Like many other substances, water can exist in all three phases or states: solid, liquid and gas. We usually refer to the solid phase of water as ice and the gaseous phase as water vapour or steam, reserving the word water itself for the liquid phase.
At normal atmospheric pressure at sea level (1013 mb) the freezing point of pure water is 0oC and its boiling point 100oC. Solids dissolved in the water usually lower the freezing point and raise the boiling point; higher pressures raise the boiling point and lower the freezing point, while lower pressures lower the boiling point and raise the freezing point.
The freezing point of pure water at atmospheric pressure is 0oC, but salty water freezes at a lower temperature, so the sea round the British Isles very seldom freezes. Adding salt (sodium cloride) to ice may lower the freezing point of the mixture to below its current temperature so that the ice melts, but the latent heat (see the last paragraph on this page) taken in as the ice melts will also cause a lowering of the temperature of the mixture. We can therefore make a freezing mixture by adding salt to ice - the greater the amount of salt the lower the temperature of the mixture. The lowest temperature we can reach in this way is about -20oC. This is about the temperature of a domestic freezer and was intended by Fahrenheit to be zero on the temperature scale he had invented, although in fact it is about -4oF. For more about Fahrenheit and temperature scales please click here 
We can melt ice on roads by adding salt, but the amount of salt needed depends upon the temperature of the ice, and as the ice/salt mixture is very corrosive on the undersides of motor vehicles, and the greater the amount of salt the more corrosive the mixture, councils in Britain seldom put down sufficient salt to cope with temperatures of below -5oC.
Ice has a very high coeficient of friction - it is slippery only when the surface of the ice is covered with a thin film of water. Ice skates have thin metal blades which exert a very high pressure on the ice. This lowers the freezing point of the water by two or three degrees Celsius and the ice under the blade melts. The skate is thus sliding on a film of water. Once the pressure of the skate is removed the water refreezes. If the temperature of the ice is below about -5oC the pressure of the skates on the ice is not sufficient to lower the melting point enough, the ice does not melt, and skating is impossible.
You can do an experiment to demonstrate this at home - to find out how click here
Snow is made up of tiny crystals of ice. We can make a snowball by squeezing a handful of snow. The high pressure melts the ice crystals, and then when we release the pressure the water refreezes as a block of ice rather than as individual crystals. But if the snow is too cold we cannot make a snowball.
Skis work in exactly the opposite way to skates: they are designed to exert a very low pressure on the snow so they do not sink into it.
Ice is a very poor conductor of heat. If we put an ice-cube straight from the freezer (-20oC) into a bowl of water the outside of the cube will warm up (not melt) and expand much more rapidly than the inside and the stresses caused by the uneven expansion will shatter it with a loud cracking sound. If the ice has been out of the freezer for a few minutes it will have warmed up slightly and the effect will not be noticed. The same effect takes place whenever any solid with a low thermal conductivity is heated or cooled very rapidly, for example by dropping a stone heated in a bonfire into a bucket of water, but to do this experiment with any material except ice is very dangerous indeed.
We usually reserve the word steam to mean water in the gaseous state unmixed with other gases, and at a temperature at or above 100oC. When the water in the gaseous state is mixed with other gases, particularly air, or when it is below 100oC, we usually call it water vapour.
The boiling point of water depends upon the pressure. At atmospheric pressure (1013 mb) the boiling point of pure water is 100oC, but at higher pressures it is higher. Inside a pressure cooker the pressure is about twice atmospheric pressure and the boiling point of the water, and so the temperature of the steam used to cook the food, is about 120oC. This is of course why food cooks more quickly inside a pressure cooker. Most modern steam engines and steam turbines use steam at pressures much higher than atmospheric.
At the top of Mount Everest (about 10 000 m) the pressure is only about 260 mb and the boiling point of water is about 69oC
Once the steam has been produced by boiling water it may be heated to raise its pressure even further in the same way as for any other gas: steam that has been heated in this way is called superheated steam. Superheated steam is at a temperature far higher than the boiling point of water at the same pressure.
Steam is used for many purposes: in steam engines and turbines, for cleaning and for heating and cooking. Most modern steam turbines are powered by high pressure superheated steam.
Water vapour mixed with other gases can exist at any temperature, from below 0oC to above 100oC. It is present in the atmosphere and in the gases produced by the burning of substances containing hydrogen compounds - these include almost all fuels.
