2017 H2 Chem P1 Q5 — Gases Solution

The answer is D.
In a gas the molecules are far apart, so the short-range intermolecular forces of attraction between them are negligible. Increasing the pressure reduces the volume the gas occupies and pushes the molecules much closer together. At these short distances the intermolecular attractions become effective enough to hold the molecules together, and the gas condenses into a liquid. This is also why a real gas deviates from ideal behaviour at high pressure.
• A is wrong: increasing the pressure does not lower the temperature of the gas (compressing a gas, if anything, warms it). The gas liquefies because the molecules are closer together, not because it is cooled.
• B is wrong: the average kinetic energy of the molecules depends only on temperature, not on pressure, so increasing the pressure does not reduce it.
• C is wrong: the size of a molecule is fixed and is not changed by pressure. Only the distance between the molecules decreases.