Water hardness is primarily caused by the presence of dissolved multivalent cations, specifically calcium (Ca2+) and magnesium (Mg2+) ions. These ions originate from the dissolution of minerals from rocks and soil through which water passes.
Sources of Calcium and Magnesium Ions
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Limestone and Chalk: These rocks are largely composed of calcium carbonate (CaCO3). Rainwater, slightly acidic due to dissolved carbon dioxide (CO2), can dissolve the limestone and chalk, releasing calcium ions into the water.
CaCO3 (s) + H2O (l) + CO2 (g) ⇌ Ca2+ (aq) + 2HCO3- (aq)
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Dolomite: Dolomite is a calcium-magnesium carbonate mineral (CaMg(CO3)2). Its dissolution releases both calcium and magnesium ions.
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Gypsum and other minerals: Gypsum (CaSO4·2H2O) and other minerals containing calcium or magnesium sulfates, chlorides, or other salts also contribute to water hardness.
How Hardness Manifests
The presence of these ions in water leads to several noticeable effects:
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Soap Scum: Hard water reacts with soap, forming an insoluble precipitate (soap scum) which reduces the cleaning effectiveness of soap and leaves a residue on surfaces. Sodium or potassium soaps react with calcium and magnesium ions to produce insoluble calcium and magnesium soaps.
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Scale Formation: When hard water is heated, calcium and magnesium carbonates can precipitate out of solution, forming a scale or limescale deposit in pipes, boilers, and other appliances. This scale reduces heating efficiency and can damage equipment.
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Taste: High concentrations of calcium and magnesium can give water a slightly bitter or metallic taste.
Types of Hardness
Water hardness is generally categorized into two types:
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Temporary Hardness: This is caused by dissolved calcium and magnesium bicarbonates (Ca(HCO3)2 and Mg(HCO3)2). Temporary hardness can be removed by boiling the water, which converts the bicarbonates into insoluble carbonates that precipitate out.
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Permanent Hardness: This is caused by dissolved calcium and magnesium sulfates, chlorides, and nitrates. Boiling does not remove permanent hardness.
Summary
In essence, the hardness of water is primarily due to the geological landscape through which water flows. The dissolution of calcium and magnesium-containing minerals contributes these ions to the water, resulting in the issues associated with hard water.
