CDS (I) 2021 General Knowledge · Question 114 of 120

Correct — B, Sodium hydrogen carbonate. A soda-acid extinguisher holds a solution of sodium hydrogen carbonate — baking soda, NaHCO₃ — with a sealed bottle of sulphuric acid suspended inside it. Inverting or striking the cylinder breaks the seal, the acid mixes with the solution, and the reaction releases carbon dioxide at once. The gas builds pressure and drives the liquid out through the nozzle in a jet, and the carbon dioxide itself smothers the fire by cutting off oxygen, being heavier than air. Baking soda earns its place because the carbonate group gives up carbon dioxide readily on contact with acid, and because it is cheap, stable and harmless — the same property makes it the raising agent in baking and an ingredient in antacids.

• (a)Sodium chloride — Common salt does not react with acid to give a gas. It is the raw material from which baking soda, washing soda and bleaching powder are made, but it plays no part in the extinguisher itself.
• (c)Calcium hydroxide — Slaked lime is a base and would neutralise the acid, but the reaction produces a salt and water and no gas. With no carbon dioxide there is no pressure to expel the contents and nothing to smother the flame.
• (d)Acetic acid — An acid, and the extinguisher already has one. The device needs a carbonate to react with the acid; putting a second acid in its place leaves nothing to produce the gas.

A fire needs fuel, heat and oxygen, and an extinguisher removes at least one of them. The soda-acid design removes oxygen by blanketing the fire with carbon dioxide, and uses the same gas as the propellant that pushes its liquid out. The chemistry is the ordinary reaction of an acid with a hydrogen carbonate: NaHCO₃ + H₂SO₄ gives a sodium salt, water and carbon dioxide.

Reason from what the device has to produce rather than from what it contains. Something must make a gas, and only a carbonate or hydrogen carbonate on this list will do that with an acid, so the answer is fixed before any recall of the apparatus. Two of the wrong options are worth separating carefully. Calcium hydroxide is chemically the opposite of what is wanted here even though it is a base — a neutralisation without gas is useless for this purpose. And baking soda must be kept distinct from washing soda, sodium carbonate, which appears in the next question of this same paper for a completely different job.

• A soda-acid extinguisher pairs a sodium hydrogen carbonate solution with a sealed container of sulphuric acid.
• The reaction is NaHCO₃ + H₂SO₄ → sodium sulphate, water and carbon dioxide.
• The carbon dioxide both expels the liquid and smothers the fire by keeping oxygen away.
• Baking soda is sodium hydrogen carbonate, NaHCO₃; washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O.
• Soda-acid extinguishers must not be used on electrical fires, because the liquid jet conducts.

• Confusing sodium hydrogen carbonate with sodium carbonate — the first is baking soda, the second washing soda.
• Choosing calcium hydroxide because it is a base; neutralising the acid without producing gas achieves nothing here.
• Assuming any extinguisher is safe on any fire; the soda-acid type is unsuitable for electrical fires.

Either as this ingredient question, or as a full composition item naming both the hydrogen carbonate and the acid.

• practice — not a real PYQ

  The gas that is released when baking soda reacts with an acid, and which smothers a fire, is

  • (a)oxygen
  • (b)hydrogen
  • (c)carbon dioxide
  • (d)nitrogen

  Answer(c) carbon dioxide — heavier than air, it settles over the burning material and cuts off the oxygen supply.

• practice — not a real PYQ

  Baking soda is chemically

  • (a)sodium carbonate
  • (b)sodium hydrogen carbonate
  • (c)sodium hydroxide
  • (d)calcium carbonate

  Answer(b) sodium hydrogen carbonate — NaHCO₃, to be kept distinct from washing soda, which is sodium carbonate decahydrate.