Chemistry of Water: Hard Water, Heavy Water, Purification

Water is the one compound that appears in every section of a General Science paper: its structure in chemistry, its anomalous behaviour in physics, hardness and softening in applied chemistry, and purification and water-borne disease in everyday science. These notes cover the composition and the unusual properties of water, hard and soft water with the methods of softening, heavy water and its use in nuclear reactors, and the stages by which water is made fit to drink.

Composition and structure of water

Water is a compound of hydrogen and oxygen with the formula H2O, in which the two elements are combined in the fixed mass ratio of one to eight, the standard illustration of the law of constant proportions. The molecule is bent, with a bond angle of about 104.5 degrees, and because oxygen pulls the shared electrons towards itself the molecule is polar. That polarity, and the hydrogen bonds between neighbouring molecules, explain almost every property of water that papers ask about. Pure water is neutral, with a pH of seven at twenty-five degrees Celsius, and it conducts electricity very poorly until a salt or an acid is dissolved in it.

The anomalous properties of water

• Water has an unusually high boiling point of one hundred degrees Celsius at normal pressure, because hydrogen bonds must be broken before a molecule can escape.
• Its specific heat capacity is about 4.18 joule per gram per kelvin, the highest of the common liquids, which is why oceans moderate climate and why water is used as a coolant.
• The latent heat of fusion of ice is about 334 joule per gram and the latent heat of vaporisation of water about 2,260 joule per gram, so steam scalds far worse than boiling water.
• Water is densest at four degrees Celsius, not at its freezing point. Between zero and four degrees it contracts on heating, which is called the anomalous expansion of water.
• Because ice is less dense than water, it floats, and a sheet of ice on a lake insulates the water below so that fish survive the winter.
• Water is called the universal solvent because its polarity and high dielectric constant let it dissolve more substances than any other common liquid.

Hard water and soft water

Water that lathers freely with soap is soft; water that does not is hard. Hardness is caused by dissolved salts of calcium and magnesium, picked up as water passes over limestone, chalk and gypsum. With soap, which is the sodium salt of a fatty acid, these dissolved salts form insoluble calcium and magnesium stearates, the greyish scum seen in a wash basin, so soap is wasted before any lather appears. Detergents work even in hard water because their calcium and magnesium salts remain soluble. Hard water also leaves scale in boilers and kettles, which wastes fuel and can be dangerous in an industrial boiler. Hardness is reported in parts per million of calcium carbonate equivalent, and rain water is the softest water found in nature, while distilled water contains no dissolved salts at all.

In the zeolite or Permutit process, water is passed through sodium aluminium silicate, which exchanges its sodium ions for the calcium and magnesium ions of the water. A modern ion exchange plant uses a cation resin and an anion resin in series and can bring the hardness down to almost nothing, which is why demineralised water is produced this way. The Calgon process adds sodium hexametaphosphate, which locks up the calcium and magnesium in a soluble complex instead of removing them.

Heavy water

Heavy water is deuterium oxide, D2O, in which both hydrogen atoms are the isotope deuterium, with one proton and one neutron in the nucleus. Deuterium was discovered by Harold Urey in 1931, for which he received the Nobel Prize in Chemistry in 1934, and nearly pure heavy water was first obtained by Gilbert Newton Lewis in 1933 by the prolonged electrolysis of ordinary water. Heavy water occurs in ordinary water to the extent of roughly one part in six thousand, and it is still obtained industrially by repeated electrolysis or by fractional distillation, since its boiling point of 101.4 degrees Celsius and freezing point of 3.8 degrees Celsius differ from those of ordinary water. Its molecular mass is twenty against eighteen, and its density about 1.105 gram per cubic centimetre, so a cube of ordinary ice sinks in heavy water. Its great use is in a nuclear reactor, where it serves as a moderator, slowing down fast neutrons without absorbing many of them, and in Indian reactors also as the coolant; the pressurised heavy water reactor is the mainstay of the Indian nuclear programme and the Heavy Water Board under the Department of Atomic Energy produces the heavy water for it. Heavy water is also used as a tracer in chemical and biological research, but it is harmful to living organisms in large amounts.

How water is made fit to drink

A municipal water works treats river or reservoir water in a fixed sequence, and papers ask for the purpose of each step.

1. Screening holds back floating leaves, rags and other large matter.
2. Aeration drives off dissolved gases and odour and adds oxygen.
3. Sedimentation lets the heavier suspended particles settle under gravity.
4. Coagulation adds alum, that is aluminium sulphate or potash alum, which makes the fine clay particles clump into flocs heavy enough to settle.
5. Filtration through beds of sand and gravel removes the remaining suspended matter and most bacteria.
6. Disinfection kills the pathogens that are left, usually by chlorination with chlorine gas or bleaching powder, which is calcium oxychloride; ozonation and ultraviolet light are the alternatives.

At home, boiling is the simplest and surest method against bacteria and viruses, and a candle filter removes suspended dirt. A reverse osmosis unit forces water through a semi-permeable membrane at a pressure greater than the osmotic pressure and so removes dissolved salts as well, which an ultraviolet purifier cannot do, since ultraviolet light kills microbes but leaves every dissolved salt behind. Activated charcoal removes colour, odour and organic matter. Distillation gives the purest water, and the desalination of sea water is carried out by reverse osmosis or by multi-stage flash distillation. Water that has not been treated carries cholera, typhoid, hepatitis A, dysentery and polio, while dissolved impurities cause their own diseases: excess fluoride causes fluorosis, arsenic causes arsenicosis and nitrate in drinking water causes blue baby syndrome in infants. The organic pollution of a water body is measured by its biochemical oxygen demand, and clean water is marked by a high level of dissolved oxygen.