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.
Some people consider that plasmas are a fourth state of matter, but plasmas are not considered further on this Page.
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.
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.
Inside a house a comfortable humidity is about 50%. 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 methane gas is a compound of hydrogen so 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 burning a fuel containing hydrogen, but not an electric cooker or a charcoal barbeque.
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 a long time. When they surface the air in their lungs is very warm and almost saturated. 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, ready for the next night.
Dew ponds are small Man-made ponds used for providing drinking water for cattle and sheep on higher ground, away from rivers. 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. Until quite recently it was believed that the special (and in earlier times secret) way in which they were built meant that they collected dew, but this has now been disproved, their design and construction just make them very good at catching and retaining rain.
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!
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 
Before refrigerators were invented Kings and Emperors and other very rich and powerful people had “ice houses”. In the winter blocks of ice from frozen lakes were cut out and stored in special underground chambers. These were so well insulated that the ice melted only very slowly, and so ice was available all the year round. The ice was not pure enough to put into drinks, but you could cool wine bottles in it. You could even add salt to the ice to make a freezing mixture for making ice cream!
We can melt ice on roads by adding salt, but the amount of salt needed depends upon the temperature of the ice, and the ice/salt mixture is very corrosive on the undersides of motor vehicles. Councils usually use a mixture of salt and grit or sand, hence of course “gritting lorries”.
When water freezes it turns into ice. If you can watch a pond, or even better a birdbath, starting to freeze you will see long needles of ice forming, although these soon join together to form a single sheet of ice. You might see patterns of lines in the ice but these are actually trapped air. You can see the effect more clearly in ice cubes from the freezer: because the water freezes from the outside all the trapped air ends up in the middle.
When water vapour freezes the very tiny crystals do not join together as they are formed so you get the most wonderful individual crystals, all based upon a hexagon but every one unique. This is how snowflakes are formed inside a cloud. You can see pictures of snowflakes in many books or on many web pages. You can see that they are all very “pointy”. Snow itself is millions of these tiny snowflakes which have been blown around inside the cloud so that all the pointy bits have got tangled together. We can make a snowball by squeezing a handful of snow. Even quite a small force can produce a very high pressure at the points, high enough to lower the melting point of the ice slightly so that the crystals melt. Then when we release the pressure the water refreezes, but as a block of ice rather than as individual crystals. This is called regelation, which just means refreezing. But if the snow is too cold we cannot lower the freezing point enough so cannot make a snowball. The same thing happens when we walk on snow, hence the icy pavements in winter.
In some very cold places, for example in the arctic and antarctic and on high mountains, some of the snow which falls in one winter is still laying on the ground at the start of the next winter, so there are permanent snow fields. If “new” snow falls on “old” snow something similar to regelation happens because of the weight of the new snow. This is called firnification (from the German for from last year) and is the way glaciers, and of course the Greenland and Antarctic icecaps, are formed.
In parts of Antarctica the ice is more than three kilometres thick and the layers of ice go back more than eight hundred thousand years. You can drill into it with a special hollow drill and produce an ice core, as a cook uses a special kitchen tool to core an apple. The sections of the core are kept in a special freezer. You can see the markings for each year, like the growth rings in the trunk of a tree. Air was trapped in the snow as it fell, so these ice cores tell us the composition of the atmosphere, including of course carbon dioxide levels, for hundreds of thousands of years into the past.
The only permanent snow field in Britain is on the North Face of Ben Nevis, but it is not quite big enough to become a glacier.
Most books and web sites, including most school science text books, explain ice skating by regelation: the very high pressure exerted by the metal skate blade on the sheet of ice lowers its melting point so the skate is sliding on a layer of water. In fact however the pressure needed to change the melting point is about 10 MPa (10 million pascals) per degree Celsius. (Pressure units are described on the Page on Pressure Units.) Atmospheric pressure at sea level is about 100 kPa so to lower the melting point by one degree Celsius we need a pressure of a hundred times atmospheric pressure, and not even a sumo wrestler on a pair of child’s skates could produce that! Skates actually work because for reasons described in the next Section above about -10°C the surface of a sheet or block of ice is always covered with a thin layer of water.
Ice is a very poor conductor of heat but has quite a high coefficient of expansion. 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.Ordinary glass cannot be put into an oven or freezer for the same reason. Ovenware such as Pyrex® is made of a special type of glass which is both a much better conductor of heat and has a much lower coefficient of expansion.
A sheet or block of ice is slippery because it is always covered with a thin layer of water, even at atmospheric pressure and at temperatures far below 0°C.
