Precipitation of Insoluble Magnesium Carbonate

Steps:

• Sodium carbonate (Na₂CO₃) dissociates to provide CO₃²⁻ ions in solution.
• Tap water contains dissolved ions, including potential hardness contributors like Mg²⁺.
• CO₃²⁻ reacts with Mg²⁺ to form MgCO₃, an insoluble white precipitate.
• Other common tap water ions do not form insoluble carbonates under these conditions.

Why B is correct:

• Magnesium ions (Mg²⁺) form magnesium carbonate (MgCO₃), which is insoluble in water per solubility rules for Group 2 carbonates.

Why the others are wrong:

• A. Chloride ions (Cl⁻) do not react with CO₃²⁻ to form any precipitate; sodium chloride is highly soluble.
• C. Potassium ions (K⁺) form soluble potassium carbonate (K₂CO₃) with CO₃²⁻.
• D. Sulfate ions (SO₄²⁻) do not form an insoluble compound with CO₃²⁻; sodium sulfate is soluble.

Final answer: B