In which of these groups will all the substances be insoluble in water?

Analyze the question: We need the group where every substance is insoluble (or sparingly soluble) in water. This uses the solubility rules for salts.

Key solubility rules:

• All sodium, potassium and ammonium salts are soluble.
• All nitrates are soluble.
• Chlorides are soluble, except AgCl and PbCl2 (PbCl2 dissolves only in hot water).
• Sulphates are soluble, except BaSO4, PbSO4 and CaSO4 (CaSO4 is sparingly soluble).
• Carbonates are insoluble, except those of Na, K and NH4.

Check each option:

• A. Na2CO3, KNO3, (NH4)2CO3: All are soluble, because they are sodium, potassium or ammonium salts and KNO3 is a nitrate. ✗
• B. NaHCO3, PbSO4, CaSO4: NaHCO3 is soluble. ✗
• C. K2CO3, CaSO4, AgNO3: K2CO3 and AgNO3 are soluble. ✗
• D. PbCl2, NaCl, ZnCl2: NaCl and ZnCl2 are soluble. ✗
• E. ZnCO3, BaSO4, AgCl: • ZnCO3 is a carbonate of a transition metal, so it is insoluble. • BaSO4 is an insoluble sulphate. • AgCl is an insoluble chloride.

All three are insoluble. ✓

Answer: E

Tip: Remember "SNAP" for what is always soluble: Sodium, Nitrates, Ammonium, Potassium salts. Then memorise the few exceptions: AgCl, PbCl2, BaSO4, PbSO4 and CaSO4. If an option contains a Na, K or NH4 salt or a nitrate, you can cross it out immediately when the question asks for "all insoluble".