Softening of hard water is done by A. Aeration B. Boiling C. Lime soda D.…

Concept

Hardness of water is caused by dissolved calcium and magnesium salts, i.e. by the Ca2+ and Mg2+ ions carried in the water. Softening means taking those two ions out of solution — either by precipitating them as an insoluble solid or by exchanging them for an ion that does not cause hardness. A treatment that never acts on Ca2+ or Mg2+ cannot soften water, however valuable it is for some other water-quality goal such as removing gases, taste or microbes.

Temporary hardness comes from the bicarbonates Ca(HCO3)2 and Mg(HCO3)2, which decompose on heating. Permanent hardness comes from the sulphates and chlorides of the same two metals, which heat cannot touch and which therefore need a chemical precipitation route or an exchange route.

Applying the concept to the five listed processes

• A. Aeration — water is brought into contact with air in a cascade, spray or diffuser. This strips dissolved gases such as CO2 and H2S and oxidises dissolved iron and manganese into filterable solids. The dissolved Ca2+ and Mg2+ are untouched, so the hardness of the water is exactly what it was before.

• B. Boiling — heat decomposes the bicarbonates and drives the metals out as insoluble solids:

  Ca(HCO3)2 → CaCO3↓ + H2O + CO2↑

  Mg(HCO3)2 → Mg(OH)2↓ + 2CO2↑

  The precipitated CaCO3 and Mg(OH)2 are filtered or settled off, so the temporary hardness is removed. Its limitation is that the sulphates and chlorides behind permanent hardness are unaffected.

• C. Lime soda — slaked lime Ca(OH)2 and washing soda Na2CO3 are dosed so that both metals are precipitated:

  Mg2+ + Ca(OH)2 → Mg(OH)2↓ + Ca2+

  Ca2+ + Na2CO3 → CaCO3↓ + 2Na+

  Because the reagents act on the ions themselves and not on the bicarbonate linkage, both temporary and permanent hardness are removed. This is the standard municipal and boiler-feed softening process.

• D. Ozonation — ozone (O3) is dosed as a powerful oxidant to disinfect the water and to break down organic matter, colour, taste and odour compounds. Its chemistry is aimed at microorganisms and organics; dissolved Ca2+ and Mg2+ salts are not oxidisable in a way that removes them, so the water leaves an ozone contactor just as hard as it entered.

• E. Ion exchange — the water is passed through a bed of zeolite (permutit) or a synthetic cation-exchange resin, written here as NaR:

  Ca2+ + 2NaR → CaR2 + 2Na+

  The hardness-causing ions are held on the bed and sodium ions take their place, so both temporary and permanent hardness are removed; the bed is later regenerated with brine.

Cross-check: what each process actually targets

Three of the five listed processes act on the hardness-causing ions: boiling, lime–soda treatment and ion exchange. Aeration and ozonation improve the water in other ways but leave its Ca2+ and Mg2+ content unchanged. The correct combination is therefore B, C and E Only.