(a) Give one disadvantage of: (i) hard water (ii) soft water (b) Explain why the degree of hardness in a sample of clear lime water is higher than in anothe...
Question 1 Report
(a) Give one disadvantage of:
(i) hard water
(ii) soft water
(b) Explain why the degree of hardness in a sample of clear lime water is higher than in another sample has that been turned milky by carbon (IV) oxide.
Answer Details
(a) One disadvantage of:
- (i) Hard water: it wastes soap because it forms an insoluble scum with soap before it can lather (it can also fur/scale kettles and boilers).
- (ii) Soft water: it dissolves lead from lead pipes, giving water that can cause lead (plumbo) poisoning (soft water is more corrosive to pipes).
(b) Clear lime water contains dissolved calcium hydroxide, Ca(OH)2, which supplies Ca2+ ions responsible for hardness. When carbon(IV) oxide is bubbled through and the lime water turns milky, the Ca2+ ions are precipitated as insoluble calcium trioxocarbonate(IV):
\[ \text{Ca(OH)}_2 + \text{CO}_2 \to \text{CaCO}_3\downarrow + \text{H}_2\text{O} \]Removing Ca2+ from solution as the insoluble carbonate lowers the concentration of hardness-causing ions. Hence the milky sample has a lower degree of hardness than the clear lime water.
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