To press a gas into a liquid we must use temperature and __________.

• A. Temperature
• B. Pressure
• C. Humidity
• D. Volume

Answer: (B)

Liquefying a gas depends on reaching conditions where the vapor can no longer be sustained as a separate gas phase, which is controlled by temperature and pressure. Lowering temperature reduces molecular motion so the gas can condense into a liquid at a given pressure. Increasing pressure packs molecules closer together, promoting liquid formation at a given temperature. Humidity isn’t a factor in this phase change, and changing volume alone doesn’t guarantee liquefaction because it primarily affects pressure through the gas-law relationship. So the necessary additional factor is pressure.