The two ions responsible for hardness in water are

Step 1: Analyze the question The question asks which two ions cause hardness in water.

Step 2: Concept Hard water is water that does not easily form lather with soap. Hardness is caused by dissolved salts of calcium and magnesium, which are present as Ca²⁺ and Mg²⁺ ions.

• Temporary hardness is caused by calcium or magnesium hydrogencarbonates, Ca(HCO₃)₂ and Mg(HCO₃)₂. It can be removed by boiling.
• Permanent hardness is caused by calcium or magnesium sulphates and chlorides, CaSO₄, MgSO₄, CaCl₂ and MgCl₂. It needs a chemical method such as adding washing soda (Na₂CO₃) or ion exchange.

Why soap fails to lather: Soap is sodium stearate, C₁₇H₃₅COONa. Ca²⁺ and Mg²⁺ react with it to form an insoluble precipitate called scum:

Ca²⁺(aq) + 2C₁₇H₃₅COO⁻(aq) → Ca(C₁₇H₃₅COO)₂(s)

Soap is wasted until all the Ca²⁺ and Mg²⁺ have been precipitated, and only then does lather form.

Step 3: Eliminate the options

• B (K⁺ and Mg²⁺): K⁺ does not cause hardness. Its soaps are soluble.
• C (Ca²⁺ and Li⁺): Li⁺ does not cause hardness.
• D (Ca²⁺ and Na⁺): Na⁺ soaps are the soluble soaps themselves, so Na⁺ does not cause hardness.
• A (Ca²⁺ and/or Mg²⁺) is correct.

Step 4: Answer

Exam tip: Remember that hardness comes from "Ca and Mg", the two metals that form insoluble soaps. Group 1 ions (Na⁺, K⁺, Li⁺) never cause hardness because their soaps are soluble.