What is protection? Explain with an example.

Hint: We know that the minimum mass of colloid which is added to prevent the coagulation of $10\,ml$ red gold is called a gold number. The gold number and the protecting power are inversely proportional which means smaller the gold number higher the protective power.

Complete step by step answer:First, we see what colloids are.
A phase separated mixture is called colloid in which one substance of microscopically soluble or insoluble particle is suspended throughout another substance. There are two types of colloids and they are (1) Lyophilic colloids, and (2) Lyophobic colloids.

Now, we see lyophilic colloids.
The colloidal solutions in which the dispersed particles have strong affinity towards water are called as lyophilic sol and they can be easily formed by direct heating or mixing. These sols are reversible in nature and stable, and cannot be coagulated easily. The force of attraction between dispersed phase and dispersed medium is high due to the formation of hydrogen bonds.
Let us discuss lyophobic colloids.

The colloidal solutions in which the dispersed particles have low affinity towards dispersed medium are called as lyophobic sol and they cannot be formed by direct heating or mixing. They are prepared by special methods. These sols are irreversible in nature and unstable, easily coagulated.
Lyophobic colloid is protected from coagulation by using lyophilic protective colloid. Gold number says about the protective power of the colloid.
Example: Adding Gelatin to ice-creams protects the ice particles from coagulation.


Note:
We must know that the lyophilic sols coagulated easily by the addition of a large amount of electrolyte whereas lyophobic sols are easily coagulated by the addition of a small amount of electrolyte.