NCERT Class 9 Science — Chapter 1: Matter in our surroundings

Correct Option

The correct option is

The direct transition of a gas into a solid.

Explanation

Matter changes its state (solid, liquid, gas) based on variations in temperature and pressure. These transitions are physical changes where the chemical composition of the substance remains unaltered. Deposition is a specific thermodynamic process where a substance transitions directly from a gaseous state to a solid state without passing through the intermediate liquid phase.

Option Analysis

• The direct transition of a solid into a gas. is incorrect: The direct transition of a solid into a gas is known as Sublimation. Examples include dry ice (solid CO₂) turning into gas or camphor disappearing over time.
• The direct transition of a gas into a solid. is correct: The direct transition of a gas into a solid is called Deposition (also known as desublimation). A common example is the formation of frost on a cold surface, where water vapour in the air turns directly into solid ice crystals.
• The transition of a liquid into a solid. is incorrect: The transition of a liquid into a solid is termed Freezing or Solidification.
• The condensation of vapour into liquid droplets. is incorrect: The transition of vapour (gas) into liquid droplets is called Condensation.

Key Takeaway: Deposition is the reverse process of sublimation, characterized by the direct phase change from gas to solid, bypassing the liquid state.