CDS (I) 2023 General Knowledge · Question 49 of 120

Correct — A, Solution of sodium hydrogen carbonate and sulfuric acid. The name of the device is the answer. The 'soda' is baking soda, sodium hydrogen carbonate (NaHCO₃), held in solution in the body of the cylinder; the 'acid' is concentrated sulfuric acid, sealed in a small glass vial suspended inside it. Knocking the plunger, or inverting the cylinder, breaks that vial. Acid meets bicarbonate and carbon dioxide comes off at once — 2NaHCO₃ + H₂SO₄ → Na₂SO₄ + 2H₂O + 2CO₂ — and the gas pressure drives the water out through the nozzle in a jet. Almon M. Granger's American patent of 1880 describes exactly this pairing of sulfuric acid with a sodium bicarbonate solution, and the earliest patent for the type belongs to François Carlier of France in 1866.

• (b)Solution of sodium carbonate and sulfuric acid — Sodium carbonate is washing soda, Na₂CO₃, not baking soda. Acid would liberate carbon dioxide from it too, so the chemistry is not absurd, but a washing-soda solution is strongly alkaline and the extinguisher was never charged with it. In everyday chemistry vocabulary 'soda' in this device means the hydrogen carbonate.
• (c)Solution of carbon dioxide and sulfuric acid — Carbon dioxide is what the extinguisher makes, not what you put in. A solution of carbon dioxide in water is carbonic acid, and mixing one acid with another releases nothing.
• (d)Solution of sodium chloride and sulfuric acid — Common salt has no carbonate in it, so no carbon dioxide can be produced. What this pair would give is sodium hydrogen sulfate and hydrogen chloride fumes — corrosive, and useless as a propellant.

A soda-acid extinguisher is a water extinguisher with its own built-in pump. The water sits in the cylinder with sodium hydrogen carbonate dissolved in it; a stoppered vial of concentrated sulfuric acid hangs inside, kept apart until the moment of use. Once the two meet, the carbonate is decomposed by the acid and the carbon dioxide released has nowhere to go but against the water surface, forcing the water up the discharge tube. The gas is a propellant here, not the extinguishing agent — the water does the cooling.

Two of the four options can be thrown out on sight, because neither sodium chloride nor carbon dioxide can be the source of fresh carbon dioxide. That leaves a straight choice between the carbonate and the hydrogen carbonate, and the tie-breaker is the word 'soda'. In Indian school chemistry 'baking soda' is sodium hydrogen carbonate and 'washing soda' is sodium carbonate decahydrate; UPSC has tested that exact pair of names more than once. One thing follows from the mechanism rather than from the chemistry: what this device delivers is a jet of water, so it belongs to burning wood, paper and cloth and not to burning oil or to a live electrical circuit.

• The charge is a sodium hydrogen carbonate solution plus a sealed vial of concentrated sulfuric acid: 2NaHCO₃ + H₂SO₄ → Na₂SO₄ + 2H₂O + 2CO₂.
• The carbon dioxide is the propellant; the water thrown out is what actually puts the fire out.
• François Carlier of France patented the type in 1866; Almon M. Granger patented a sulfuric-acid-and-bicarbonate version in the United States in 1880.
• Baking soda is NaHCO₃ and washing soda is Na₂CO₃·10H₂O — the distinction the second option is built on.
• Sodium hydrogen carbonate is also the active powder in the dry-chemical extinguishers used on burning liquids.

• Reading 'soda' as sodium carbonate. In this device it means baking soda, the hydrogen carbonate.
• Treating carbon dioxide as an ingredient. It is the product of the reaction inside the cylinder.
• Assuming the carbon dioxide smothers the fire. In this type it mainly pushes the water out; a carbon dioxide extinguisher is a different device.

Either as the composition of the charge, as here, or as a statement item on baking soda's uses alongside washing soda, quicklime and gypsum.

• practice — not a real PYQ

  Which gas is responsible for expelling the liquid from a soda-acid fire extinguisher?

  • (a)Oxygen
  • (b)Carbon dioxide
  • (c)Hydrogen
  • (d)Sulfur dioxide

  Answer(b) Carbon dioxide — sulfuric acid decomposes the sodium hydrogen carbonate, and the gas released builds pressure that drives the water out.

• practice — not a real PYQ

  Washing soda differs from baking soda in that washing soda is

  • (a)sodium hydrogen carbonate
  • (b)sodium hydroxide
  • (c)hydrated sodium carbonate
  • (d)sodium sulfate

  Answer(c) hydrated sodium carbonate — washing soda is Na₂CO₃·10H₂O, while baking soda is NaHCO₃.