Freezing
Have you walked along a road on a winter's night and been sprayed by a gritter lorry. As the lorry moves along it spreads out sand, grit and salt across the road. This mixture is designed to make the roads easier to travel on in freezing weather.

We describe weather as freezing because it is cold enough to make water freeze. Water, like all liquids, is made up of tiny particles, which slide around on each other. The sliding of the particles makes the liquid flow. As the temperature falls, the particles in all liquids move more slowly. When the temperature reaches 0°C a change takes place in the water particles. They stop moving and link together. This makes the liquid water turn into a solid. The name of this solid is ice. The temperature at which the water turns into ice is called the freezing point.

When ice forms on a road it makes the road surface slippery. Car and bicycle tyres cannot grip it firmly. If a driver has to brake suddenly, the tyres lose their grip of the road and the car skids and perhaps even crashes. When the salt from the gritter hits the road surface, it dissolves in any water that is present. The salt and water make a mixture. This mixture does not freeze when the temperature reaches 0°C, so no ice forms on the road and tyres keep their grip. The freezing point of the mixture is about -5°C. This temperature is not often reached in many areas so salting the roads is a useful way of keeping traffic moving safely.

The first sign of gritters on the roads makes many car drivers check their engines. A car engine produces a great deal of heat as it burns petrol, to give it power. The heat is removed by water. This moves around inside the engine then passes through a part called the radiator. This is usually at the front of the engine so that cool air can pass over it as the car moves along. When water passes through the radiator it releases heat to the passing air and the engine is prevented from getting too hot.

When a car is parked its engine cools down. The water in the engine also cools. If it cools down to 0°C it turns to ice. Water has an unusual property. When it changes from a liquid to a solid, it expands. If water in an engine expands to form ice it pushes on the metal around it and causes it to crack. Motorists avoid this damage by adding antifreeze to the water. Mixtures of water and antifreeze can be made to avoid freezing in the coldest weather.

Next time you see a gritter lorry think about how the salt will stop the water freezing. If there is a car driver in your family you might remind them to make sure they have enough antifreeze in the engine.

Why does a gritter send out sand and grit as well as salt?
The sand and grit are to help the tyres grip the road in case the temperature falls lower than -5°C. They do not affect the temperature at which the salt and water mixture freezes.

Why does ice form on the top of a puddle and not on the bottom?
When almost all liquids cool down they contract until they freeze and when they freeze they contract a little more. Water is unusual. It contracts down to 4°C then starts to expand again. When water freezes it expands even more. In a puddle the water cools from the surface. At first the cool water sinks as it contracts but once it reaches below 4°C it begins to expand again. This means that the water does not sink. It floats on the warmer water at the bottom of the puddle whose temperature is 4°C. The water on the surface of the puddle continues to cool and expand until it turns to ice.

Why don't puddles always freeze solid?
The ice acts as a heat insulator. It stops heat passing through it. The ice on the top of the puddles stops heat escaping from the water beneath. If the heat cannot escape the water cannot freeze. In periods of very cold weather all the heat can pass from the water in the puddle and the puddle can be frozen solid.

When something freezes doesn't it become full of coldness?
No. When something freezes it loses heat. There is no such thing as coldness which can enter something, although many people think there is. You only become cold because you are losing heat not because coldness is entering your body.

How does frost form?
There are four kinds of frost. You may see shapes on windows that look like the frond or leaf of a fern. This kind of frost is called fern frost. It is made when water vapour turns to drops of liquid water with a flat surface. The water then loses heat to the cold surface and forms crystals, which spread out to look like a fern leaf. The spiky frost you may see on leaves is called hoarfrost. It forms when water vapour turns to water very quickly and freezes straight away. The spikes are made of ice crystals sticking up into the air. Fog is made from tiny water droplets in the air. If fog forms in very cold weather and the droplets touch a cold surface they form an icy coating called rime. The fourth kind of frost is called glazed frost. This forms when rain falls onto a freezing surface. This kind of frost can also produce icicles.

How do hailstones form?
Hailstones form in very large clouds like storm clouds. It is so cold in most clouds that ice crystals form at the top of them but in a storm cloud the ice crystals join together to make a small ball called a hailstone. This falls through the cloud towards the ground. In a large cloud there are winds, which blow upwards. They are strong enough to blow a hailstone back up again. A hailstone can move up and down several times as it is blown by the winds. When it is low in the cloud it is covered with clear ice. When it is high in the cloud frosty ice coats its surface. As it moves up and down the hailstone can gather many coats of ice. Eventually it becomes too heavy for the wind to lift and it falls to the ground. If a hailstone is carefully cut open the coating of frost will be seen like the layers of leaves inside an onion. A very large hailstone may be the size of a tennis ball and weigh one kilogram.

Why is some snow good for making snowballs while some is useless?
Snow is made from ice crystals. They form high in a cloud. A snowflake is made by many ice crystals joining together. If the temperature of the air is about 0°C snow falls as snowflakes and makes wet snow. This is good for making snowballs. If the temperature of the air is below 0°C the ice crystals do not join together to make snowflakes and dry powdery snow forms. This is useless for making snowballs.