Question

When caustic soda is added to an aqueous solution of copper sulphate, a blue precipitate of $Cu{{(OH)}_{2}}$ is obtained. How will you separate $Cu{{(OH)}_{2}}$ from the mixture?
A. Distillation
B. Filtration
C. Evaporation
D. Crystallization

Answer 1

Hint: Think about all the methods of separation listed in the options and under what conditions they are used. Consider the solubility of copper hydroxide in water and also its boiling point before narrowing down the answer.

Complete step by step solution:
We know that copper sulphate is a bright blue, crystalline solid which is soluble in water. When sodium hydroxide, also known as caustic soda, reacts with this copper sulphate, a double displacement reaction occurs. One molecule of copper sulphate combines with two molecules of caustic soda, or sodium hydroxide, to give one molecule of copper hydroxide and one molecule of sodium sulphate. The reaction is as follows:
\[CuS{{O}_{4}}+2NaOH\to Cu{{(OH)}_{2}}+N{{a}_{2}}S{{O}_{4}}\]
Here, the product formed, i.e. copper hydroxide, is pale greenish or bluish and is sparingly soluble in water. Thus, it forms a precipitate and settles at the bottom.
Now, let us look at the given processes, in short, to see which one we can apply here.
- Distillation
This method is used when the product we have to separate is liquid and is completely miscible in the solvent. It also has to have a significant difference in boiling points.
- Filtration
This method is used when the product that we have to separate is a solid which is insoluble or sparingly soluble in the solvent.
- Evaporation
This method is used when the product we have to filter out is solid but is completely soluble in the solvent.
- Crystallization
This method is used for the separation of solid impurities from the solid product that we have to obtain.
From the descriptions of these processes, we can easily deduce that the required process to separate the sparingly soluble copper hydroxide from water is filtration.

Hence, the correct answer is B ‘Filtration’

Note: Note that after filtration, we can make the obtained copper hydroxide to undergo recrystallization to filter out any impurities that might be present due to the reaction that occurs. We can determine whether the product is pure or not using thin-layer chromatography.