so fast that they separate from each other and form water vapour.

What happens when a solid melts?
In a solid, the particles are held firmly together. This gives the solid its fixed shape. However, the particles can still move slightly in their positions. This movement is called vibrating. When a solid is heated, the particles take up energy and begin to move or vibrate more quickly. Eventually they vibrate so much that they lose their grip on each other. When this happens, particles slide around each other and the solid loses its shape and becomes a liquid. We call this change melting. It is reversible because no new substance has formed. If the liquid is cooled it forms the same solid again.

Doesn't clay melt in the kiln?
No, it doesn't. A clay particle that you can see is made up from thousands of particles that you cannot see. These are the particles that can only be seen using an electron microscope. These particles are called atoms. There are different kinds of atoms and they join together in different ways. When the clay is heated some of the atoms receive enough energy to change the way they are joined together. When this happens they form a new substance - the glassy substance that holds the clay together. When the clay is cooled, the atoms stay in their new arrangement because they have formed a new substance. They have not simply slid round each other like particles do when they form a liquid.

If clay particles have gaps between them why does a mug hold water?
After the mug has cooled down, it is coated with a liquid clay mixture called glaze and heated up again. This time, the glaze forms a glassy substance, which gives the mug a smooth, watertight surface.

When food is cooked do its atoms change the way they are arranged?
Yes, they do. In most substances atoms are arranged into groups called molecules. When the molecules receive heat energy, the atoms in them can rearrange themselves to form new molecules with new properties. For example, egg white is clear and colourless before it is heated but afterwards it becomes white and opaque. These changes are due to the irreversible change that has taken place in the egg white molecules.