Which of the following statements is incorrect?

Clouds are not a colloidal system

The correct answer is option 4. Clouds are not a colloidal system.

Let us delve deeper into what colloidal systems are and why clouds, fog, and mist qualify as such.

A colloidal system (or colloid) is a mixture where one substance is dispersed uniformly throughout another. The dispersed substance is known as the dispersed phase, and the substance in which it is dispersed is known as the dispersion medium. The particle size in colloids typically ranges from 1 nanometer (nm) to 1 micrometer (µm). These particles are small enough that they do not settle out under gravity and large enough to scatter light, creating a phenomenon known as the Tyndall effect.

1. Clouds

Clouds are composed of tiny water droplets or ice crystals suspended in the air (which is the dispersion medium). They form when warm, moist air rises and cools, leading to condensation of water vapor into tiny droplets or ice crystals. The water droplets or ice crystals are in the size range typical for colloids (1 nm to 1 µm). They remain suspended in the air because of their small size and the constant movement of air particles. Saying "Clouds are not a colloidal system" is incorrect because clouds fit the definition of a colloidal system.

2. Fog

Fog is similar to clouds but occurs at ground level. It consists of tiny water droplets dispersed in the air. Fog forms when air near the ground cools to its dew point, causing water vapor to condense into tiny droplets. Like clouds, the droplets in fog are in the colloidal size range and remain suspended in the air. "Fog is a colloidal system" is correct because fog fits the definition of a colloid.

3. Mist

Mist is also composed of tiny water droplets dispersed in the air but generally has larger droplets than fog. Mist typically forms under conditions similar to fog, often in the early morning or after rain. The droplets in mist are still in the colloidal size range, allowing them to stay suspended in the air. "Mist is a colloidal system" is correct because mist fits the definition of a colloid.

Explanation of Colloids in Clouds, Fog, and Mist

Dispersed Phase: In clouds, fog, and mist, the dispersed phase is liquid water droplets (or ice crystals in the case of cold clouds).

Dispersion Medium: The dispersion medium in all three cases is air.

Particle Size: The size of the water droplets in clouds, fog, and mist is typically in the range of 1 nm to 1 µm, which is characteristic of colloidal particles.

Suspension: Due to their small size, these droplets do not settle under gravity but remain suspended due to Brownian motion (random movement caused by collisions with air molecules).

Tyndall Effect: These systems can scatter light, which is why clouds, fog, and mist often appear whitish or grayish when light passes through them.

From the analysis, it is clear that clouds, fog, and mist all qualify as colloidal systems because they consist of fine water droplets dispersed in air. The statement "Clouds are not a colloidal system" is therefore incorrect.