Hard water is water with high concentrations of dissolved ions, in particular calcium and

Step 1: Analyze the question The question asks which ion, along with calcium, is mainly responsible for water hardness.

Step 2: The concept Hard water contains dissolved salts of calcium (Ca²⁺) and magnesium (Mg²⁺). These usually come from water flowing over limestone, chalk, or dolomite rocks, which dissolve as bicarbonates, sulphates, or chlorides.

• Temporary hardness: caused by calcium and magnesium hydrogencarbonates, Ca(HCO₃)₂ and Mg(HCO₃)₂. It can be removed by boiling.
• Permanent hardness: caused by calcium and magnesium sulphates and chlorides, CaSO₄, MgSO₄, CaCl₂ and MgCl₂. It cannot be removed by boiling, but it can be removed by adding washing soda (Na₂CO₃) or by ion exchange.

Hard water does not form lather easily with soap. Instead it forms scum, because the soap reacts with Ca²⁺ and Mg²⁺ ions:

Ca²⁺ + 2C₁₇H₃₅COO⁻ → Ca(C₁₇H₃₅COO)₂ (scum)

Step 3: Evaluate the options

• A. Magnesium ions: correct. Mg²⁺ is the second major cause of hardness.
• B. Nitrogen ions: not a cause of hardness.
• C. Phosphorus ions: not a cause of hardness.
• D. Helium ions: helium is an inert noble gas and does not form ions in water.

Step 4: Answer The correct option is A. magnesium ions.

Exam tip: Remember "Ca and Mg make water hard." Whenever you see a hardness question, think of calcium and magnesium salts, and remember that boiling removes only the temporary kind (the hydrogencarbonates).