What Makes Water “Hard”
Hard water doesn’t lather easily with soap; soft water does. Hardness comes from dissolved calcium and magnesium ions, and comes in two kinds.
Temporary Hardness
Caused by calcium and magnesium hydrogencarbonates (bicarbonates) dissolved in the water, and removable simply by boiling.
- Boiling: the hydrogencarbonate decomposes, precipitating out as carbonate — this is what deposits “fur”/scale inside kettles and boilers. Ca(HCO₃)₂(aq) —heat→ CaCO₃(s) + H₂O(l) + CO₂(g)
- Adding calcium hydroxide (a calculated amount): precipitates the calcium out as carbonate. Ca(HCO₃)₂(aq) + Ca(OH)₂(aq) → 2CaCO₃(s) + 2H₂O(l)
Effects: furs up kettles and boilers (removable with acid); over geological time, the same carbonate-depositing chemistry forms stalactites and stalagmites in caves.
Permanent Hardness
Caused by calcium and magnesium sulphates and chlorides — these don’t decompose on heating, so boiling alone can’t soften this kind of hardness.
- Adding sodium carbonate (washing soda) — works on both temporary and permanent hardness alike, precipitating out the calcium or magnesium ion responsible: CaSO₄(aq) + Na₂CO₃(aq) → CaCO₃(s) + Na₂SO₄(aq); Ca(HCO₃)₂(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2NaHCO₃(aq)
- Ion exchange: water is passed through a resin that swaps its calcium/magnesium ions for sodium ions, which don’t cause hardness.
Advantages and Disadvantages of Hard Water
Advantages: a more pleasant taste than soft water; the calcium it supplies helps build bones and teeth; well suited to brewing beer.
Disadvantages: wastes soap (forms scum instead of lather); furs up and blocks hot water pipes.
Testing for the Presence of Water
Anhydrous copper(II) sulphate is white and turns blue in the presence of water. Anhydrous cobalt(II) chloride is blue and turns pink in the presence of water. Either serves as a simple chemical test for water.
