Change of State: Evaporation MCQs Quiz | Class 9

This Class IX Science (Code 086) quiz focuses on Unit I: Matter – Its Nature and Behaviour, specifically covering the process of evaporation, its nature as a surface phenomenon, and the factors affecting it. Test your understanding, review the detailed notes below, and download your result PDF.

Overview of Evaporation

Evaporation is the process by which a liquid changes into a gas (vapour) at any temperature below its boiling point. Unlike boiling, which is a bulk phenomenon occurring throughout the liquid, evaporation is a strictly surface phenomenon. Particles on the surface with higher kinetic energy break away from the forces of attraction of other particles and get converted into vapour.

Factors Affecting Evaporation

The rate of evaporation is influenced by several environmental and physical factors:

  • Surface Area: Since evaporation occurs at the surface, increasing the surface area increases the rate of evaporation (e.g., spreading out clothes to dry).
  • Temperature: Higher temperatures increase the kinetic energy of particles, allowing more of them to escape into the vapour state, thus increasing the rate.
  • Humidity: Humidity is the amount of water vapour present in the air. If the air is already saturated with water vapour, the rate of evaporation decreases.
  • Wind Speed: Moving air carries away water vapour particles, maintaining a concentration gradient that favors faster evaporation. Thus, an increase in wind speed increases the rate of evaporation.

Cooling Effect of Evaporation

Evaporation causes cooling. The particles of liquid absorb energy from the surroundings (or the liquid itself) to regain the energy lost during evaporation. This absorption of energy lowers the temperature of the surroundings.

Factor Change Effect on Evaporation Rate
Surface Area Increase Increases
Temperature Increase Increases
Humidity Increase Decreases
Wind Speed Increase Increases

Common Examples

Acetone on palm: When you pour acetone (nail polish remover) on your palm, particles gain energy from your palm and evaporate, causing the palm to feel cool.

Earthen pots (Matka): Water seeps through the pores of the pot and evaporates from the surface, cooling the remaining water inside.

Cotton clothes in summer: Cotton is a good absorber of water. It absorbs sweat and exposes it to the atmosphere for easy evaporation, which cools the body.