Hard water is defined as water, which contains the salts of both magnesium and calcium principally as chlorides, bicarbonates, and sulfates. Often present and oxidised to ferric form is ferrous iron, which appears as a reddish-brown stain on enamel surfaces and washed fabrics. Since boiling transforms the bicarbonate into insoluble carbonate, the hardness of water caused by calcium bicarbonate is referred to as temporary. And, the hardness from the other salts is referred to as permanent.
Ions in Hard water
The higher fatty acids of soap combine with the magnesium and calcium ions in hard water to create an insoluble gelatinous curd, resulting in soap waste. This particular objectionable reaction does not occur with modern detergents.
Hard Water in Boilers
In boilers, the magnesium and calcium in hard waters form an adherent and hard scale on the plates. As a result of the scale's poor heat conductivity, fuel consumption can be increased, and also, the boiler rapidly deteriorates through the external overheating of the plates. If present, sodium carbonate hydrolyzes to form free alkali that causes caustic embrittlement and failure of boilerplates. Also, water is softened on a small scale by the addition of borax, or trisodium phosphate, ammonia, together with the sodium carbonate (otherwise called washing soda).
The calcium in water is then precipitated as carbonate, and the magnesium is precipitated as hydroxide. And, on a wide scale, water is softened by adding only enough lime to precipitate calcium as carbonate and magnesium as hydroxide, after which the sodium carbonate can be added to extract the remaining calcium salts. Home water softeners, which use natural or artificial zeolite mineral properties, are used in places where the water is rough.
Snow, rainwater, and other sources of precipitation usually contain low levels of multivalent cations including Magnesium and Calcium. They can contain small concentrations of ions such as Chloride, Sulphate, and Sodium derived from the wind action over the sea. Whereas precipitation falls in the drainage basins that are formed of impervious, calcium-poor, and hard rocks, only very low concentrations of the multivalent cations can be found, and the water is known as Soft water. Examples in the UK include the Western Highlands in Scotland and Snowdonia in Wales.
Areas that have complex geology can produce differential degrees of the hardness of water over short distances.
The permanent hardness of water can be determined by the concentration of multivalent cations present in the water. The positively charged metal, that complexes with a charge greater than 1+, is referred to as a multivalent cation. In general, the cations hold a charge of 2+. Common cations found in the hard water include Mg2+ and Ca2+. These particular ions enter a water supply by leaching from the minerals within an aquifer. Gypsum and Calcite are given as the common calcium-containing minerals. A common magnesium mineral is given as dolomite (which also holds calcium). Distilled water and rainwater are soft because they have fewer ions.
The below equilibrium reaction describes the formation and dissolving of calcium carbonate and calcium bicarbonate:
CaCO3 (s) + CO2 (aq) + H2O (l) ⇌ Ca2+ (aq) + 2 HCO3 (aq)
The reaction can go in either of the directions. Rain that contains dissolved carbon dioxide can react with the calcium carbonate compound and carries the calcium ions away with it. The calcium carbonate can be re-deposited as Calcite as the CO2 is lost to the atmosphere, sometimes forming stalagmites and stalactites.
With the hard water, solutions of soap form a white precipitate (otherwise called soap scum) instead of producing the lather, due to the reason the 2+ ions destroy the surfactant properties of the soap by producing a solid precipitate (which is the soap scum). Calcium stearate is a primary component of such scum that arises from sodium stearate, the major component of soap:
2C₁₇H₃₅COO⁻(aq) + Ca²⁺(aq) ➝ (C₁₇H₃₅COO)₂Ca(s)
As a result, hardness can be defined as a water sample's soap-consuming capacity or soap precipitation capacity as a water property that prevents soap from lathering. At the same time, synthetic detergents do not form that kind of scum.
Since soft water contains a few calcium ions, the soaps' lathering action is not inhibited, and no soap scum can form during normal washing. In the same way, soft water produces zero calcium deposits in water heating systems.
Often, it is more desirable to soften the hard water. Most of the detergents contain ingredients that counteract the hard water effects on the surfactants. For this specific reason, softening of water is much often unnecessary. Where softening of water is practised, often it is recommended in hard water softener that only the water that is sent to domestic hot water systems so as to either prevent or delay damage and inefficiencies due to the scale formation in water heaters. A common method for water softening includes the ion-exchange resin use, which replaces ions such as Ca2+ with twice the number of mono cations such as potassium or sodium ions.