Question

(a) Where would it take longer to boil (cook) an egg? In Melbourne or at the Col du Galibier? (b) What is the principle of operation of the pressure cooker?

Note: Please explain with a detailed answer and provide illustration with figures and examples. Please do not attempt if you cannot complete these requirements.

Answer 1

a) Col du Galibier is a mountain pass, so it is at a reasonably higher altitude than Melbourne.

We know that boiling point of water is affected by air pressure. Higher the pressure, higher the boiling point and vice versa. Air pressure decreases with altitude. Therefore air pressure will be lesser in Col du Galibier, meaning that boiling point of water will be lower at Col du Galibier. Therefore an egg will take longer to boil in Col du Galibier because now water will evaporate before it could provide enough heat to the egg, so the egg will take a longer time to absorb enough heat from the water to get cooked.

b) Pressure cooker works on the principle of steam pressure. As the water heats inside a pressure cooker and reaches the boiling point, part of it evaporates but is unable to leave the sealed cooker. This is how steam is built inside the cooker which has comparatively higher temperature than boiling water (around 120⁰ C) . Pressure inside the cooker rises due to this steam and makes the boiling point of water to rise so now the water can absorb more heat before it gets evaporated, meaning, it can transfer more heat to the food inside the cooker.As the pressure of the steam continues to rise, it triggers the pressure regulator at the top of the lid, preventing the cooker from bursting due to high pressure. This is how a pressure cooker can cook food much faster than other conventional means of cooking with water.