Question

Gold number of protective colloids $A$, $B$, $C$ and $D$ are $0.50$, $0.01$, $0.10$ and $0.005$ respectively. The correct order of their protective power is
A: $A < C < B < D$
B: $B < D < A < C$
C: $D < A < C < B$
D: $C < B < D < A$

Answer 1

Hint: Gold number is the amount of protective colloid which is just sufficient to prevent the coagulation of $10mL$ of a standard hydro gold sol when $1mL$ of $10\% $ $NaCl$ solution is added to it. Coagulation of gold sol is indicated by color change from red to blue/purple when particle size just increases.

Complete step by step answer:
Coagulation is the colloidal phenomenon that changes the size distribution of a large number of small particles to a small number of large particles. In this process some chemical substances are added that cause small particles to clump together in water.
Protective colloid is a lyophilic colloid (liquid loving) that when present in small quantities keeps lyophobic (liquid hating) colloids from precipitating under the coagulating action of electrolytes. Gold number is the amount of protective colloid which is just sufficient to prevent the coagulation of $10mL$ of a standard hydro gold sol when $1mL$ of $10\% $ $NaCl$ solution is added to it.
This means strong the protective power less will be the gold number and less will be the protective power more will be the gold number. Among the given colloids $D$ has the lowest gold number. This means the protective power of $D$ is highest and the gold number of $A$ is highest this means protective power of $A$ is lowest. In the option A protective power of $A$ is lowest and $D$ is highest.

So, correct answer is option A.

Note: Hydrophobic sols can coagulate easily. These sols are stabilized due to charge on the particle. If the charge on the particle is removed by adding any electrolyte the particles of these sols tend to come closer. Macromolecular colloids can’t be coagulated easily.