If we add a teaspoonful of sugar to a cup of tea the sugar will dissolve and we shall get sweet tea, but if we add a teaspoonful of tea to a cup of sugar we do not get very sweet tea we just get slightly damp sugar. There is a limit to how much sugar will dissolve in a given volume of tea. Similarly there is a limit to the amount of water vapour that can be contained in a given volume of air. When the amount of water vapour in the air is at this level the air is said to be saturated. The maximum amount of water vapour the air can contain depends upon the temperature: the higher the temperature the greater the amount.
The relative humidity (or just humidity) of the air is the amount of water vapour the air actually does contain expressed as a percentage of the amount needed to saturate it at that temperature. Totally dry air would have a r.h. of 0% and saturated air a r.h. of 100%.
Because the amount of water needed to saturate the air depends upon the temperature, changing the temperature changes the humidity of the air even if no water is added to or taken from it, and this has very many implications.
Central heating systems warm the air without adding moisture, and so lower the humidity (dry the air). This can adversely affect wooden furniture and many other things, particularly musical instruments, and things affected by static electricity. It also dries the throat and can sometimes lead to skin problems. Burning coal or other fuels in a fireplace with a chimney or flue does not have the same effect as, in a well-ventilated room, the hot gases escape up the chimney and cold moist air is drawn into the room to take their place. Paraffin and other portable heaters without chimneys or flues to carry the fumes away actually increase the humidity because water (vapour) is one of the products of combustion. The burning of a litre of paraffin produces more than a litre of water and if the room is not well ventilated it will become very damp indeed.
Air conditioning systems lower the temperature of the air, and this by itself would raise its humidity. They must therefore not only cool the air but also remove water vapour from it so as to maintain the humidity at a comfortable level.
If we take air containing water and cool it, its humidity increases. At some point it will become saturated, and if we cool it any further some of the water vapour will turn back into water (condense). This is how we get dew, and the temperature at which dew (condensation) starts to form is called the dew point. The drier the air the lower the dew point.
In a warm house on a cold day the window panes, and also the cold water pipes, may be below the dew point and so we may get condensation on them. People who wear glasses may find them steaming up as they come into the house.
Out-of-doors on a cold day the air in our lungs will be far warmer and more humid than the air around us, and as we breathe out our breath will mix with the colder air and some of the water vapour in it may condense into a cloud of droplets of water. We can see the same effect if we watch steam coming out of the spout of a boiling kettle: the initial jet of steam is invisible but as it mixes with the air it cools and condenses. We often refer to this cloud of droplets of water as clouds of steam but this is incorrect - steam is a clear colourless gas and we cannot see it.
When we open the door of an upright freezer we may say we see clouds of water droplets pouring down out of it, but what we are actually seeing is the cold dry air from the freezer mixing with the warm moist air in the room. The air from the room is cooled below its dew point and so condensation occurs.
If we put a saucepan of cold water on a gas hob, or a beaker of cold water on a bunsen burner, for the first few seconds there may be condensation on the outside. This is because the products of combustion of the gas contain water vapour and the hot gases are being cooled to below their dew point. Note that this applies to any heater actually burning a fuel, but not electric cookers.
The clouds of water droplets coming out of the exhaust pipes of cars are caused the same way. The smoke from the funnel of a steam train is of course almost entirely the clouds of water droplets condensed from the steam which has been used to drive the train.
Whales are mammals and breathe air, but they can stay under water for very long times. When they surface the air in their lungs is very warm and moist. They must empty and refill their lungs very quickly - a whale’s spout is the condensing of the water in the exhaled breath as it mixes with the cold air.
Many deserts are very hot during the day but they are also very cold at night, and even though during the day the air is very dry it may still fall below its dew point at night. Some desert plants have adapted to use this dew: they have a system of roots spreading out up to 100 m from the base of the plant but only 1 or 2 mm below the surface. One desert mammal spends the day in a burrow deep under the sand, where it is much cooler. It stores small stones in its burrow. In the evening when it is getting cooler but before the air has quite reached its dew point it brings these stones to the surface. They are below the dew point so they collect condensation. The animal drinks this. It leaves the stones out until the coldest part of the night, to make them as cold as possible, then returns them to its burrow.
Dew ponds are designed to collect dew. They were once common all over the Downs of Kent and Sussex and in other parts of Southern England, and some of them date back to Neolithic times. To read about them visit one of the Dew Pond web sites .
