Question

Large crystals of pure substances can be formed from their solutions during _______.
A.Crystallization
B.Evaporation
C.Expansion
D.Contraction

Answer 1

All the methods given in the question are used for the separation of pure substances from a mixture. To answer this question, you must recall the concepts of each process.

Complete answer:
The process used to separate large crystals of pre substances from their solutions is Crystallization.
The process by which the molecules of a pure substance arrange themselves in a definite three-dimensional lattice forming crystals and consequently, minimizing the total energy of the system is termed as crystallization. When a pure substance is subjected to crystallization, its atoms or molecules bind together at well-defined angles forming a crystal lattice structure.
Crystallization is primarily used as a separation technique for obtaining pure crystals of a substance from a mixture. Crystallization is also used to obtain pure common salt (sodium chloride) from seawater. It can also be used for obtaining pure alum crystals from impure alum. In such scenarios where we need to separate crystals of the substance from a solution, crystallization is more effective than evaporation since it also removes the soluble impurities present in the solution from the required crystals.

Thus, the correct answer is A.
Note:
The steps involved in the process of crystallization are:
First, the solution is heated in an open container and the solvent particles start to evaporate leaving behind the solutes. As the solution begins to cool, crystals of the substance begin to accumulate on the surface of the solution. The crystals thus formed are collected and dried. The size of the crystals formed depends on the rate of cooling of the solution.