NDA (II) 2023 General Ability Test · Question 87 of 146

Correct — A, when atmospheric pressure becomes equal to the vapour pressure. A liquid boils at the temperature at which its saturated vapour pressure has climbed to match the pressure pressing on its surface. Below that temperature any bubble of vapour that tries to form inside the liquid is crushed at once, because the surrounding pressure is greater than the pressure of the vapour inside the bubble. The moment the two pressures are equal, bubbles can form and survive right through the bulk of the liquid and rise to the surface — and that is what boiling is. Option (a) states exactly that equality, which is why heating a liquid further does not raise its temperature while it is boiling; the extra heat goes into turning liquid into vapour.

• (b)When atmospheric pressure becomes less than vapour pressure — This is not the condition that starts boiling — it is what you would have to arrange from outside, by pumping the air away. At a fixed external pressure the liquid's vapour pressure stops rising once boiling begins, because the temperature stops rising, so it never overtakes the atmospheric pressure on its own.
• (c)When atmospheric pressure becomes higher than the vapour pressure — This is the ordinary state of a liquid that is below its boiling point. The excess external pressure collapses every vapour bubble that forms inside the liquid, so the liquid only evaporates quietly from its free surface and does not boil.
• (d)When vapour pressure becomes equal to the air pressure — This restates the same equality but swaps in 'air pressure', and the official key does not mark it. The quantity a boiling liquid has to match is the total pressure standing above its surface, which need not be the pressure of air alone — in a sealed vessel that space also holds the liquid's own vapour. The textbook statement therefore uses atmospheric pressure, the wording of option (a).

Any liquid in a closed space sets up an equilibrium with its own vapour, and the pressure of that vapour is the saturated vapour pressure. It rises steeply with temperature. Boiling is the point on that curve where the saturated vapour pressure equals the pressure holding the liquid down, so vapour bubbles can survive inside the liquid instead of only at its surface. The normal boiling point is the temperature at which this happens at one standard atmosphere.

Two options in this set say the same thing from opposite ends, and that is the whole difficulty. The examiner's key marks (a), the phrasing that matches the standard school statement, in which the external load on the liquid is named as atmospheric pressure. Option (d) reads as a near-twin, and its wording is close enough that a careful student can feel the tension; the defensible separation is that the pressure a boiling liquid has to overcome is the total ambient pressure and not the air component of it. In the exam the safe move is to pick the option worded the way the textbook writes the definition, which here is option (a). The physics behind it is worth more than the wording quarrel: lower the external pressure and the boiling temperature falls, which is why water boils below 100 degrees Celsius on a mountain; raise it and the boiling temperature climbs, which is how a pressure cooker works.

• A liquid boils when its saturated vapour pressure becomes equal to the external pressure on its surface.
• The normal boiling point is measured at one standard atmosphere, 101.325 kPa; for water that is 100 degrees Celsius.
• Boiling is a bulk phenomenon — bubbles form throughout the liquid — while evaporation happens only at the free surface and at any temperature.
• A domestic pressure cooker runs at a gauge pressure of roughly 0.8 to 1 bar, at which water boils at about 121 degrees Celsius.

• Assuming the vapour pressure must exceed the atmospheric pressure to push bubbles out; equality is enough, and the temperature stops rising once boiling starts.
• Skimming past options (a) and (d), which state the same equality in different words — read to the end of the list before marking.
• Confusing boiling with evaporation; evaporation needs no equality of pressures and happens at every temperature.

Asked as a one-line definition item — identify the pressure condition that defines boiling.

• practice — not a real PYQ

  Water boils at a temperature below 100 degrees Celsius at a hill station mainly because

  • (a)the atmospheric pressure there is lower
  • (b)the air there is colder
  • (c)the vapour pressure of water is higher there
  • (d)the water there contains fewer dissolved salts

  Answer(a) the atmospheric pressure there is lower — the vapour pressure needs to climb less far to match it, so the equality is reached at a lower temperature.

• practice — not a real PYQ

  Which one of the following statements about boiling and evaporation is correct?

  • (a)Both are surface phenomena
  • (b)Boiling is a bulk phenomenon while evaporation is a surface phenomenon
  • (c)Both are bulk phenomena
  • (d)Evaporation is a bulk phenomenon while boiling is a surface phenomenon

  Answer(b) Boiling is a bulk phenomenon while evaporation is a surface phenomenon — bubbles form throughout a boiling liquid, whereas evaporation only removes molecules from the free surface.