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Difficulty: HardFiltration, Evaporation, and Crystallization

A student aims to recover pure hydrated zinc tetraoxosulfate(VI) crystals (ZnSO4⋅7H2O\text{ZnSO}_4 \cdot 7\text{H}_2\text{O}) from a mixture containing dissolved zinc tetraoxosulfate(VI) and insoluble fine sand. After filtering off the sand, why must the filtrate be concentrated by gentle heating to saturation and allowed to cool, rather than evaporated directly to complete dryness?

  1. Evaporating to complete dryness destroys the water of crystallization, producing an anhydrous powder or decomposed residue instead of hydrated crystals.Answer
  2. B
    Direct evaporation to dryness causes the dissolved salt to distill over along with the water vapor.
  3. C
    Heating the solution causes the salt to react chemically with water to form an insoluble metal oxide precipitate.
  4. D
    Crystallization is only applicable when the solute has a higher boiling point than the solvent, whereas evaporation requires lower boiling points.

Answer

Evaporating to complete dryness destroys the water of crystallization, producing an anhydrous powder or decomposed residue instead of hydrated crystals.
Hydrated salts contain water of crystallization chemically integrated into their crystal lattice structure. Heating a solution of such a salt to complete dryness drives off this essential water, leaving behind an anhydrous powder or causing thermal decomposition rather than yielding the intended hydrated crystals.

Step-by-Step Solution

1
Identify the structural requirements of the target product.
The target product is hydrated zinc tetraoxosulfate(VI), ZnSO4⋅7H2O\text{ZnSO}_4 \cdot 7\text{H}_2\text{O}, which requires chemically bound water of crystallization to maintain its crystalline lattice.
Retaining water of crystallization necessitates gentle thermal treatment rather than harsh heating.
2
Analyze the impact of evaporating to complete dryness.
Heating a hydrated salt solution to complete dryness removes all water molecules, converting the hydrated salt into an amorphous anhydrous powder (ZnSO4\text{ZnSO}_4) or decomposing it.
High thermal energy breaks the coordination bonds holding water molecules in the crystal structure.
3
Evaluate the correct procedural approach.
Evaporating the filtrate only to the point of saturation and allowing it to cool slowly enables pure hydrated crystals to form as solubility decreases.
Crystallization preserves the stoichiometric water of crystallization and excludes soluble impurities.

Key Concept

Distinction between evaporation to dryness and crystallization for hydrated salts
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Topics

Filtration, Evaporation, and Crystallization