Which polyatomic ionic compound is a white, crystalline powder used in fire extinguishers and to neutralise acids and bases?

Question

Accepted Answer

Step 1: Identify distinctive uses and appearance

- The compound is described as a white, crystalline powder used in fire extinguishers and to neutralise acids/bases.

Step 2: Match each option to common properties

- (1) Sodium bisulphite (commonly $NaHSO_3$): a reducing agent/preservative, not typical in fire extinguishers.
- (2) Sodium thiosulphate ($Na_2S_2O_3$): used in photography (fixer) and as a dechlorinating agent; not used in extinguishers.
- (3) Sodium chromate ($Na_2CrO_4$): yellow crystalline salt and is toxic/oxidising, not used for neutralisation or extinguishers.
- (4) Sodium bicarbonate ($NaHCO_3$): a white crystalline powder (baking soda) widely used in dry chemical fire extinguishers and as an antacid/neutraliser. It reacts with acids, e.g. $NaHCO_3 + HCl \rightarrow NaCl + CO_2 + H_2O$, removing acidity.

Step 3: Conclusion

The description fits sodium bicarbonate best, so the correct choice is option (4).

FINAL_ANSWER: d

Answer

Answer

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