An example of a crystalline substance that does not possess water of crystallization is

Analyze the question: We need a crystalline salt that is anhydrous in its normal crystalline form, meaning it has no water of crystallization.

Concept: Water of crystallization is the fixed number of water molecules that some salts hold within their crystal structure. Examples are:

• Na₂SO₄·10H₂O (Glauber's salt)
• FeSO₄·7H₂O (green vitriol)
• Na₂CO₃·10H₂O (washing soda)

Salts like KNO₃, NaCl, and KCl crystallize without water.

Going through the options:

• A. Potassium trioxonitrate (V), KNO₃: It crystallizes in anhydrous form. No water of crystallization. ✔
• B. Sodium tetraoxosulphate (VI), Na₂SO₄: The common crystalline form is the decahydrate, Na₂SO₄·10H₂O. It does have water of crystallization. ✘ (The anhydrous form exists, but it is not the typical crystalline form, and it is not what the question is after.)
• C. Iron (II) tetraoxosulphate (VI), FeSO₄: Crystallizes as FeSO₄·7H₂O. ✘
• D. Sodium trioxocarbonate (IV), Na₂CO₃: Crystallizes as Na₂CO₃·10H₂O (washing soda). ✘

Answer: The salt that has no water of crystallization is potassium trioxonitrate (V), KNO₃, which is option A.

Exam tip: Remember that nitrates and chlorides of Na and K (KNO₃, NaNO₃, NaCl, KCl) are usually anhydrous. The hydrated ones that WAEC commonly tests are:

• Na₂CO₃·10H₂O
• FeSO₄·7H₂O
• CuSO₄·5H₂O
• MgSO₄·7H₂O
• Na₂SO₄·10H₂O

Keep this list handy, and any salt not on it is likely your anhydrous one.