Water often has ions dissolved in it: post 16.39 explains why and gives more information on ions. Two common ions found in drinking water are calcium ions (Ca2+) and magnesium ions (Mg2+). When these ions interact with soap, they form a soft, insoluble deposit called scum.
Water that forms scum, in this way, is called hard water. Of course, it isn’t really hard in the sense that solids can be hard. Steel is harder than soap, because steel can scratch soap; diamond is harder than steel because diamond can scratch steel. Liquids, like water, can’t scratch anything. The “hard” in “hard water” is another unfortunate example of using one word to mean two completely different things (post 17.25).
Why does scum form? Soap is a mixture of sodium ions (Na+) and stearate ions, that dissolves in water (post 16.48). However, calcium ions bind to stearate ions to form an insoluble solid.
Why are there calcium and magnesium ions in drinking water? When rain seeps through soil and rock, it dissolves any solids that are soluble in water. Deposits of calcium sulfate (the mineral gypsum) and magnesium sulfate (the mineral epsomite, also called Epsom salts) are found in rocks; they are not very soluble but some will dissolve to make water hard.
Another source of calcium and magnesium ions in water are calcium carbonate (the mineral calcite that occurs in chalk and limestone) and a mixed calcium and magnesium carbonate (the mineral dolomite that often occurs in limestone). The way in which their calcium and magnesium ions dissolve in water is more complicated.
When rain falls, it dissolves some of the carbon dioxide in the air. Some of the dissolved carbon dioxide reacts with the water (see post 16.33 to find out about chemical reactions) to form molecules of carbonic acid; a molecule of carbonic acid has the structural formula (see post 16.30) shown below.
Carbonic acid is an acid (post 17.49) because some of its molecules dissociate into a hydrogen ion and a hydrogen carbonate (sometimes called a bicarbonate) ion, as shown below (see post 16.40 to find out more about ions like this). There are arrows pointing in two directions because the reactions can go in either direction, giving an equilibrium mixture of reactants and products.
Calcium hydrogen carbonate and magnesium hydrogen carbonate are more soluble in water than calcium carbonate and magnesium carbonate. So, when rain-water trickles through limestone, calcium and magnesium ions can pass into solution as calcium hydrogen carbonate and magnesium hydrogen carbonate.
When a solution containing hydrogen carbonate ions is heated, the ions decompose to give carbonate ions, water and carbon dioxide:
2HCO3– → CO32- + H2O + CO2.
Now the calcium and magnesium ions can’t dissolve and are deposited as calcium carbonate and magnesium carbonate.
These insoluble deposits of calcium and magnesium carbonate can block hot-water pipes, boilers and kettles when that are called scale. When scale forms, the water is no longer hard – because the calcium and magnesium ions have been removed. So, water hardness caused by hydrogen carbonates is called temporary harness. Hardness resulting from calcium and magnesium sulfates, nitrates and chlorides, which don’t decompose on heating, is called permanent hardness. Of course, these compounds will be deposited if the water evaporates.
Scale formation is the major disadvantage of hard water.
Most people like to drink hard water. However, some people prefer to drink water that is as pure as possible, because they believe tap and bottled water contain impurities that could be dangerous to health. The most common forms of purer water are deionized water (that has its ions separated from it) and distilled water (that is evaporated, leaving all the ions behind, and its vapour then cooled to form liquid water again); water can also be purified by reverse osmosis (post 18.29). Since most people have several sources of calcium and magnesium ions in their diet, drinking purified water probably does no harm – provided it doesn’t become contaminated with harmful bacteria or viruses. Is there any evidence that hard water is harmful? I have searched the scientific literature to find any harmful effects or benefits of consuming hard water – just as I did for fats in post 16.47. There appears to be no reliable evidence for hard water being harmful for most people and it is possible that the calcium and magnesium ions it contains may be a useful source of these essential components of our diet (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3775162/).