NDA (II) 2022 General Ability Test · Question 64 of 150

Correct — A, increasing the boiling point of water. A liquid boils when its vapour pressure equals the pressure pressing down on it. A sealed cooker traps the steam it makes, so the pressure inside climbs above one atmosphere and the water can no longer boil at 100 °C — it must reach a higher temperature first. Domestic cookers work at a gauge pressure of roughly 0·8–1 bar, at which water boils at about 121 °C, and food cooks faster simply because the cooking chemistry runs much quicker at that higher temperature.

• (b)decreasing the boiling point of water — That is what happens on a mountain, where the air pressure is lower — water boils below 100 °C and food takes LONGER to cook. A cooker does the opposite.
• (c)increasing the melting point of water — The melting point is the ice-to-water change at 0 °C and has nothing to do with cooking. In water's case raising the pressure actually lowers the melting point slightly, so even the direction is wrong.
• (d)decreasing the melting point of water — Pressure does depress water's melting point a little, but the water in a cooker is already liquid and heading for boiling — the melting point plays no part in how fast food cooks.

Boiling is not fixed at 100 °C — that figure belongs to one atmosphere of pressure. A liquid boils at whatever temperature makes its vapour pressure equal to the pressure above it, so raising the external pressure raises the boiling point and lowering it lowers the boiling point. A pressure cooker is a sealed vessel with a weighted valve that lets steam escape only above a set pressure, so it holds the water hotter than 100 °C while still liquid.

The cooker does not push more heat into the food; the flame's temperature is irrelevant to the equilibrium. What it does is raise the temperature ceiling at which the water sits, and reaction rates for softening starch, collagen and pulses climb steeply with temperature — which is why dal that needs an hour in an open pan is done in minutes. The same physics run backwards explains slow cooking at hill stations, and it is the reason option (b) is the classic trap for candidates who remember only that 'pressure changes the boiling point'.

• A liquid boils when its vapour pressure equals the surrounding pressure, so higher pressure means a higher boiling point.
• Domestic pressure cookers run at about 0·8–1 bar gauge (11·6–15 psi), at which water boils at roughly 121 °C.
• Food cooks faster because reaction rates rise sharply with temperature, not because the cooker delivers extra heat.
• At high altitude the atmospheric pressure is lower, water boils below 100 °C, and cooking is correspondingly slower.
• Water is unusual in that increasing pressure LOWERS its melting point, the opposite of what pressure does to its boiling point.

• Confusing melting point with boiling point when the question mentions pressure.
• Thinking the cooker raises the flame's heat rather than the water's boiling temperature.
• Reversing the altitude relationship — pressure falls with height, so the boiling point falls too.

As a one-line cause question like this, as an assertion-and-reason pair on altitude and boiling point, or as a statement set on what actually fixes the cooking temperature inside the cooker.

• practice — not a real PYQ

  Food takes longer to cook at a hill station mainly because

  • (a)the air there is colder
  • (b)atmospheric pressure is lower, so water boils below 100 °C
  • (c)the water there is harder
  • (d)fuel burns less efficiently at height

  Answer(b) atmospheric pressure is lower, so water boils below 100 °C — a lower cooking temperature means slower cooking.

• practice — not a real PYQ

  Increasing the pressure on the surface of water

  • (a)raises its boiling point
  • (b)lowers its boiling point
  • (c)leaves its boiling point unchanged
  • (d)makes it boil at 0 °C

  Answer(a) raises its boiling point — the water must get hotter for its vapour pressure to match the higher external pressure.