Question

A negatively charged suspension of clay in water will need for precipitation of the minimum amount of:
A. Aluminum chloride
B. Potassium sulfate
C. Sodium hydroxide
D. Hydrochloric acid

Answer 1

A negatively charged suspension of clay would need a positive counterpart for precipitation. And precipitation is the chemical process where the colloidal particles are settled down in the solution.

Complete answer:
There are many ways by which one can precipitate the suspended colloidal particle present in the solution. One of them is by adding electrolyte. When the electrolyte of the opposite charge is added to a solution it neutralizes the solution and results in their coagulation.

Therefore, from all the given options only the positive ions will be used to neutralize the solution and form precipitation. Hence, only ${\text{A}}{{\text{l}}^{{\text{3 + }}}}{\text{, }}{{\text{K}}^{\text{ + }}}{\text{, N}}{{\text{a}}^{\text{ + }}}{\text{, }}{{\text{H}}^{\text{ + }}}$ ions will take part in precipitation.
Now, according to the Hardy-Schulze rule, the greater the valence of flocculating ions added, the greater is its power to cause precipitation. Also, we have studied that the greater the coagulating power, the lesser is the quantity needed. Which means, valence is directly proportional to coagulating power, and coagulating power is inversely proportional to coagulating value( amount of ions required)
$\Rightarrow \text{Valency} \,{\rm{\propto }}\, \text{Coagulating power} \,\propto$ $\dfrac{{\text{1}}}{{{\text{coagulating value}}}}$

Now, in the coagulation of the negative clay, the amount ${\text{A}}{{\text{l}}^{{\text{3 + }}}}$ will be required the least, as it has the greatest valence of +3, which will result in greater coagulating power.

**Therefore, the correct answer is option (A) i.e., Aluminium chloride

Additional information:**
The minimum concentration of an electrolyte in millimoles per litres required to cause precipitation of a sol in two hours is called coagulating value.

Note:
This electrolyte method could confuse the electrophoresis, where the colloidal particle moves towards the oppositely charged electrode, hence electricity is used in this method. While in electrolysis the oppositely charged ions attract each other and settle down in the form of a precipitate.