The first person to explain this was the English scientist Michael Faraday (1791 - 1867). During the nineteenth century scientists were beginning to understand the particle nature of matter (atoms and molecules etc) which is the foundation of all modern science. In ice (and all solids) the molecules are packed tightly together in a lattice and cannot move out of their position: they are held in place by the attraction of the molecules surrounding them on every side. But in water (and all other liquids and gases) the molecules are free to move around. In 1859 Faraday suggested that at the very surface of a sheet of ice the molecules are less firmly held in the lattice because they are not completely surrounded by other water molecules. This means that they can move out of their position in the lattice, that is, they have become as the molecules in a liquid. The surface of a sheet of ice is therefore covered by a very thin layer of liquid water, whatever the temperature of the ice and the pressure of the atmosphere. The conditions in this layer depend upon the temperature of the ice: at temperatures above about -10°C the conditions are such that the surface will feel slightly wet.
Ice has a very high coefficient of friction, it is slippery at temperatures above about -10°C only because it is covered with a thin layer of water. Here is an experiment you may do for yourself to prove it.
For this experiment you will need an empty large round plastic soft drinks bottle (2 litres is about right) and a freezer. Ask permission before you use the freezer.Fill the bottle not quite to the top with tap water, put the top on, and put it upright in the freezer. Leave it for 24 hours to freeze.
Take it out and carefully cut away the plastic bottle. This will leave you with a cylinder of ice. Wipe it with a cloth to remove the surface water and put it back in the freezer for at least two hours.
Now you can try the experiment on your friends. You can do the experiment as often as you like provided you wipe the cylinder after use and keep it in the freezer.
Take the cylinder out of the freezer and stand it on its end. At this point its temperature is about -20°C You should have no difficulty in picking it up with your bare hands and keeping it vertical by wrapping your hands round the sides.
Now wait five minutes and then try again. By now the temperature of the outside will have risen to about -10°C and the outside will feel slightly wet, and keeping it vertical while picking it up without damaging it will be if not impossible at any rate very difficult.
This experiment is quite safe because ice is a very poor conductor of heat. But do not try to pick up a metal object in your bare hands straight from the freezer - your hands will freeze to it and you may get bad freezer burns.
You can show that even though the surface of the cylinder is slippery its temperature is still well below 0°C by carrying out one more simple experiment, but this is best carried out at school rather than at home: ask your science teacher for permission to do it.
You will need your ice cylinder, a block of metal, a block of hardwood and an empty drinks can. Fill the drinks can with water and put them all in the freezer for 24 hours.
When you take them out of the freezer - use a cloth to hold the block of metal - they will be below the dew point of the air so water vapour will condense onto the blocks of metal and wood and the drinks can and they will become covered with crystals of ice. The ice will still be on them long after the ice cylinder has become slippery.
Faraday could not of course prove his theory: it was another hundred years before scientists had developed instruments which could actually “see” the molecules in the surface layer and so show that he was right. Since then and as a result there have been great advances in our understanding of ice and snow.
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 water (or in fact almost any 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 instantly 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 bottle 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 throughout the liquid. This does not increase the pressure, but it does provide thousands of nuclei and when the bottle is opened there is a very much more rapid evolution of gas than usual and the drink will all fizz up.
Today you can buy fresh fruit and vegetables all the year round, but before this was possible fruit and 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 0°C and the fruit and vegetables were not damaged by the frost.(Today you can buy frozen fruit and vegetables in the shops, but these have been frozen in a special way which does not damage them. Not all fruits and vegetables can be frozen in this way without damaging them.)
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.
Salty water freezes at a lower temperature than fresh water, so if we add salt to ice at 0°C the ice will melt, but the latent heat needed to melt the ice lowers the temperature of the mixture.
Sweating keeps us cool because as the sweat evaporates the latent heat needed is taken from our skin: we can feel this most readily by putting a few drops of water onto the back of our wrist and blowing on it. We have sweat glands over most of our body, but dogs do not. In hot weather they keep cool by hanging their very moist tongues out and blowing over them to increase the rate of evaporation - this is called panting.
This is an example of evaporative cooling, but there are lots of others, and it is how most refrigerators and freezers work.
Pottery is made from clay which has been shaped and then fired (heated very strongly in a special kiln) so that it keeps its shape even if it gets wet - this is explained more fully on the Clay Page. Unglazed pottery has a quite rough surface and allows liquids to soak into it: a terra cotta flower pot is unglazed pottery. We can glaze a pot by coating it with special substances and then firing it again - cups and saucers at home have been glazed in this way. You cannot store liquids in unglazed containers, but the Ancient Egyptians used to drink beer from unglazed cups: the beer soaked into the cup and some of it evaporated from its surface so cooling the beer!
At normal atmospheric pressure (about 1013 mb or 100 kPa) pure water boils at 100° and freezes at 0°C. If we raise the pressure we raise the boiling point and lower the freezing point; if we lower the pressure we lower the boiling point and raise the freezing point.
If we lower the pressure enough (lots and lots and lots) we can lower the boiling point and raise the freezing point until they are the same! This is the triple point, and occurs at a pressure of 611 Pa or 6 mb - less than one hundredth of atmospheric pressure. At this pressure the boiling point and freezing point are both 0.01°C.
For most people the triple point of water is of no importance whatever, but the triple point of carbon dioxide is, even if they do not realise it, and you can read about why here.
© Barry Gray May 2016
