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Q78 (IAS/2021) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Thermal physics Official Key

In a pressure cooker, the temperature at which the food is cooked depends mainly upon which of the following? 1. Area of the hole in the lid 2. Temperature of the flame 3. Weight of the lid Select the correct answer using the code given below.

Result
Your answer: —  ·  Correct: C
Explanation

The correct answer is Option 3 (1 and 3 only). The boiling point of water inside a pressure cooker is determined by the internal pressure, not the intensity of the external heat source.

  • Area of the hole (1): Pressure is defined as Force divided by Area (P=F/A). The size of the nozzle hole determines the area over which the steam pressure acts to lift the weight.
  • Weight of the lid (3): The "whistle" or weight provides the downward force. Equilibrium is reached when the upward steam pressure equals the downward force of the weight. Higher weight or smaller hole area increases the internal pressure, thereby raising the boiling point and cooking temperature.
  • Temperature of the flame (2): While a higher flame increases the rate of steam formation (speeding up the process), it does not change the maximum temperature attained. Once the required pressure is reached, the excess steam escapes, keeping the internal temperature constant at the boiling point corresponding to that pressure.
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Q. In a pressure cooker, the temperature at which the food is cooked depends mainly upon which of the following? 1. Area of the hole in the…
At a glance
Origin: Books + Current Affairs Fairness: Low / Borderline fairness Books / CA: 3.3/10 · 6.7/10
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This is a classic 'Applied Science' trap. It tests if you understand the *mechanism* (Physics: P = F/A) rather than just the *phenomenon*. Most aspirants fail because they confuse 'rate of heating' (flame) with 'maximum temperature achievable' (thermodynamics).

How this question is built

This question can be broken into the following sub-statements. Tap a statement sentence to jump into its detailed analysis.

Statement 1
Does the temperature at which food is cooked in a pressure cooker mainly depend on the area of the vent hole in the lid?
Origin: Web / Current Affairs Fairness: CA heavy Web-answerable

Web source
Presence: 5/5
"The steam trapped inside the pressure cooker builds up pressure to twice that of our normal atmosphere, so the temperature of the liquid can rise above the boiling point of water (100C/212F)."
Why this source?
  • Explains that trapped steam raises internal pressure, which raises the liquid temperature.
  • Attributes higher cooking temperature directly to the pressure built up inside the cooker (sealed system).
Web source
Presence: 5/5
"In a basic pan, water can boil only at a temperature of 100°C, while inside the pressure cooker, water boils approximately at 121°C because of an increased pressure in the sealed container."
Why this source?
  • States the higher boiling temperature inside a pressure cooker is due to increased pressure in the sealed container.
  • Gives a specific example of boiling temperature (≈121°C) tied to increased internal pressure.
Web source
Presence: 5/5
"As the pressure increases inside the cooker, the temperature also increases, which in turn helps in cooking food at a better pace."
Why this source?
  • Links cooking temperature and faster cooking to the increase of pressure inside the cooker.
  • Emphasizes that temperature rises as pressure increases within the cooker (sealed environment).

Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 7: Particulate Nature of Matter > 7.2.2 Liquid state > p. 105
Strength: 5/5
“The temperature at which a liquid boils and turns into vapour at atmospheric pressure is called its boiling point. The movement of particles becomes so vigorous that they move apart from each other, resulting in a decrease in the interparticle forces of attraction. Eventually, the constituent particles can escape from the liquid state. The liquid is converted into vapour or the gaseous state. I have seen that spilled water disappears after some time, and it happens at any temperature! At the boiling point, the formation of vapour is very fast and occurs not only at the surface but also within the liquid.”
Why relevant

Defines boiling point as the temperature at which a liquid boils at a given atmospheric pressure, linking phase-change temperature to pressure.

How to extend

A student could use this rule plus the fact that sealed cookers raise internal pressure to infer that cooking temperature depends on internal pressure rather than vent geometry alone.

Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 22: Vertical Distribution of Temperature > Example 2: Vehicle tube > p. 296
Strength: 4/5
“• In a fully inflated vehicle tube, volume remains constant (mostly). When more air is blown, pressure increases and hence the temperature. We are usually advised not to have fully inflated tubes because when the vehicle travels on a road, the friction between the tire and the road increases the temperature of the air in the tube. As the temperature is directly proportional to pressure, an increase in temperature leads to an increase in pressure and at a certain pressure threshold, the tire bursts.• The above examples explain the relation between Pressure, Temperature and Volume. But the processes are non-adiabatic as there is (will be) some heat exchange between the system and the external environment.”
Why relevant

States that in a (mostly) fixed-volume vessel more air -> higher pressure and hence higher temperature (pressure–temperature relation).

How to extend

Apply the gas-law idea to a pressure cooker: changing how much pressure is retained (e.g., by a vent) will change internal pressure and so cooking temperature.

Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 22: Vertical Distribution of Temperature > A Parcel of Rising Air > p. 297
Strength: 4/5
“• When an air parcel receives more heat than the surrounding air, its temperature increases leading to an increase in volume (an increase in volume implies the air parcel is getting less dense). The air parcel becomes lighter than the surrounding air, and it starts to rise. This process is non-adiabatic (there is heat exchange between the air parcel and the external environment).• When the air parcel starts to rise, the ambient pressure on it starts to fall (the atmospheric pressure decreases with height, so the pressure on the air parcel decreases with height). With the fall in ambient pressure, the volume of the air parcel increases and hence the temperature of the air parcel falls (gas law).”
Why relevant

Explains how ambient pressure affects volume and temperature of a gas (gas-law behaviour of a parcel under changing pressure).

How to extend

Use this pattern to reason that the cooker’s internal gas behaviour (and thus temperature) will respond to pressure changes controlled by venting.

Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 4: Earths Interior > UPSC Prelims 2009] In the structure of planet Earth, below the mantle, the core is mainly made up of which one of the following? > p. 56
Strength: 4/5
“• Scientists have determined the temperature near the Earth's centre to be 6000° C, 1000° C hotter than previously thought. At 6000° C, this iron core is as hot as the Sun's surface, but the crushing pressure caused by gravity prevents it from becoming liquid.• Remember: when the ambient pressure increases, the melting point of solids increases and vice versa. One exception is Ice. In the case of ice, an increase in ambient pressure will lower its melting point.”
Why relevant

Gives a general rule that phase-change temperatures (melting point here) shift with ambient pressure.

How to extend

Generalise from melting to boiling: a rise in ambient (or vessel) pressure should raise the boiling/ cooking temperature inside a cooker.

Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > Activity 4.4 > p. 70
Strength: 3/5
“mixture is burnt to give a clean blue flame. If you observe the bottoms of cooking vessels getting blackened, it means that the air holes are blocked and fuel is getting wasted. Fuels such as coal and petroleum have some amount of nitrogen and sulphur in them. Their combustion results in the formation of oxides of sulphur and nitrogen which are major pollutants in the environment.”
Why relevant

Notes that blocked air holes alter combustion and flame quality (affecting temperature delivered by the heat source).

How to extend

Extend this to recognise two effects of vent area: it can influence internal cooker pressure (affecting boiling temp) and separately the stove flame/heat input (affecting cooking rate).

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