Water normally boils at 100 degrees Celsius, and no matter how hard you heat an open pot, the water cannot get hotter than that; the extra heat just turns more water to steam. That sets a limit on how fast food cooks.
A pressure cooker is sealed, so the steam produced cannot escape and the pressure inside rises. For a fixed amount of gas, raising the pressure is linked to a higher temperature, and higher pressure also pushes the boiling point of water above 100 degrees. The water can now reach perhaps 120 degrees before boiling, so the food sits in hotter surroundings and cooks noticeably faster, saving fuel.
The same relationship, in reverse, explains why water boils below 100 degrees on a high mountain, where the air pressure is lower.
In SPM you should relate pressure, temperature and the gas laws, and connect boiling point to the pressure above a liquid.
Common misconceptions
- A pressure cooker cooks faster because the flame is hotter -> The flame is the same; the water and steam get hotter because the boiling point rises with pressure.
- Water always boils at 100 degrees Celsius -> Boiling point depends on the pressure above the liquid, so it is higher under pressure and lower on a mountain.
- Sealing the pot just traps heat like a blanket -> Sealing raises the pressure, which is what allows a higher temperature and faster cooking.
The physics behind it
For a fixed mass of gas kept at constant volume, the pressure law states that pressure is directly proportional to absolute temperature: P/T is constant, or P₁/T₁ = P₂/T₂, with temperature measured in kelvin (K). Pressure P is in pascals (Pa), and 0 °C equals 273 K.
Inside a sealed pressure cooker the trapped steam cannot escape, so as the temperature rises the pressure climbs with it. Suppose the gas starts at 100 kPa and 300 K and is heated to 350 K at constant volume.
The new pressure is P₂ = P₁ × T₂/T₁ = 100 kPa × 350 K ÷ 300 K = 116.7 kPa.
That higher pressure pressing on the water surface raises the boiling point above the usual 100 °C, so the water and steam can become hotter before they boil away. Food surrounded by water and steam at this higher temperature cooks faster.
The key chain is simple: sealed volume, rising pressure, higher boiling point, hotter cooking.
See it in daily life
Think of a tough cut of beef for rendang, or a pot of dried beans, that would need a couple of hours of gentle boiling in an ordinary covered pot. In an open pot the water is stuck at 100 °C no matter how high the flame, so softening the meat or beans is slow.
Seal the same food in a pressure cooker and the trapped steam pushes the pressure up, lifting the boiling point above 100 °C. Now the food sits in water and steam that are hotter than a normal pot can ever reach, and the same rendang or beans turn tender in a fraction of the time, using less gas.
You can hear the physics at work: the little weight on top jiggles and hisses as it releases excess steam, holding the pressure, and therefore the temperature, at a steady raised level. When cooking finishes, letting the steam out drops the pressure, the boiling point falls back to 100 °C, and it is safe to open the lid.
How this comes up in SPM
In Paper 2 this idea is examined with command words such as state, explain, relate and solve. You may state the pressure law, explain why a sealed pot cooks faster, or use P₁/T₁ = P₂/T₂ with temperatures converted to kelvin.
Within the same Heat chapter it belongs to the gas laws, alongside Boyle's law linking pressure and volume, and Charles's law linking volume and temperature. It also connects to specific latent heat, since boiling is a change of state, and to the idea that a liquid's boiling point depends on the pressure above it.
A frequent task is to describe a gas-law experiment and to calculate an unknown pressure, volume or temperature, always converting Celsius to kelvin first. Remember that a temperature in the gas laws must be absolute, so add 273 to the Celsius value before substituting.
Carrying the correct units and using kelvin are the two habits that keep these answers correct.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)