Solubility rules determine water solubility of ionic compounds

Steps:

• Recall general solubility rules: most nitrates and sulfates are soluble; chlorides are soluble except for Ag+, Pb2+, Hg2+; carbonates are insoluble except for group 1 and NH4+ salts.
• Evaluate A (CaCl2): Ca2+ chloride is soluble as Ca2+ is not an exception.
• Evaluate B (Pb(NO3)2): All nitrates are soluble.
• Evaluate C (MgCO3): Carbonates of group 2 metals like Mg2+ are insoluble.
• Evaluate D (K2SO4): Group 1 sulfates are soluble.

Why C is correct:

• Magnesium carbonate (MgCO3) is insoluble per the rule that alkaline earth metal carbonates have very low solubility in water (Ksp ≈ 3.5 × 10^-8).

Why the others are wrong:

• A: Calcium chloride dissolves readily as chlorides of group 2 metals are soluble.
• B: Lead nitrate is fully soluble since all nitrates dissolve in water.
• D: Potassium sulfate is highly soluble as group 1 sulfates dissolve easily.

Final answer: C