I encountered a question that asks for the reason that a perfectly fitting pot and its lid often stick after cooking when it cools down. The answer in the solution manual was that the pressure decreases when the temperature decreases. I understand that point, but my problem is the following: The pot before being heated during cooking is influenced only by the atmospheric value, and the food inside that pot is the atmospheric pressure also. So this is the initial state for the pressure inside the pot now. When heated the pressure inside the pot increases. When we let the pot cool down to the room tempreture, would the pressure return to its initial value which is the atmospheric pressure? Why does the solution manual consider a vacuum inside the pot?
Thermodynamics – Why Does a Perfectly Fitting Pot and Lid Stick After Cooking?
pressurethermodynamics
Related Solutions
The interior of the bubbles is mostly steam. When you raise the lid, cold (well, below $100^\circ\text{C}$) air flows in and the steam in the bubbles condenses and collapses the bubbles.
I don't know of anyone who has done the experiment, but you should find if you lift the lid when the outside temperature is $100^\circ\text{C}$ or greater, the foam will not collapse, or at least it will collapse more slowly. Presumably you'd still get evaporation from the foam surface.
Edit: just out of curiosity I worked out how much air would have to enter the pan to condense the steam in the foam.
Density of steam at atmospheric pressure $\rho_s = 5.90\times10^{-4}\text{ g}/\text{cm}^3.$
Latent heat of steam at atmospheric pressure $= 2.26\text{ kJ}/\text g \implies 0.00133\text{ kJ}/\text{cm}^3$
Suppose your pan contains $100\text{ cm}^3$ of steam in the foam then the heat given off when you condense it to water at $100^\circ\text{C}$ is $0.133\text{ kJ}$.
The specific heat of air is about $1\text{ J}/(\text{g}\cdot\text{K})$ so assuming the air in your kitchen is about $20^\circ \text C$, i.e. a temperature change of $80^\circ\text C$ when it hits the steam, you need $1.66\text{ g}$ of air. The density of air is about 1.2 g/litre so you need about 1.4 litres of air to fully condense the steam. I must admit that this seems a lot.
However you probably don't have to condense all the steam to collapse the foam. I suspect (but can't prove) that condensing the steam just inside the bubble wall would be enough to destabilise it and cause the bubble to break. The remainder of the steam in the bubble would simply escape and wouldn't need to be condensed, so the volume of air needed would be greatly reduced. Still, I have to concede that evaporation could well be an important factor.
When cooking, keeping the lid on a pot does a few things if you think about it:
- it decreases air circulation significantly so the air in the pot stays hotter, this cooks the food faster.
- if it's snug, it should increase the air pressure.*
- The pot lid captures condensation, so it will return water back into what you're cooking. That is, I think, the largest effect.
I can't prove that with science, it's just my observation. I cook a lot. Often when I lift a lid off a stew or broth, the lid is wet and dripping. Curiously, what this means is that water will get to a boil faster with the lid on, but it will boil away faster with the lid off.
* On Air pressure: I have a hard time believing that, unless it's a pressure cooker, this would be a significant increase, as air pressure is 14.4 lbs per square inch and what does the average lid weigh - a pound? But there might also be a small decrease in pressure (er, I think), similar to Bernouli's paper lift.
The boiling of water creates a wind direction of sorts off what you're cooing up through the pot and that wind direction should decrease air pressure, but that effect would likely also be small. Perhaps, the boiling point - no lid might be 98 degrees C and with lid, 101 degrees C - that's a guesstimate, but it shouldn't be a huge difference I would think.
Best Answer
You've made the wrong assumption about what a perfectly fitting lid does. Rather than sealing perfectly, it will vent gas when the interior is at a higher pressure (the lid will lift), but seal when the exterior pressure is higher (the lid is pressed down).