CDS (I) 2020 General Knowledge · Question 17 of 120

Correct — C, carbon dioxide. Limewater is a solution of calcium hydroxide, and carbon dioxide turns it milky by forming a white precipitate of calcium carbonate — the reaction NCERT writes as calcium hydroxide in solution plus carbon dioxide gas giving solid calcium carbonate and water. Exhaled air carries roughly four per cent carbon dioxide against the 0.04 per cent in ordinary air, which is more than enough to cloud the solution in seconds. That is why blowing through a straw into limewater is the standard classroom demonstration that respiration releases carbon dioxide. Keep bubbling gas through and the milkiness clears again as the insoluble carbonate turns into soluble calcium hydrogencarbonate.

• (a)water vapour — Exhaled air is certainly moist, but water added to limewater simply dilutes it. No precipitate forms, so nothing turns milky.
• (b)oxygen — Exhaled air still holds about 16 per cent oxygen, yet oxygen does not react with calcium hydroxide. The test is specific to carbon dioxide.
• (d)carbon monoxide — Carbon monoxide is not a normal product of human respiration, and it is a neutral oxide that does not react with limewater at all.

Limewater is a saturated aqueous solution of calcium hydroxide, a base. Carbon dioxide is an acidic oxide, so it reacts with the base to give a salt and water. The salt, calcium carbonate, is insoluble, and it is the suspended solid particles that make the clear solution look milky. Passing excess carbon dioxide converts the carbonate to calcium hydrogencarbonate, which is soluble, and the solution turns clear again.

The item is really the respiration experiment written out as a chemistry question. Two habits settle it. First, a milky appearance always means an insoluble solid has appeared, so ask which of the four gases can produce a precipitate with a base — only the acidic oxide can. Second, remember what exhaled air actually contains: less oxygen than inhaled air, a great deal more carbon dioxide, plenty of water vapour, and no carbon monoxide. Water vapour is the option built to catch a candidate who reasons from 'what is obviously in my breath' rather than from what reacts.

• Limewater is calcium hydroxide solution; carbon dioxide precipitates calcium carbonate in it, which is the milkiness.
• The reaction is calcium hydroxide solution plus carbon dioxide gas giving solid calcium carbonate and water.
• Excess carbon dioxide dissolves the precipitate again as soluble calcium hydrogencarbonate.
• Exhaled air holds about 4 per cent carbon dioxide against roughly 0.04 per cent in ordinary air.
• The same limewater test identifies the gas released when a carbonate or a hydrogencarbonate meets a dilute acid.

• Choosing water vapour because breath is visibly moist.
• Forgetting that exhaled air is still rich in oxygen, which makes oxygen look like a candidate.
• Missing the second half of the demonstration, where excess carbon dioxide clears the milkiness.

As the limewater test in either direction, or as part of a statement set on the reactions of slaked lime.

• practice — not a real PYQ

  Limewater turns milky when carbon dioxide is passed through it. The white precipitate responsible is

  • (a)calcium oxide
  • (b)calcium carbonate
  • (c)calcium hydrogencarbonate
  • (d)calcium chloride

  Answer(b) calcium carbonate — insoluble in water, so it appears as a suspension and clouds the solution.

• practice — not a real PYQ

  On passing an excess of carbon dioxide through milky limewater, the solution

  • (a)becomes more milky
  • (b)turns clear again
  • (c)turns blue
  • (d)remains unchanged

  Answer(b) turns clear again — the insoluble carbonate converts to soluble calcium hydrogencarbonate.