Evaporation
Have you sat next to someone while they painted their fingernails with nail varnish? It would not be long before you could smell the varnish. You may have moved away because of the strong smell and told the person to use something with less smell. While the nail varnish may have given the fingernails a different colour it will also have released a gas into the air. The release of this gas is through evaporation.

Nail varnish is a mixture. It is made from coloured pigments, which are mixed with the smelly liquid. The purpose of the liquid is to let the pigments spread out evenly over the fingernails. When it has done its job it leaves the pigments in place by evaporating. This is much better than having to wash off the liquid as that might remove pigments and leave the fingernails in a blotchy condition.

The liquid in nail varnish is made from particles, which slide around each other. When they are spread out on the nail they receive heat from the finger. This gives them more energy to move. The particles in liquids hold each other close as they slide by. When they receive more energy they begin to move faster and hold each other less strongly. If they receive enough energy they let go of each other completely and fly off into the air. When this happens the particles have formed a gas and the process of evaporation has taken place.

When the particles of gas enter the air they gather above the surface from which they have just evaporated. This makes it more difficult for other particles to enter the air.
If you have stayed in the room with the person painting their fingernails, you may see them blow on their nails to dry them. The movement of the air helps to whisk away particles of the nail varnish gas and create more space for others to leave the liquid. The heat in the breath also provides more energy to the particles in the liquid so they can move faster and leap into the air.

In a short time all the nail varnish liquid will have evaporated and the pigment will have formed a hard, colourful coat on the fingernails. The smell of the nail varnish will have gone too as the particles will have spread out completely among the particles of the other gases in the air.

Nail varnish provides a smelly example of evaporation. Slower but odourless examples of evaporation can be found at the surface of water anywhere. It may be the surface of a drink or the water in a vase of cut flowers. Evaporation occurs on the surface of puddles, lakes, oceans and pools in rocks when the tide goes out. It is a very important process in the movement of water on the Earth.

Why do some liquids evaporate faster than others do?
The particles in a liquid pull on each other as they slide by. In some liquids, the particles pull strongly on each other. This means that the particles need more energy to pull themselves away from each other. They will only evaporate slowly at normal temperatures. In other liquids, particles only pull weakly on each other. They do not need large amounts of energy to help them move apart. These liquids, like the nail varnish, evaporate quickly at normal temperatures.

Does petrol evaporate quickly?
Yes, it does. You can sometimes smell it if you pass a busy petrol station. The nozzles of the petrol pump are long and go deep into the tube leading to the petrol tank so there is little chance of petrol evaporating and mixing with the air. When the nozzle is removed from the tube a flap closes over the top of the tube to prevent petrol vapour escaping from the petrol tank.

Why is petrol vapour to be avoided?
When air and petrol mix they can explode if they are heated. You will see notices at a petrol station telling people not to smoke or to light a match. Some people put petrol on a bonfire. This is very dangerous because the petrol evaporates quickly and forms an explosive mixture with the air in the pile of wood. If a match is thrown onto the bonfire an explosion can take place which blows logs of the bonfire in all directions.

When you have an injection your skin is rubbed with surgical spirit. Why does it feel cold?
The particles in surgical spirit only hold each other loosely. When the liquid is rubbed on your skin the particles readily take up heat from your skin and evaporate. This rapid uptake of heat makes your skin feel cold.

Why is surgical spirit rubbed in the skin?
To kill germs on the skin before the injection is made.

When water evaporates what gas does it form?
Water forms the gas called water vapour. This gas has no colour and no smell and is invisible. The rate at which water and all other liquids evaporate depends on the temperature of the surroundings. In cold weather the rate of evaporation is slow. When the weather is warm the rate of evaporation is faster.

What is the rate of evaporation?
It is the speed with which water turns from a liquid into a gas. It is the speed of evaporation.

Does anything else affect the rate of evaporation?
Yes, it does. The movement of the air and the humidity of the air. If the air is still, the particles gather just above the surface from which they have evaporated and only move slowly away. This stops other particles from the liquid entering the air. If the air is moving, it carries the particles of water vapour away and this makes room for more water particles to evaporate.

The humidity of the air is a measure of how much water vapour it contains. Air with a high humidity has a large amount of water vapour in it and feels clammy. Air with a low humidity has a small amount of water vapour in it. If the air has a high humidity, it contains a large number of water particles and there is not much room for others to enter from a water surface. This means that air with high humidity causes a low evaporation rate. If the air has a low humidity there is plenty of space for other water particles to enter it. This means that air with a low humidity causes a high evaporation rate.

Are there machines which can speed up evaporation?
Yes, there are. They are hair dryers and tumble dryers. They produce a current of warm dry air. These are the three things that are needed for a high evaporation rate.