The chemicals used to soften hard water involves the addition of

Analyze the question: It asks what chemical softening of hard water involves. The options are worded awkwardly, and "from" is a typo for "form".

Concept: Hard water contains dissolved Ca²⁺ and Mg²⁺ ions. These ions react with soap to form scum, so they have to be removed. In chemical softening, we add a soluble sodium compound, such as washing soda (Na₂CO₃) or sodium phosphate. The sodium compound dissolves in the water. Its anions then react with Ca²⁺ and Mg²⁺ to form insoluble precipitates, which are removed.

Step-by-step solution:

1. Add washing soda, which is soluble in water: Na₂CO₃(aq) → 2Na⁺(aq) + CO₃²⁻(aq)

2. The carbonate ions react with the hardness ions: Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s) Mg²⁺(aq) + CO₃²⁻(aq) → MgCO₃(s)

3. The insoluble CaCO₃ and MgCO₃ precipitate out. Only the sodium ions, which do not form scum with soap, stay in solution.

Checking the options:

• A: Wrong. The sodium compound must be soluble, and the Ca²⁺ and Mg²⁺ compounds formed must be insoluble, not soluble.
• B: Wrong. If Ca²⁺ and Mg²⁺ stayed in solution, the water would still be hard.
• C: Correct. A soluble sodium compound forms insoluble precipitates with calcium and magnesium ions.
• D: Incomplete. It mentions only the precipitates and not the added soluble sodium compound.

Tip: Remember "soluble in, insoluble out". You add a soluble sodium salt, and the hardness ions leave as an insoluble precipitate. The same idea appears in the Calgon (sodium hexametaphosphate) process, although Calgon works by forming soluble complex ions. For JAMB, focus on washing soda.