7.2 Water
Name two compounds in each case which can cause: (a) temporary hardness (b) permanent hardness.
(a) Temporary hardness is caused by the presence of dissolved calcium hydrogen carbonate and magnesium hydrogen carbonate. (b) Permanent hardness is caused by the presence of dissolved calcium sulphate and magnesium sulphate.
Degree of the hardness of water
Degree of hardness of water is the number of parts of mass of CaCO3 (calcium carbonate), equivalent to various calcium and magnesium salts present in one million parts by mass of water (ppm).
Hard water
Hard water is water which will not readily form a permanent lather with soap.
How will you test for hardness in water?
Hardness in water will be tested with soap. If the water becomes cloudy instead of forming lather, the water is hard.
Explain how you will distinguish between temporary and permanent hardness in water.
If the hardness of water can be removed by boiling, the water is temporary hard water. Temporary hard water contains calcium and magnesium hydrogen carbonates. These substances are decomposed by boiling and producing calcium and magnesium sulphate and chloride. These substances are not decomposed when heated.
Permanent hardness
Permanent hardness is caused by dissolved calcium or magnesium chloride and sulphate which cannot be removed by boiling.
Permutit method
Permutit method is a process in which hard water containing calcium or magnesium salt is passed through a layer of sodium zeolite. The calcium or magnesium is removed and the corresponding sodium salt passes in solution.
Soft water
Soft water is water which readily gives a permanent lather with soap. •
Temporary hardness
Temporary hardness is caused by dissolved calcium hydrogen carbonate which is removed by boiling.
How do you soften temporary hard water?
Temporary hardness is easily removed from water by boiling. When heated, the calcium hydrogen carbonate decomposes, producing insoluble calcium carbonate. calcium hydrogen carbonate →∆ calcium carbonate + water + carbon dioxide Ca(HCO₃)₂(aq) →∆ CaCO₃(s) + H₂O(l) + CO₂(g)
What are the effects of temporary and permanent hardness of water?
The effects of temporary and permanent hardness of water are (i) furring in kettles and (ii) blockages in hot water pipes.
Water hardness
Water hardness is the amount of dissolved calcium ions, magnesium ions or both in the water.
Water pollution
Water pollution is caused by the pollutants such as sewage, industrial wastes, chemical fertilisers and detergents. The treatment needed to make water fit to drink depends on the source of the water. The process of water treatment involves both filtration and chlorination.
Write chemical equations, how temporary hard water reacts with soap.
sodium stearate (soap) + calcium hydrogen carbonate → calcium stearate (scum) + sodium hydrogen carbonate 2C₁₇H₃₅COONa(s) + Ca(HCO₃)₂(aq) → (C₁₇H₃₅COO)₂Ca(s) + 2NaHCO₃(aq)
