Which of the following represent the suitable condition for the liquefaction of gases ?

Question

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Correct — B, Low temperature, high pressure. A gas becomes a liquid when the attractive forces between its molecules finally overcome their kinetic energy, and the two conditions in the answer attack those two quantities separately. Lowering the temperature reduces the average kinetic energy of the molecules, so they no longer have enough speed to escape one another's pull. Raising the pressure squeezes them into a smaller volume, so the average intermolecular distance falls and the weak van der Waals attractions — which fade steeply with distance — become strong enough to hold the molecules together. Neither condition alone is sufficient in general, and the temperature condition is the one that cannot be bargained away: every gas has a critical temperature above which no pressure whatever will liquefy it, because above that temperature the molecules simply have too much energy for any attraction to bind them. That is why the two 'high temperature' options fail outright rather than merely being inefficient. Thomas Andrews established this in 1869 by compressing carbon dioxide and finding that above 31 degrees Celsius it refused to condense at any pressure he could apply. It is also why the classic 'permanent gases' — hydrogen, helium, nitrogen, oxygen — resisted liquefaction for most of the nineteenth century: their critical temperatures lie far below room temperature, so chemists were applying pressure to gases that had to be deeply cooled first.

Why the other options are wrong — (a) The temperature half is right and the pressure half is wrong. Cooling alone does work if you cool far enough — liquid nitrogen is made at essentially atmospheric pressure — but at low pressure the molecules stay far apart, so a much deeper chill is needed. Pressure is what lets liquefaction happen at a manageable temperature, which is why industrial plants use both.  (c) This is the exact recipe for keeping a substance gaseous, and is how liquids are made to evaporate. Both variables push the wrong way: heat gives molecules more kinetic energy to escape each other, and low pressure lets them spread out. No gas has ever been liquefied under these conditions.  (d) The tempting half-truth, chosen by candidates who remember that pressure liquefies gases and stop there. Above a gas's critical temperature, pressure is powerless — carbon dioxide above 31 °C will not liquefy however hard it is squeezed. Compressing a hot gas simply produces a dense supercritical fluid, not a liquid with a visible surface.

Concept: The critical temperature is the single most useful idea here: it is the highest temperature at which a substance can exist as a liquid, and above it the distinction between liquid and gas disappears. The pressure needed to liquefy a gas exactly at its critical temperature is the critical pressure, and the two together define the critical point, first described by Thomas Andrews for carbon dioxide in 1869. Below the critical temperature, the liquid-vapour boundary is a curve on the pressure-temperature diagram: cool the gas or compress it and you cross that curve. Industrially, liquefaction exploits the Joule-Thomson effect — a compressed gas cooling as it expands through a throttle — used in Carl von Linde's air-liquefaction process of 1895 and refined by Georges Claude, who added an expansion engine that also extracts external work.

State PCS 2024 Q64 — Science & Technology PYQ with Answer

State PCS 2024 Q64 — Science & Technology PYQ with Answer