The chemicals used to soften hard water involve the addition of

Correct answer: soluble sodium compounds forming insoluble precipitates of calcium and magnesium ions.

Explanation: Hardness is caused by Ca²⁺ and Mg²⁺ in water. Softening chemicals are typically soluble sodium salts (e.g., washing soda Na₂CO₃, trisodium phosphate Na₃PO₄) that react with these ions to form insoluble solids such as CaCO₃ and Mg(OH)₂/Mg₃(PO₄)₂, which can then be removed.

• Insoluble sodium compounds forming soluble Ca/Mg – Incorrect; insoluble sodium salts won’t react effectively, and making Ca/Mg more soluble doesn’t soften water.
• Soluble sodium compounds forming soluble Ca/Mg – Incorrect; leaves hardness ions in solution.
• Insoluble precipitates of Ca/Mg ions – This describes the products, not what is added. What’s added are the soluble sodium compounds that produce those precipitates.