Question

Four different colloids have the following gold number, which one has the most effective action?
[A] 5
[B] 10
[C] 15
[D] 13

Answer 1

Hint: We know gold number as a measure of efficiency of protective colloids and it is different for every substance. As the gold number increases, the requirement of the amount of the colloid also increases. Therefore, particles with lower gold number are preferred.

Complete answer:
We come across the term gold number in colloidal chemistry. It is generally used to explain the strength of lyophilic substances.
As we know that lyophilic sol is more stable than lyophobic sol, addition of even a little amount of electrolyte causes the lyophobic sol to coagulate. To prevent this, we add a lyophilic sol to the lyophobic sol.
By addition of lyophilic sol, the lyophobic sol becomes less sensitive towards the addition of electrolyte.
Therefore, we call lyophilic sols as protective colloids. The protecting power of the lyophilic sol is defined by a term which we know as gold number.
The gold number is defined as the “The number of milligrams of the protective colloid which must be added to 10ccs of a given gold sol to prevent its precipitation by addition of 1cc of 10% NaCl solution” –according to Zsigmondy.
The gold number of every substance is different. Gold number is inversely proportional to the protecting power of a colloid. Therefore, we can write that, higher the gold number of a particle, lower will be its protecting power.
Here, among the four colloids, the lowest gold number is 5 i.e. it will have the most effective action.

Therefore, the correct answer is option [A] 5.

Note: The most commonly used protective colloid is gelatin. It is commonly used as a protective colloid because its gold number is very low $\sim$0.005-0.1 mg/cL. A gelatin solution stops the precipitation of many insoluble solutions.