Solidifying
When you have your next birthday don't blow out the candles on your cake straight away. Let the candles burn a while and you may see some wax solidify.

The heat of a candle flame melts the wax at the top of the candle. The molten wax forms a little pool around the wick. Most of it flows up the wick and turns into a gas, which burns in the air. If the candle flame is large, a great amount of heat may be created and so much molten wax produced that the pool on the candle top cannot hold it. When this happens, the pool overflows and the wax moves down the side of the candle. You may think that the wax would run quickly to the bottom - just like water. But the wax behaves in a different way. At the top of the candle, the wax flows quickly, but as it goes further and further down the candle, it moves slower and slower. Eventually the wax stops moving. It has turned to a solid.

If we were considering ice instead of wax we would say that the ice has frozen but we do not usually use these words when other liquids turn to solids. We use the word solidify. However, the temperature at which any liquid turns into a solid is called its freezing point. We do not often use the term freezing point with substances other than water. As the temperature at which a substance freezes (its freezing point) is the same as the temperature at which it melts (its melting point) we tend to use the term melting point all the time.

As you watch the molten wax flow down the candle you may wonder what is going on inside it. In molten wax the particles slide freely over each other. When the wax flows away from the top of the candle it begins to lose heat. It loses some to the air and some to the solid wax in the candle. As the molten wax loses heat, its particles lose energy to keep them moving. Eventually the particles lose so much energy that they come to rest close to each other and link up. The linked up particles form a solid.

Probably the most spectacular example of a liquid turning to a solid occurs on the side of a volcano. Deep in the ground underneath a volcano is a huge chamber of hot molten rock. It is filled by molten rock from under the Earth's crust. When the chamber can hold no more molten rock, the volcano erupts. The molten rock escapes from the top of the volcano and flows away down its sides. It begins to cool in just the same way as wax on a candle. Sometimes the surface cools and forms a hard crust while underneath a torrent of red-hot rock flows by. Eventually the rock cools so much it solidifies. By the time is has stopped flowing it can have crossed roads, covered fields, destroyed woods and demolished buildings. However, inside the rock tiny particles will have stopped sliding about and will have become locked together. They may stay like that for millions of years.

Can it be useful to melt a substance then let it solidify?
Yes, it can be very useful. You can use the process to change the shape of a substance. A solid has a fixed shape. You can only change it by cutting bits off or sticking more pieces to it. When you melt a solid it turns into a liquid. Liquids do not have fixed shapes. They take the shape of the container into which they are poured. The shape does not have to be simple like a cup. It can be complicated like a car engine. It does not matter what the shape is: the liquid will flow into it. When the liquid solidifies the solid has the shape of the container.

What do they call the shape that a molten substance is poured into?
It is called a mould. A common example of a mould is a jelly mould. It may be in the shape of a rabbit or even a fish. Whatever its shape it may have details such as eyes, fur or scales. Jelly is a solid substance, which is melted by adding hot water to it. When the hot water and jelly mixture is poured into the mould it flows into every part. When the jelly solidifies or sets, it takes up the shape of the mould. If you carefully turn out the solid from its mould you can have a wobbly, jelly animal to eat.

Metals can be molten. Can they be moulded?
Yes, they can. The moulding of metals is called casting. As metals are only liquid at high temperatures the mould has to be made of materials such as sand which will not be destroyed. Imagine you wanted to make a casting of the first letter of your name. You would make a model of the letter. Put it in the middle of a box and add sand. You would pat down the sand firmly then remove the model. You would then have a cavity in the sand in the shape of your letter. Molten metal could now be poured into the cavity and it would take up the shape of the letter without damaging the sand. When the metal had cooled and solidified you could pull your metal letter out of the sand. This is a very simple way of casting. By using more complicated casting methods statues and car engines can be made.

Why is casting so important for making car engines?
The main part of a car engine is called the cylinder block. This has lots of cavities in it, which form cylinders and pipes. Petrol, air and exhaust fumes run through these cavities. It would be very time consuming to make them all separately and stick them together. Making a model of the block, then using the model to make a mould, saves a great deal of time and allows a large number of engine blocks to be made quickly.

Is window glass made by moulding?
No. The glass is molten but it is not placed in a mould. Glass is made by mixing sand, powdered limestone and soda ash then heating the mixture in a furnace. This produces molten glass. If the glass is to be used for making windows it is allowed to flow out on a bath of molten tin. This molten metal has a very smooth surface and when the molten glass rests on it, it acquires a smooth surface too. The molten glass is allowed to cool and solidify on the tin then is moved away on rollers.

Are glass bottles made by moulding?
Yes, they are. A bottle is made from a lump of molten glass that is taken from the furnace. The lump is called a gob. It is placed in a mould but the glass is too thick to flow and take up the shape of the mould. A hollow is made in the gob with a plunger then air is blown into the hollow. The push of the air makes the glass spread out and cover the walls of the mould. As the mould is in the shape of a bottle, when the glass has cooled and solidified it has taken up the shape of the bottle too.