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Water that has a high mineral content

Hard water is water that has a high mineral content (in contrast with "soft water"). Hard water is formed when water percolates through deposits of limestone, chalk or gypsum, which are largely made up of calcium and magnesium carbonates, bicarbonates and sulfates. Drinking hard water may have moderate health benefits. It can pose critical problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling towers, and other equipment that handles water. In domestic settings, hard water is often indicated by a lack of foam formation when soap is agitated in water, and by the formation of limescale in kettles and water heaters. Wherever water hardness is a concern, water softening is commonly used to reduce hard water's adverse effects.

Temporary hardness is caused by the presence of dissolved bicarbonate minerals (calcium bicarbonate and magnesium bicarbonate). When dissolved, these types of minerals yield calcium and magnesium cations (Ca2+, Mg2+) and carbonate and bicarbonate anions (CO32− and HCO3−). The presence of the metal cations makes the water hard. However, unlike the permanent hardness caused by sulfate and chloride compounds, this "temporary" hardness can be reduced either by boiling the water or by the addition of lime (calcium hydroxide) through the process of lime softening. Boiling promotes the formation of carbonate from the bicarbonate and precipitates calcium carbonate out of solution, leaving water that is softer upon cooling.

In some cases it is desirable to soften hard water. Most detergents contain ingredients that counteract the effects of hard water on the surfactants. For this reason, water softening is often unnecessary. Where softening is practised, it is often recommended to soften only the water sent to domestic hot water systems to prevent or delay inefficiencies and damage due to scale formation in water heaters. A common method for water softening involves the use of ion exchange resins, which replace ions like Ca2+ by twice the number of mono cations such as sodium or potassium ions. Washing soda (sodium carbonate, Na2CO3) is easily obtained and has long been used as a water softener for domestic laundry, in conjunction with the usual soap or detergent. Water that has been treated by a water softening may be termed softened water. In these cases, the water may also contain elevated levels of sodium or potassium and bicarbonate or chloride ions.

The World Health Organization says that "there does not appear to be any convincing evidence that water hardness causes adverse health effects in humans". Some studies have shown a weak inverse relationship between water hardness and cardiovascular disease in men, up to a level of 170 mg calcium carbonate per litre of water. The World Health Organization has reviewed the evidence and concluded the data was inadequate to recommend a level of hardness. Other studies have shown weak correlations between cardiovascular health and water hardness. The prevalence of atopic dermatitis (eczema) in children may be increased by hard drinking water. Living in areas with hard water may also play a part in the development of AD in early life. However, when AD is already established, using water softeners at home does not reduce the severity of the symptoms. It is equivalent to mg/L without chemical compound specified, and to American degree. Grain per gallon (gpg) is defined as 1 grain (64.8 mg) of calcium carbonate per U. S. gallon (3.79 litres), or 17.118 ppm. a mmol/L is equivalent to 100.09 mg/L CaCO3 or 40.08 mg/L Ca2+. A degree of General Hardness (dGH or 'German degree (°dH, deutsche Härte))' is defined as 10 mg/L CaO or 17.848 ppm. A Clark degree (°Clark) or English degrees (°e or e) is defined as one grain (64.8  mg) of CaCO3 per Imperial gallon (4.55 litres) of water, equivalent to 14.254 ppm. A French degree (°fH or °f) is defined as 10 mg/L CaCO3, equivalent to 10 ppm.

Several indices are used to describe the behaviour of calcium carbonate in water, oil, or gas mixtures.

The Puckorius scaling index (PSI) uses slightly different parameters to quantify the relationship between the saturation state of the water and the amount of limescale deposited.

Other indices include the Larson Skold Index, the Stiff Davis Index, and the Oddo Tomson Index.

The hardness of local water supplies depends on the source of water. Water in streams flowing over volcanic (igneous) rocks will be soft, while water from boreholes drilled into porous rock is normally very hard.

+ Hardness water level of major cities in England and WalesAreaPrimary sourceLevelManchesterLake District Pennines 1.750 °clark / 25 ppmBirminghamElan Valley Reservoirs3 °clark / 42.8 ppmBristolMendip Hills 16 °clark / 228.5 ppmSouthamptonBewl Water18.76 °clark / 268 ppmLondon (EC1A)Lee Valley Reservoir Chain19.3 °clark / 275 ppmWrexham (LL11)Hafren Dyfrdwy4.77 °clark
Information from the British Drinking Water Inspectorate shows that drinking water in England is generally considered to be 'very hard', with most areas of England, particularly east of a line between the Severn and Tees estuaries, exhibiting above 200 ppm for the calcium carbonate equivalent. Water in London, for example, is mostly obtained from the River Thames and River Lea both of which derive a significant proportion of their dry weather flow from springs in limestone and chalk aquifers. Wales, Devon, Cornwall and parts of northwest England are softer water areas and range from 0 to 200 ppm. In the brewing industry in England and Wales, water is often deliberately hardened with gypsum in the process of Burtonisation. Generally, water is mostly hard in urban areas of England where soft water sources are unavailable. Several cities built water supply sources in the 18th century as the industrial revolution and urban population burgeoned. Manchester was a notable such city in North West England and its wealthy corporation built several reservoirs at Thirlmere and Haweswater in the Lake District to the north. There is no exposure to limestone or chalk in their headwaters and consequently the water in Manchester is rated as 'very soft'. In this section, reference to original EU documentation is given, which sets out no limit for hardness. The handbook also gives no "Recommended or Mandatory Limit Values" for hardness. The handbook does indicate that above the midpoint of the ranges defined as "Moderately Hard", effects are seen increasingly: "The chief disadvantages of hard waters are that they neutralise the lathering power of soap[ ] and, more important, that they can cause blockage of pipes and severely reduced boiler efficiency because of scale formation. These effects will increase as the hardness rises to and beyond 200 mg/L ."

A collection of data from the United States found that about half the water stations tested had hardness over 120 mg per litre of calcium carbonate equivalent, placing them in the categories "hard" or "very hard".