Hint: Colloids are the particles whose size ranges from 1 to 1000 nanometre. Richard Adolf first used the concept of gold no. Coagulation is the phenomenon of the particles that come together to form a bigger molecule.
> The gold number is defined as the minimum mass of the colloid in milligram that is added to the 10ml of red gold sol to protect it from the coagulation when 1 ml of 10% NaCl is added.
> Coagulation occurs when the positively and negatively charged colloid particles come together and form a bigger molecule. Just like the blood cells come together to form a clot.
> The colloid which is added is a lyophilic colloid i.e. water-loving colloid.
> Moreover, lyophilic colloids are that colloid which has more affinity towards the dispersion medium and are stable in the water.
> The protective power of a colloid is determined by the reciprocal of a gold number. According to Zsigmondy, the smaller is the value of the gold number, more will be it's protective power to prevent the coagulation.
> The protective colloid is those colloids which are lyophilic and they help prevent the precipitation of lyophobic colloids. For example, egg albumin, casein, gelatin, etc are the protective colloids.
> The gold sol in which the protective colloid is added is negatively charged sol.
Note: Gelatin is considered as the best protective colloid because the value of gold number for it is the smallest i.e. 0.001. If the gold sol is coagulated then it can be determined by the colour that is from red to blue and the size of the particle also increases.