What is liquefaction?

• A. Liquefying of atmospheric air by compression and cooling
• B. The process of turning a liquid into a solid
• C. The chemical reaction between oxygen and hydrogen
• D. The method of filtering gases through a membrane

Answer: (A)

Liquefaction is turning a gas into a liquid by removing energy or applying pressure until the molecules come together enough to flow as a liquid. When atmospheric air is liquefied, it’s cooled and compressed so the components—mostly nitrogen and oxygen—condense into liquids. This step is often used before separating the gases by fractional distillation.

The other ideas describe different processes: turning a liquid into a solid is freezing or solidification, not liquefaction; a chemical reaction between oxygen and hydrogen is a chemical change that forms new substances (combustion or synthesis), not a simple phase change; and filtering gases through a membrane is a separation method based on physical barriers, not changing the state of matter.