UPSC Prelims 2021 Q68: Pressure Cooker Temperature — Explained
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.
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Answer: (c) 1 and 3 only
Statement-wise explanation
- 1: correct — The hole in the lid is covered by the weighted valve, also called the whistle. Steam lifts the weight only when the steam pressure acting over the hole's area balances the weight. So the gauge pressure held inside is roughly the weight divided by the hole area. With a smaller hole, the same weight needs a higher pressure to lift, which raises the boiling point and the cooking temperature.
- 2: incorrect — The boiling point of water depends on the pressure above it, not on how hot the heat source is. Once the weight lifts, the extra steam escapes, so the pressure stays at the set value and the temperature does too. A bigger or hotter flame only makes the cooker reach that pressure sooner and whistle more often.
- 3: correct — Here 'weight of the lid' means the weight placed on the vent, which holds the steam in. A heavier weight keeps the vent shut until the pressure is higher, so the water boils at a higher temperature. At about 1 kgf/cm² above atmospheric pressure, water boils at roughly 120 °C, against 100 °C in an open pan at sea level.
How to eliminate
You can rule out Statement 2 with basic physics. In an open pan at sea level, water does not go above 100 °C however strong the flame, because pressure sets the boiling point. Only one option leaves out Statement 2, so you can reach the answer without knowing how the valve is designed. The phrase 'weight of the lid' makes many candidates hesitate. Read it as the pressure weight that sits on the vent. The fact to remember: cooker pressure ≈ weight ÷ vent-hole area, and that pressure fixes the boiling point.
Know more
Why a pressure cooker cooks faster
A liquid boils when its vapour pressure equals the pressure on its surface. If the pressure on the surface rises, the water must get hotter before it can boil. A pressure cooker traps steam to raise that pressure.
How the set pressure is fixed
The weight sits on a narrow vent tube. It lifts when pressure × vent area = weight. So the extra (gauge) pressure is about W ÷ A. Changing the weight or the hole size changes the cooking temperature. The amount of heat does not. It only decides how fast steam is made and how often the weight lifts.
| Gauge pressure (approx.) | Boiling point of water (approx.) |
|---|---|
| 0 (open pan, sea level) | 100 °C |
| 0.5 kgf/cm² | ~111 °C |
| 1.0 kgf/cm² | ~120 °C |
Britannica gives about 130 °C as the upper range for domestic cookers.
Parts and what they do
| Part | Role |
|---|---|
| Gasket (rubber ring) | Seals the lid so steam cannot leak |
| Vent tube + weight | Sets the working pressure |
| Safety valve / fusible plug | Releases steam if the vent is blocked |
The same idea in reverse. At high altitude the air pressure is lower, so water boils below 100 °C and food cooks slowly. This is why people in the hills need pressure cookers.
Regulation. The Domestic Pressure Cooker (Quality Control) Order came into force on 1 February 2021. Under it, domestic cookers must meet IS 2347 and carry the BIS Standard Mark.