Question

Why is alum added to water containing suspended impurities?
A. To make a colloidal solution
B. To coagulate the suspended impurities
C. To remove the impurities of calcium and magnesium
D. To protect the colloidal solution from getting precipitated

Answer 1

Hint:Colloidal solutions are a heterogeneous mixture of dispersed particles whose size is more than the particles present in the true solution. Though the size of the particles is large but still cannot be seen with naked eyes. The phenomenon which involves the precipitation of a colloidal solution on adding an electrolyte is called coagulation or flocculation.

Step-by-step explanation:
Step 1
All the colloidal particles have similar charges, so they repel each other and are unable to combine to form larger particles. However, if the charge of the colloidal particles is destroyed, they are free to come closer and grow in size. When they grow sufficiently large, they get precipitated. This type of phenomenon where the precipitation of the colloidal particles takes place is known as coagulation or flocculation. The coagulation of a colloidal solution is achieved by adding an electrolyte. When the electrolyte is added in a small amount it helps in the stabilization of the colloidal particles but when a substantial amount of the electrolyte is added the oppositely charged electrolyte neutralizes the charge on the colloidal particles and coagulation takes place.
Step 2
The greater the valency of the oppositely charged ion of the electrolyte added to a colloidal solution, the faster is the coagulation of the colloidal solution.
Step 3
In the water treatment plants, water is purified by the addition of some electrolytes like potash alum, aluminum sulfate, etc by the process of coagulation. The impure water where we find suspended particles contains dispersed colloidal particles which cannot be removed by the normal method of filtration. As we add potash alum to the impure water, the colloidal particles which carry a negative charge of impurities get coagulated by the action of \[A{l^{3 + }}\] furnished by the alum. The coagulated substance can be removed by filtration or decantation.
Hence, alum is added to water containing suspended impurities to coagulate the suspended particles.
So, option b) is the correct answer to the given question.

Note: There is a various application of colloids like in foodstuffs and medicines, sewage disposal, smoke precipitator, causing artificial rain, etc.