Which of the following colloidal system represents a foam?

Gas in liquid

The correct answer is option 1. Gas in liquid.

A foam is a type of colloidal system characterized by the dispersion of gas bubbles in a liquid medium. This colloidal system is commonly observed in various everyday situations, such as the foam on the surface of a carbonated beverage or the lather produced when washing with soap.

In a foam, the gas bubbles are dispersed throughout the liquid phase. The liquid acts as the continuous phase, surrounding and stabilizing the gas bubbles. The stability of the foam is often attributed to the presence of surface-active agents, such as surfactants, which lower the surface tension of the liquid and help to maintain the integrity of the gas bubbles.

Foams can exhibit a range of properties, including viscosity, stability, and texture, depending on factors such as the composition of the liquid phase, the size and distribution of the gas bubbles, and the presence of any additives or surfactants.

In summary, a foam is a colloidal system where gas is dispersed in a liquid medium, making "Gas in liquid" the appropriate option.