Animated Solution for Chemistry - Solutions: A pressure cooker reduces cooking time for food because

Have you ever wondered why a pressure cooker is an absolute necessity in almost every kitchen? It is not just a heavy metal pot; it is a brilliant application of physical chemistry, specifically the relationship between vapour pressure, external pressure, and boiling point. Let's dive deep into the science of why food cooks so much faster inside this sealed vessel.

To understand the pressure cooker, we first need to understand what it actually means for a liquid to boil. In everyday language, we say water boils at $100^{\circ }\mathrm{C}$. But physically, boiling is a very specific condition.

A liquid boils when its vapour pressure (the pressure exerted by the gas molecules escaping from the liquid) becomes exactly equal to the external atmospheric pressure pushing down on the liquid's surface.

At sea level, the atmospheric pressure is $1\mathrm{atm}$. When water is heated to $100^{\circ }\mathrm{C}$, its vapour pressure reaches $1\mathrm{atm}$. Because the internal push of the vapour matches the external push of the atmosphere, bubbles of vapour can form freely within the bulk of the liquid. This is boiling.

Trapping the Steam

Now, imagine you are cooking in an open pot. No matter how high you turn up the flame, the temperature of the boiling water will never exceed $100^{\circ }\mathrm{C}$. Any extra heat energy simply converts more liquid water into steam, which escapes into the kitchen. The food cooks at a maximum temperature of $100^{\circ }\mathrm{C}$.

Enter the pressure cooker. A pressure cooker is a completely sealed system. When you heat water inside it, the water begins to vaporize. However, because the lid is locked tight, the steam cannot escape. As more and more steam is generated, it gets trapped in the empty space above the water.

This accumulation of trapped steam drastically increases the pressure inside the cooker. The internal pressure ($P_{\mathrm{inside}}$) quickly becomes much greater than the normal atmospheric pressure ($P_{\mathrm{atm}}$).

Because the external pressure pushing down on the water surface inside the cooker is now much higher than $1\mathrm{atm}$, the water's vapour pressure must also reach this new, higher value for boiling to occur.

How does water increase its vapour pressure? By absorbing more heat and reaching a higher temperature!

Inside a typical pressure cooker, the pressure can reach about $2\mathrm{atm}$. At this elevated pressure, water does not boil at $100^{\circ }\mathrm{C}$. Instead, it must be heated to approximately $120^{\circ }\mathrm{C}$ before its vapour pressure matches the high internal pressure.

Faster Cooking

This is the core secret of the pressure cooker. The food inside is not cooking faster because the pressure is physically "crushing" it. It is cooking faster because it is submerged in water that is at $120^{\circ }\mathrm{C}$ instead of $100^{\circ }\mathrm{C}$.

Chemical reactions involved in cooking (like the breakdown of tough fibers in meat or the softening of lentils) are highly temperature-dependent. A $20^{\circ }\mathrm{C}$ increase in cooking temperature can cut the cooking time down to a quarter of what it would be in an open pot.

Understanding this concept also explains a classic geographical phenomenon. If you try to cook food on a high mountain, you will face a frustrating problem. At high altitudes, the atmospheric pressure is significantly lower than at sea level.

Because the external pressure is low, water's vapour pressure equals the atmospheric pressure at a much lower temperature (for example, $90^{\circ }\mathrm{C}$). The water boils, but it is not very hot! Cooking food at $90^{\circ }\mathrm{C}$ takes an agonizingly long time. This is exactly why pressure cookers are not just a convenience, but an absolute necessity for people living in hilly or mountainous regions.