Question

A student is given a mixture of barium sulphate, copper (II) sulphate and water. The table shows information about barium sulphate and copper (II) sulphate.

Substance	Solubility in water	State at room temperature
Barium sulphate	insoluble	Solid
Copper (II) sulphate	soluble	solid

How does the student obtain copper (II) sulphate crystals from the mixture?
[A] Crystallisation followed by distillation.
[B] Crystallisation followed by filtration.
[C] Distillation followed by crystallisation.
[D] Filtration followed by crystallisation.

Answer 1

Hint: In the mixture formed by water, barium sulphate and copper (II) sulphate, we can find barium sulphate as a precipitate as it is insoluble in water. After separating the two we can try to obtain the required crystals.

Complete step by step solution: In the question, we are given a mixture of copper (II) sulphate and barium sulphate in water.
As we can see from the table that barium sulphate is insoluble in water but copper (II) sulphate is soluble in water. Therefore, in the mixture, we can see barium sulphate in the solution but not copper (II) sulphate and we have to obtain the crystals of copper (II) sulphate.
Before answering the question, let us understand the terms mentioned in the option.
Firstly, we have crystallization. It is a process by which we obtain a highly organised structure of the solid which is known as a crystal. Crystals are formed as a precipitate in the solution. Every compound gives us different types of crystals. Some are coloured while others are not. Some form needle-like crystals and some are powder-like. We can obtain crystals by precipitating from a solution or freezing.
Next, we have distillation. It is a process of separating certain components or substances from a solution thus purifying it by using selective boiling and condensation. The mixture of liquids having different boiling points can be separated through distillation. It is also used to separate liquids from non-volatile solids.
And lastly, we have filtration. It is a process by which we can separate a mixture of solids and liquids bypassing the solution from a porous medium, generally a filter paper.
Here, we have a mixture of barium sulphate and copper (II) sulphate. As we can understand from the above discussion that the student can separate a mixture of solid and liquid through filtration, therefore to obtain the required crystals he would have to separate barium sulphate from the solution through filtration. Then he can obtain the crystals of copper (II) sulphate by crystallization.

Therefore, the correct answer is option [D] Filtration followed by crystallisation.

Note: The copper sulphate crystals are bright blue. The crystals can be formed by mixing excess copper sulphate to very hot water until no more copper sulphate dissolves. The bright blue crystals appear in a few hours. The blue coloured crystals are generally formed by copper sulphate pentahydrate. It is the common form in which it generally exists but there are other variants too like dihydrate or tetrahydrate.