Pure, demineralized water, free from all soluble mineral salts is obtained by which of the following method?

Correct — C, passing water through a cation exchange and an anion exchange resin bed. Demineralised water is water with its dissolved ions taken out, so the method has to remove ions rather than particles. A cation exchange resin in its hydrogen form swaps every cation in the water — calcium, magnesium, sodium and the rest — for H⁺, and an anion exchange resin in its hydroxide form swaps every anion — chloride, sulphate, bicarbonate and the rest — for OH⁻. The H⁺ and OH⁻ released then neutralise each other to give water, so what leaves the second bed is free of soluble mineral salts. This is the standard route to deionised water for laboratories and boilers.

• (a)Passing water through microfiltration membrane — Microfiltration works at pore sizes near a tenth of a micrometre, which stops suspended solids, algae and most bacteria. Dissolved ions are thousands of times smaller and pass straight through; it is reverse osmosis, not microfiltration, that rejects them.
• (b)Calgon's method — Calgon is sodium hexametaphosphate, and it works by locking calcium and magnesium into a soluble complex so they cannot deposit as scale. The ions stay in the water, and sodium is added to it, so the water is conditioned rather than demineralised.
• (d)By boiling — Boiling decomposes bicarbonates and so removes temporary hardness alone. Sulphates and chlorides survive, and as water evaporates their concentration rises.

Ion exchange rests on a resin carrying fixed charged groups and mobile counter-ions. A cation resin holds acidic groups with H⁺ as the mobile ion; when hard water passes through, the more strongly held Ca²⁺ and Mg²⁺ displace H⁺ into solution. An anion resin holds basic groups with OH⁻ as the mobile ion and takes up chloride, sulphate and bicarbonate in the same way. Run in series the two beds strip both halves of every dissolved salt, and the exhausted resins are regenerated with dilute acid and dilute alkali respectively.

The examiner is separating three ideas that ordinary language runs together — filtration, softening and demineralisation. Filtration removes what is suspended. Softening removes or sequesters only the hardness ions, which is what washing soda, the zeolite process and Calgon do. Demineralisation removes every dissolved ion, and only ion exchange, distillation or reverse osmosis achieves it. Reading the stem closely settles the question: the words are free from all soluble mineral salts, and all is doing the work.

• Demineralised or deionised water contains no dissolved ionic salts; softened water may still be rich in sodium salts.
• A cation exchange resin in the hydrogen form releases H⁺ in exchange for the cations in the water; an anion resin in the hydroxide form releases OH⁻ in exchange for the anions.
• The H⁺ and OH⁻ released combine to give water, which is why the treated stream leaves neutral and salt-free.
• Exhausted beds are regenerated with dilute hydrochloric or sulphuric acid and with dilute sodium hydroxide.
• Temporary hardness is due to bicarbonates of calcium and magnesium and is destroyed by boiling; permanent hardness is due to their sulphates and chlorides and is not.

• Treating softening and demineralisation as the same thing.
• Assuming any membrane removes dissolved salts, when only the tightest ones do.
• Forgetting that boiling leaves permanent hardness untouched.

Asked as a which-method item where three of the four do something real to water, so the examiner is testing whether you can match the process to the exact words of the stem.

• practice — not a real PYQ

  Temporary hardness in water is caused mainly by which of the following?

  • (a)Sulphates of calcium and magnesium
  • (b)Bicarbonates of calcium and magnesium
  • (c)Chlorides of sodium and potassium
  • (d)Suspended silica

  Answer(b) Bicarbonates of calcium and magnesium — they decompose on boiling, which is why this form of hardness is called temporary.

• practice — not a real PYQ

  In a cation exchange resin used for water treatment, which ion is released into the water as the hardness ions are taken up?

  • (a)OH⁻
  • (b)H⁺
  • (c)Na⁺ only
  • (d)Cl⁻

  Answer(b) H⁺ — the resin in its hydrogen form gives up H⁺ in exchange for calcium and magnesium.