Question

When a hot, saturated solution of two substances ‘ ’X' and ' Y ' at 80$°$C is cooled slowly, X first crystallizes and settles down at 30$°$C, whereas Y remains in the solution. Identify the true statement which complies with the above experimental result.

• A. X is more soluble than Y at 80$°$C.
• B. X is more soluble than Y at 30$°$C.
• C. Solubility of Y at 30$°$C is more than that of X.
• D. Solubility of Y is more than that of X from 80$°$C to 30$°$C.

Answer 1

The correct option is C

Solubility of Y at 30$°$C is more than that of X.

The explanation for the correct option C):

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. 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.
3. As X starts crystallizing first and settles at 30$°$C, X is less soluble than Y at 30$°$C.
4. Hence, the solubility of Y at 30$°$C is more than that of X.

The explanation for the incorrect option:

A) At 80$°$C, both X and Y are equally soluble in water.

B) At 30$°$C, X crystallizes out. Thus, X is less soluble at 30 $°$C.

D) Solubility of X and Y are similar around 80 $°$C,

Conclusion: The correct option is C) Solubility of Y at 30$°$C is more than that of X.