Hardness of water is due to which of the following?

Concept

Water hardness is a water-quality property caused by dissolved divalent (doubly charged) metallic cations — by standard definition, calcium (Ca2+) and magnesium (Mg2+). Their bicarbonates cause temporary hardness: boiling decomposes the bicarbonate into an insoluble carbonate precipitate, removing this hardness. Their chlorides and sulphates cause permanent hardness, which persists on boiling and needs chemical or ion-exchange softening instead. Both forms react with soap to form an insoluble scum rather than lather.

Application

• Chlorides and sulphates of Na+ and K+: sodium and potassium are monovalent (+1) alkali-metal ions, so their salts remain fully dissolved and do not precipitate with soap.

• Chlorides and sulphates of Ca2+ and Mg2+: this is exactly the divalent ion-anion combination described above as the standard chemical cause of permanent hardness.

• Nitrites and nitrates of Al3+: aluminium is trivalent, but water hardness is defined by standard convention specifically in terms of calcium and magnesium salts, not aluminium compounds. Aluminium is instead used in water treatment as a coagulant, but as different salts — such as aluminium sulphate or aluminium chloride — not as the nitrite/nitrate pairing in this option.

• Carbonates of Na+ and K+: sodium/potassium carbonates are alkaline but, being salts of monovalent metals, do not contribute hardness — sodium carbonate (washing soda) is in fact used to soften hard water.

Cross-check

Calcium and magnesium bicarbonates likewise cause temporary hardness, which is removable by boiling — confirming that it is primarily the Ca2+/Mg2+ cations that make water 'hard'. The accompanying anion mainly governs the salt's solubility and whether the hardness is temporary (bicarbonate) or permanent (chloride/sulphate), rather than determining hardness itself. The fact that sodium carbonate is itself a commercial water softener further confirms that Na+/K+ salts are not hardness-causing.

Result

Therefore, the correct answer is chlorides and sulphates of Ca2+ and Mg2+.