The purging of impurities from a compound by crystallization:

occurs when the compound is put under great pressure

occurs when the compound is vaporized

occurs after the liquid melt cools back to a solid

occurs after condensation from the vapor phase

C.

This is also the only change by which crystals may be produced.

Answer;

occurs after the liquid melt cools back to a solid

The purging of impurities from a compound by crystallization occurs after the liquid melt cools back to a solid.

Explanation;Purging of impurities from a compound by crystallization is done through a method known as melt crystallization. Melt crystallization is a type of cooling crystallization process which is operated close to the melting point of the pure compound.The sample for melt crystallization id normally an impure compound. When the impure sample is cooled below the equilibrium temperature the result is formation of a solid phase which is more pure that the initial compound, while the impurities remain in the impure sample.

The mass has escaped into a gas which you can not see so the law stays true.