The atmosphere contains water vapour. As the air cools at night it may reach its dew point and we get dew. The drier the air the lower the dew point. If the dew point is below 0oC the water vapour may condense into crystals of ice rather than droplets of water: this is called hoar frost. Usually however the dew point is above 0oC and is reached quite soon after sunset, even though the minimum temperature reached later on in the night may be below 0oC. Then we get ordinary dew which freezes after it has formed.
As the Sun warms the surface of the Earth it sets up convection currents and the warm moist air starts to rise. As it rises it cools and at some height it reaches its dew point. Condensation starts to form - these are clouds, and the height at which the clouds begin to form is called the cloud-base. If the cloud-base is at ground level we have a mist or fog. As the water condenses it gives out its latent heat and this causes further convection currents inside the cloud - the convection currents inside a cloud are far stronger than those outside it.
The droplets of water are initially very tiny and their terminal velocity is very low. If their terminal velocity is less than the upcurrent they will get carried round and round in the cloud and continue to grow. Eventually they will be of a size to fall as rain - the size of the raindrops depends upon the strength of the convection currents in the cloud.
If the cloudbase is below 0oC then the water vapour may condense as crystals of ice not droplets of water and we shall get snow not rain. If however the lower part of the cloud is above 0oC but the higher parts below 0oC, the droplets of water being carried round in the cloud by the convection currents will freeze as they are carried into the higher parts, but more water will condense onto them as they are carried by the convection currents back into the lower layers of the cloud. This is how hail is formed. Each hailstone is built up of layers, like an onion - if you cut open a hailstone with a razor blade (care! - get an adult to help you) you can see the rings with a magnifying glass. The number of rings is the number of times the hailstone was carried round in the cloud before it was too big to be carried round again. In England we very seldom get hailstones bigger than peas, but in the tropics the convection currents inside storm-clouds can be so strong that the hailstones can be as big as footballs!
Droplets of water, crystals of ice and bubbles of steam cannot have zero size, so they must start to grow on a nucleus, such as a tiny particle of dust. If there are very few suitable nuclei we may be able to heat water to well above 100oC without it boiling - this is called superheating. When it does eventually boil there is a very rapid evolution of steam which may shake or even break the container. This is known as bumping. In science laboratories we often add anti-bumping granules (tiny pieces of special glass) to a liquid before heating it in order to provide nuclei so that the liquid will boil smoothly. Warning! If you forget to add the anti-bumping granules to a liquid you must always let it cool right down before adding them. If you add them to a superheated liquid you may get an explosion as the liquid suddenly boils.
At home bumping does not often occur because the insides of kettles and saucepans usually contain plenty of suitable nuclei. But you may notice if you are boiling an egg in a Pyrex saucepan that most of the bubbles of steam are coming from just a few points.
Similarly in the absence of nuclei we can supercool a liquid to well below its freezing point, or supercool air containing water vapour to well below the dew point without condensation occurring. The cloud chamber used in some experiments on radioactivity contains supercooled water vapour. The trails of ionised molecules of air produced by the alpha and beta particles given off by the radioactive source act as nuclei for condensation and you can follow the tracks of the particles by the condensation trails they leave behind.
Icing on aircraft is caused by flying through clouds of supercooled water droplets. The droplets freeze as they hit the aircraft, and the build-up of ice on the aircraft, particularly on the leading edges of the wings, can be very dangerous.
Note that superheated steam is nothing to do with this: it is steam that has been heated again after it has been formed by boiling water.
As an interesting aside, carbon dioxide is soluble in water and the greater the pressure the greater the solubility. Champagne and fizzy drinks such as Coca Cola contain, among other things, carbon dioxide dissolved in water under pressure: when the can is opened the pressure is released and bubbles of gas are formed. These bubbles can only form on nuclei and there are usually only a few nuclei in the bottle so the gas is given off only slowly. However, if the bottle is shaken before it is opened the gas that is usually at the top becomes distributed as small bubbles through the liquid. This does not increase the pressure, but it does provide thousands of nuclei and when the top is eventually taken off there is a very much more rapid evolution of gas than usual and the drink will all fizz up.
Today you can buy fresh (or frozen) vegetables all the year round, but before this was possible vegetables were stored in the cellar of the house. In frosty weather tubs of water were placed in the cellar: the latent heat given out as the water froze prevented the temperature of the cellar from dropping below 0oC and the vegetables were not damaged by the frost.
The latent heat given out when water vapour condenses inside a cloud gives rise to convection currents inside the cloud far stronger than those produced outside the cloud by the Sun’s warmth.
© Barry Gray June 2002
