Which one of the following salts is responsible for temporary hardness of water?
- (a)Sulphates of calcium
- (b)Chlorides of magnesium
- (c)Bicarbonates of magnesium
- (d)Sulphates of magnesium
Correct — C, bicarbonates of magnesium. Temporary hardness is caused by the dissolved bicarbonates of calcium and magnesium — Ca(HCO3)2 and Mg(HCO3)2. It is called 'temporary' because simply boiling the water decomposes these bicarbonates into insoluble carbonates that settle out, softening the water. The magnesium bicarbonate in option (c) is the only bicarbonate listed.
- (a)Sulphates of calcium — Calcium sulphate causes permanent hardness, which cannot be removed by boiling; it needs chemical treatment such as washing soda.
- (b)Chlorides of magnesium — Magnesium chloride is a permanent-hardness salt. Chlorides do not break down on boiling, so this hardness persists.
- (d)Sulphates of magnesium — Magnesium sulphate also causes permanent hardness; like other sulphates and chlorides, it is unaffected by boiling.
Hard water contains dissolved calcium and magnesium salts that stop soap from lathering. Temporary hardness is due to the bicarbonates of these metals and can be removed by boiling, which drives off carbon dioxide and precipitates insoluble carbonates. Permanent hardness is due to their sulphates and chlorides and needs chemical softening, such as adding washing soda (sodium carbonate).
The single word 'temporary' points straight to the bicarbonates, because only bicarbonates decompose on boiling. Scan the options for the bicarbonate: three of them are sulphates or chlorides (permanent hardness), and only magnesium bicarbonate fits temporary hardness.
- Temporary hardness is caused by the bicarbonates of calcium and magnesium and is removed by boiling.
- Permanent hardness is caused by the sulphates and chlorides of calcium and magnesium.
- Boiling converts soluble bicarbonates into insoluble carbonates, which is why kettles develop limescale.
- Permanent hardness can be removed by adding washing soda (sodium carbonate) or by ion-exchange.
- Only bicarbonates give temporary hardness; sulphates and chlorides give permanent hardness.
- Both calcium and magnesium cause hardness — do not assume it is a calcium-only phenomenon.
Hardness of water is asked as 'which salt causes temporary or permanent hardness' or 'how is temporary hardness removed' — link temporary hardness to bicarbonates and boiling.
Consider the following statements: Hard water is not suitable for I. Drinking. II. Washing clothes with soap. III. Use in boilers. IV. Irrigating crops. Which of these statements are correct?
- (a) I and III
- (b) II and III
- (c) I, II and IV
- (d) I, II, III and IV
Answer(b) II and III
The same hardness-of-water theme. UPSC tested the consequences of hardness — poor lathering with soap and scale in boilers — while this NDA item tests its cause, the bicarbonates responsible for temporary hardness; both reward knowing what makes water hard and how it behaves.
- practice — not a real PYQ
Temporary hardness of water can be removed by
- (a)adding common salt
- (b)boiling the water
- (c)adding sugar
- (d)filtering through a cloth
Answer(b) boiling the water — it decomposes the bicarbonates into insoluble carbonates.
- practice — not a real PYQ
Permanent hardness of water is caused by
- (a)bicarbonates of calcium and magnesium
- (b)sulphates and chlorides of calcium and magnesium
- (c)dissolved oxygen
- (d)dissolved carbon dioxide
Answer(b) sulphates and chlorides of calcium and magnesium — these are not removed by boiling.