Question

Which of the following mixtures can be separated by fractional crystallization?

• A. $P+NaN{O}_3$
• B. $NaN{O}_3+NaCl$
• C. $KN{O}_3+C$
• D. $NaCl+S$

Answer 1

The correct option is B

$NaN{O}_3+NaCl$

The explanation for the correct option

1. Fractional crystallization is a separation technique of components of a mixture on the basis of the differences in their solubilities in a common solvent.
2. If a solution of two or more substances is allowed to crystallize, a less soluble product will crystallize out.
3. In the procedure, a saturated solution is heated to solubilize all components completely and then cooled slowly. As the temperature decreases, the solvent can no longer hold all the components and hence the least soluble component starts to precipitate out in crystal forms followed by crystallization of other components based on their solubilities.
4. Both NaNO₃ (Sodium nitrate) and NaCl (Sodium chloride) are soluble in water but NaNO₃ is only soluble in hot solution. Thus, it crystallizes out on slow cooling.

Thus, the correct option is b).

The explanation for the incorrect option-

a) NaNO₃ (Sodium nitrate) and P (Phosphorus) react with one another when mixed together. Thus, they cannot be separated using fractional crystallization.

c) KNO₃ (Potassium nitrate) and C (Carbon) react with one another when mixed together. Thus, they cannot be separated using fractional crystallization.

d) NaCl (Sodium chloride) and S (Sulphur) are not soluble in any common solvent. NaCl dissolves in water while Sulphur dissolves in solvents like CS₂ (Carbon disulphide), CCl₄ (Carbon tetrachloride).

Conclusion: The correct option is b)