CDS (I) 2021 General Knowledge · Question 115 of 120

Correct — A, Na2CO3. Hard water is water carrying dissolved calcium and magnesium ions, picked up as rain percolates through limestone and gypsum. Sodium carbonate, sold as washing soda in its decahydrate form Na₂CO₃·10H₂O, softens it by supplying carbonate ions that take those metal ions out of solution as insoluble calcium carbonate and magnesium carbonate, which settle out and leave harmless sodium salts behind. Because the precipitation does not depend on driving off any dissolved gas, it works on permanent hardness — the kind caused by sulphates and chlorides, which simple boiling cannot touch. That is the practical point of the compound: temporary hardness can be dealt with by boiling, and washing soda is what is needed for the rest.

• (b)NaHCO3 — Baking soda. It is the raising agent of the kitchen and the active substance in a soda-acid extinguisher, and it does not precipitate the calcium and magnesium of hard water. Adding it to hard water softens nothing.
• (c)NaOH — Caustic soda, a strong alkali used for making soap and paper. It is far too corrosive to be added to household water, and softening is not its role.
• (d)Na2SO4 — Sodium sulphate, known as Glauber's salt. The sulphate ion forms no insoluble compound with calcium or magnesium worth speaking of, so nothing is removed from the water.

Water is called hard when it holds dissolved calcium and magnesium salts, which react with soap to form an insoluble scum instead of lather and leave scale in kettles and boilers. Temporary hardness comes from the hydrogen carbonates of those metals and breaks down on boiling; permanent hardness comes from their sulphates and chlorides and survives it. Softening means getting the calcium and magnesium out of solution, whether by precipitating them, by exchanging them for sodium on a resin, or by locking them up in a soluble complex.

The four options differ by one ion each, and only the carbonate makes an insoluble compound with calcium and magnesium, so the answer follows from solubility rather than from memory of a product name. The pairing to keep straight is the one this paper sets up itself: sodium hydrogen carbonate is baking soda, used in the fire extinguisher of the previous question, and sodium carbonate is washing soda, used here. Both are white sodium salts, both are called 'soda' in the kitchen, and swapping them is the commonest slip in this chapter.

• Hardness is caused by dissolved calcium and magnesium ions.
• Sodium carbonate precipitates them as insoluble calcium and magnesium carbonates, leaving soluble sodium salts.
• Washing soda is the decahydrate Na₂CO₃·10H₂O and is the standard treatment for permanent hardness.
• Temporary hardness, caused by hydrogen carbonates, can be removed simply by boiling.
• Hard water wastes soap by forming scum and leaves scale inside boilers, which is why softening matters industrially.

• Choosing NaHCO₃ because 'soda' appears in both names — baking soda does not soften water.
• Assuming any alkali will do; caustic soda is alkaline but is not a softening agent for household water.
• Forgetting that boiling handles only temporary hardness.

As a compound-to-use item, as a formula-to-common-name match, or as a statement item on which method removes which kind of hardness.

• practice — not a real PYQ

  Temporary hardness of water can be removed by

  • (a)adding sodium chloride
  • (b)boiling the water
  • (c)adding sodium sulphate
  • (d)filtering the water

  Answer(b) boiling the water — heating decomposes the dissolved hydrogen carbonates and the calcium and magnesium come out as insoluble carbonates.

• practice — not a real PYQ

  The chemical formula of washing soda is

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

  Answer(b) Na₂CO₃·10H₂O — sodium carbonate decahydrate; NaHCO₃ is baking soda and Na₂SO₄·10H₂O is Glauber's salt.