When a gas is converted into a liquid, what is this process called?

Introduction / Context:
Phase changes between solid, liquid, and gas are fundamental concepts in physics and chemistry. Each change has a specific name, such as melting, freezing, evaporation, and condensation. This question asks for the term used when a gas turns into a liquid. Recognising these terms is important for understanding weather phenomena, refrigeration, and many industrial processes, and it is frequently tested in school and competitive exams.

Given Data / Assumptions:

• A gas changes state and becomes a liquid.
• The options are condensation, evaporation, deposition, sublimation, and None of the above.
• We assume the process occurs due to cooling or compression at suitable conditions.

Concept / Approach:
When a gas loses energy (for example, when it cools) and its molecules come closer together to form a liquid, the process is called condensation. This is the reverse of evaporation, which is the conversion of liquid to gas. Deposition is the direct change from gas to solid without passing through the liquid state, and sublimation is solid to gas directly. Therefore, among the provided options, condensation is the correct term for gas turning into liquid.

Step-by-Step Solution:
Step 1: Identify the direction of phase change in the question: gas → liquid. Step 2: Recall that condensation is defined as the process by which a gas becomes a liquid. Step 3: Recognise that evaporation is the opposite process (liquid → gas), so it cannot be correct here. Step 4: Understand that deposition is gas → solid and sublimation is solid → gas, both bypassing the liquid phase. Step 5: Conclude that condensation is the only option that correctly names gas changing into liquid.

Verification / Alternative check:
Everyday examples confirm this terminology. Water vapour in the air condenses on cold surfaces as droplets; we call this condensation. In the water cycle, formation of clouds from water vapour is also described as condensation. Refrigerators and air conditioners cause moisture in air to condense on cooling coils. In all of these cases, the term condensation is used, and it always refers to gas turning into liquid, matching the question exactly.

Why Other Options Are Wrong:
Evaporation: Refers to a liquid changing into a gas at the surface, the reverse of the process described. Deposition: Describes a gas directly becoming a solid, as in frost formation, not gas to liquid. Sublimation: Refers to a solid directly becoming a gas, such as dry ice turning to CO₂ gas. None of the above: Incorrect because condensation is correctly listed and matches the definition.

Common Pitfalls:
A common mistake is confusing condensation with evaporation because both involve liquids and gases. Students sometimes remember one term but not the direction of change. Another error is mixing up deposition and condensation since both involve gas losing energy, but deposition skips the liquid phase. To avoid this, keep a clear mental map of phase changes and their names: melting, freezing, evaporation, condensation, sublimation, and deposition.

Final Answer:
The process of a gas changing into a liquid is called condensation.