BPSC 70th CCE Prelims General Studies · Question 64 of 143

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.

• (a)Low temperature, low pressure — 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)High temperature, low pressure — 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)High temperature, high pressure — 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.

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.

Two of the four options can be dismissed with a single fact — that no gas liquefies above its critical temperature — and that removes both 'high temperature' choices at once. The remaining decision, between low pressure and high pressure at low temperature, turns on what pressure actually does: it reduces the mean free path, bringing molecules within range of their mutual attraction. The clearest discriminator to carry into the exam hall is the contrast between LPG and LNG. LPG is propane and butane, whose critical temperatures are well above room temperature (butane's is about 152 °C), so a steel cylinder at a few atmospheres holds them liquid with no cooling at all. LNG is mainly methane, whose critical temperature is about −82.6 °C, so no pressure will do it — LNG tankers carry methane chilled to roughly −162 °C at near-atmospheric pressure. Same industry, same purpose, opposite methods, and the critical temperature explains both.

• Critical temperature is the temperature above which a gas cannot be liquefied by pressure alone; carbon dioxide's is about 31 °C at a critical pressure of roughly 73 atmospheres
• Thomas Andrews established the critical point in 1869 through compression experiments on carbon dioxide
• Critical temperatures of the 'permanent gases' are extremely low — oxygen about −118.6 °C, nitrogen about −147 °C, hydrogen about −240 °C and helium about −268 °C (5.19 K)
• James Dewar first liquefied hydrogen in 1898 and Heike Kamerlingh Onnes liquefied helium in 1908 at about 4.2 K, work that led him to discover superconductivity in 1911
• Carl von Linde's 1895 air-liquefaction process uses the Joule-Thomson effect — compressed gas cools on expanding through a throttle — and remains the basis of industrial cryogenics

• Believing pressure alone can liquefy any gas — above the critical temperature it cannot, no matter how high
• Assuming both variables must be extreme; ammonia liquefies under modest pressure near room temperature because its critical temperature is about 132 °C
• Confusing liquefaction of a gas with condensation of a vapour in air — the physics is the same phase boundary, but exam options usually ask about the deliberate industrial process

BPSC asks this as a bare two-variable choice, the sort of item that is decided in ten seconds by anyone who knows the direction each variable pushes. UPSC almost never asks it so directly; it embeds the same physics in an application — how a pressure cooker raises the cooking temperature, what a cryogenic engine burns, or what happens to a gas expanding through a nozzle — so understanding the phase boundary matters more than memorising a phrase.

• practice — not a real PYQ

  A gas cannot be liquefied by the application of pressure alone when it is

  • (a)below its critical temperature
  • (b)above its critical temperature
  • (c)at its triple point
  • (d)below its boiling point

  Answer(b) above its critical temperature — beyond that temperature no pressure produces a liquid, only a dense supercritical fluid.

• practice — not a real PYQ

  Natural gas has to be cooled to about −162 °C to be shipped as LNG, whereas LPG is kept liquid in ordinary steel cylinders at room temperature. The reason is that

  • (a)methane is heavier than propane and butane
  • (b)methane's critical temperature is far below room temperature while those of propane and butane are above it
  • (c)LPG cylinders are held at a much higher pressure than LNG tanks
  • (d)methane does not obey the gas laws

  Answer(b) methane's critical temperature is far below room temperature while those of propane and butane are above it — methane's is about −82.6 °C, so pressure alone cannot liquefy it.