CDS (I) 2020 General Knowledge · Question 21 of 120

Correct — A, A B C D 4 1 2 3. Take the formulae one at a time. Bleaching powder is written CaOCl₂ and is made by passing chlorine over dry slaked lime, so A goes to 4. Baking soda is sodium hydrogencarbonate, NaHCO₃, so B goes to 1. Washing soda is Na₂CO₃·10H₂O — sodium carbonate with ten water molecules of crystallisation — so C goes to 2. Slaked lime is calcium hydroxide, Ca(OH)₂, the base that limewater is made from and that bleaching powder is made out of, so D goes to 3. That is 4, 1, 2, 3 in order.

• (b)A B C D 4 2 1 3 — Gets bleaching powder and slaked lime right but swaps baking soda with washing soda. Baking soda is the hydrogencarbonate; washing soda is the carbonate with ten waters attached.
• (c)A B C D 3 2 1 4 — Makes bleaching powder calcium hydroxide and slaked lime CaOCl₂, which reverses the pair, and also exchanges the two sodas.
• (d)A B C D 3 1 2 4 — Sorts the sodium compounds correctly but exchanges the two calcium compounds. Slaked lime is the starting material and bleaching powder the product of its reaction with chlorine.

Four products of the common-salt and lime industries, each with a formula worth knowing by heart. Bleaching powder, CaOCl₂, comes from chlorine acting on dry slaked lime and is used to bleach cotton, linen and wood pulp and to disinfect drinking water. Baking soda, NaHCO₃, is made from sodium chloride and releases carbon dioxide on heating, which is what raises a cake. Washing soda, Na₂CO₃·10H₂O, is recrystallised sodium carbonate and is a basic salt. Slaked lime, Ca(OH)₂, is calcium oxide taken up with water.

Every option is a permutation of the same four numbers, so partial knowledge is enough if it is knowledge of the right pair. The two calcium compounds split the options into halves — pick 4 for bleaching powder and you are down to (a) and (b); then the sodium pair decides between them. The mnemonic that settles the sodas is the hydrogen: baking soda is the bicarbonate, sodium hydrogen carbonate, and it is the one that carries an H in the formula. Notice too that entries 3 and 4 in List-II are chemically linked — chlorine acting on Ca(OH)₂ gives CaOCl₂ — so the two calcium formulae are a pair, not two unrelated facts.

• Bleaching powder is produced by the action of chlorine on dry slaked lime and is represented as CaOCl₂.
• Baking soda is sodium hydrogencarbonate, NaHCO₃; it is also an ingredient of antacids and of soda-acid fire extinguishers.
• Washing soda is Na₂CO₃·10H₂O, obtained by recrystallising sodium carbonate.
• Slaked lime is Ca(OH)₂, formed when calcium oxide reacts with water; its solution is limewater.
• Bleaching powder is used for bleaching cotton, linen and wood pulp, as an oxidising agent, and to make drinking water free from germs.

• Swapping baking soda and washing soda; the one with hydrogen in the formula is baking soda.
• Confusing slaked lime, Ca(OH)₂, with quicklime, CaO, and with limestone, CaCO₃.
• Forgetting the ten water molecules in washing soda, which are part of its formula and not an impurity.

Almost always as a match of common name to chemical formula or chemical name, sometimes with an odd-one-out statement set instead.

• practice — not a real PYQ

  Which one of the following is the chemical formula of washing soda?

  • (a)NaHCO₃
  • (b)Na₂CO₃·10H₂O
  • (c)NaOH
  • (d)Na₂SO₄·10H₂O

  Answer(b) Na₂CO₃·10H₂O — sodium carbonate carrying ten water molecules of crystallisation.

• practice — not a real PYQ

  Bleaching powder is obtained by the action of chlorine on

  • (a)quicklime
  • (b)limestone
  • (c)dry slaked lime
  • (d)washing soda

  Answer(c) dry slaked lime — chlorine passed over Ca(OH)₂ gives CaOCl₂ and water.