WJEC GCSE Chemistry Practice Exam

Which type of water hardness can be removed by boiling?

Temporary Hard Water Permanent Hard Water Hardness of Water Soft Water

Correct Answer:

Temporary Hard Water

Explanation:

Temporary hard water contains dissolved bicarbonates of calcium and magnesium. When you boil it, these bicarbonates break down and release carbon dioxide, causing the acids to form carbonates that precipitate out as solid scale. A typical reaction is Ca(HCO3)2 -> CaCO3(s) + CO2(g) + H2O(l). With the calcium and magnesium removed from solution, the water becomes softer. Permanent hard water, made from sulfates or chlorides of these ions, doesn’t decompose on heating, so boiling doesn’t remove it. Soft water already has low hardness, so boiling isn’t needed.

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