Runny liquids
Do you know that some scientists study volcanoes? They are called volcanologists. If you are very interested in volcanoes and science you could be a volcanologist one day. Volcanologists spend some of their time in very dangerous work. They try to get as close as they can to some very hot, runny liquid. This liquid is called lava. It comes from a very hot chamber, called a magma chamber, deep inside a volcano. The temperature inside the magma chamber can be as high as 1,200°C. That is twelve times as hot as a boiling kettle. This is hot enough to melt rock and that is exactly what lava is - molten rock.

When a volcano erupts, gases, smoke and ash shoot into the air and lava flows over the land. Volcanologists can learn a great deal from the lava as it leaves the volcano so they try and get close to it so they can take samples and measure its temperature. If you were a volcanologist investigating lava, you would have to wear a silver-coloured suit. It is coated in metal which reflects heat from the lava and keeps you cool. You would have to wear a helmet or hard hat like people on a building site. This would protect your head from falling ash and hot lumps of rock, called volcanic bombs, that have been blasted into the sky. You would also wear a mask on your face. This would have filters in it to trap dust and harmful gases so that you could breathe safely.

You could get close to the lava in your suit, hat and mask but would also need a pair of heat-resistant gloves to hold your equipment. One piece of equipment is a long pole with two prongs on the end. You would use this to collect lava samples. When you were close to the red-hot runny liquid you would stick the prongs in it, twist the pole and pull out a lump of molten rock. Afterwards you would stick the end of a special thermometer into the lava to take its temperature. This thermometer is made of metal so that it can stand up to the high temperatures.

As you followed the lava down the side of the volcano, you would see it lose its red colour. This is due to the lava cooling. As the lava cools, it flows more slowly. Eventually the lava becomes so cool that it stops moving and turns to solid rock.

You don't have to be a vocanologist to study how temperature affects the way liquids run. Put a cup of treacle in a sunny window and another in the fridge. After an hour pour both of them onto a plate and see which one flows faster.

Why do hot runny liquids slow down as they cool?
All substances are made from tiny particles which can only be seen by using powerful electron microscopes. When the liquid is hot, its particles have a great deal of energy to move and can slide over each other quickly. This makes the liquid very runny. As the liquid cools down, its particles have less energy to slide over each other so the liquid simply becomes less runny.

Why are some liquids less runny than others?
Cooking oil flows more slowly than water when both are at the same temperature. The difference is not due to the energy of the particles but their shape. The particles of the water can be thought of as ball shapes. The particles of oil can be thought of as being balls with tails. The tails of the oil particles are tangled up with each other and this makes the particles move more slowly over each other.

Is the runniness of liquids important in cooking?
Yes, it is. Custards, gravies and sauces all have to have the right amount of runniness to be poured over other foods. If they are too thick they will simply stay on top of the food. If they are too thin they will flow off it completely. All the liquids are first made runnier than is needed and are then made less runny in a process called thickening. This usually involves stirring in flour or cornflour as the liquid is being heated.

Is runniness important in paints?
Yes, it is. If the paint is too thick it is difficult to brush on. If the paint is too thin it will not cover a surface properly. Most paints are made at the correct thickness but sometimes they need to be made a little thinner. Emulsion paints contain water and a little extra water may be added to thin them. Other paints such as gloss paints do not contain water and are thinned by a liquid called paint thinner or white spirit. This liquid has to be used with great care as it catches fire very easily.