Which of the following compounds crystallizes without water of crystallization?

Analyze the question: Water of crystallization is the fixed number of water molecules that become part of a crystal when a salt crystallizes from aqueous solution. Examples are CuSO₄·5H₂O and Na₂CO₃·10H₂O. We need the compound that crystallizes without any.

Check each option:

• A. MgSO₄ crystallizes as Epsom salt, MgSO₄·7H₂O. It has water of crystallization.
• B. Na₂CO₃ crystallizes as washing soda, Na₂CO₃·10H₂O. It has water of crystallization.
• C. NaCl crystallizes from solution as anhydrous cubic crystals. Common salt is NaCl with no water of crystallization.
• D. FeSO₄ crystallizes as green vitriol, FeSO₄·7H₂O. It has water of crystallization.

(In the question, "NaCI" is just NaCl, sodium chloride, written with a capital I instead of a lowercase L.)

Why NaCl is different: Water of crystallization depends on how strongly the ions are hydrated. Ions such as Mg²⁺, Fe²⁺, and the carbonate-sodium combination are strongly hydrated, so water stays in the crystal lattice. Na⁺ and Cl⁻ are large, singly charged ions with low charge density, so their hydration is weak, and the water is not retained when the crystal forms.

Exam tip: Remember the common hydrated salts: CuSO₄·5H₂O (blue vitriol), FeSO₄·7H₂O (green vitriol), MgSO₄·7H₂O (Epsom salt), Na₂CO₃·10H₂O (washing soda), ZnSO₄·7H₂O. Salts that are always anhydrous include NaCl, KCl, KNO₃, and NH₄Cl. If a question asks for a salt without water of crystallization, pick the chloride or nitrate of Na or K.