A few days ago, we were at a party at which most of us were scientists. I was trying to cook some beans which sounded impossible. A friend of mine suggested that closing the pot lid could help the bean cook sooner. I told them as it's an open pot and the water inside already boiled, closing the lid would not be a help. However, after a long discussion between physicists and chemists, we came to the conclusion that closing the pot lid even after water boiling, would increase the temperature of the water inside, helping the bean cook sooner. I still have doubts but I want to be totally understood. any scientific points here are welcome. Please notice that the pot was not a pressure-cooking and the lid had a hole.
Somayeh F. Rastegar Water boils when it´s vapor pressure equals the pressure of the surrounding athmosphere. The influence of altitude on the boiling temperature of fluides is widely documented phenomenon. Putting on the lid raises the pressure of the space above the boiling liquid up to a certain point (limited by the hole in the lid). That as a consequence raises the boiling temperature of the liquid, resulting in a faster cooking of the beans. To what extent that infuence is present remains to be determined, it is certainly lesser than an express pot, which raises the pressure to about 1,4 bars. In any case I would deem it has an influence, but I doubt it would shorten the cooking time to a drastical extent. Still worth a while in everyday cooking.
Dear Somayeh, the main advantage of closing the lit is that is significantly decrease the energy needed to cook the food. Open pans, unless you need to evaporate the liquid, is therefore not advise.
There may be a slight increase in temperature when putting the lid on but unless you are using a pressure cooker it is negligible in practical terms (industrial sterilization at 121°C is carried out at 2 atm, but home pressure cookers reach about 1.2 atm , allowing boiling point of 104 -105°C inside). If you have used a pressure cooker you have seen that their lids have specific features so that they can be fitted to be hermtically closed, stay so in spite othe the pressure, and you need care when opening them as lots of vapor escapes).
This is not about the increase of temperature per se. Water boils at 100C. However when the surface is in contact with the surrounding air (of lower temp) it will be continually cooled down. Hence closing the pot will keep the temperature at constant high even at the surface of the water. Plus it uses around 20% less energy, when the pot is closed during cooking.
Ever heard about a pressure cooker? or Autoclave? That is exactly how they work - you increase pressure and then you increase boiling temperature.
And the saying "Water boils at 100C" is not correct, water boiling temperature totally depends on pressure - on the top of Everest pure water boils at 68°C, and in the dead sea 101.1°C due to air pressure difference.
If you put the lid on you will create some pressure (from the water pressure). Also, less heat will escape the pot and yes, your bean will cook faster!
BTW, Catherine M G C Renard a domestic pressure gas stove cooker can reach 2 atm, just like an autoclave, and can be used as such if you are stuck (I have done it in the past), it's just not very convenient due to long cooling time and the need for a lot of water rather then steam as used in autoclaves.
See:
https://www.exploratorium.edu/food/pressure-cooking#:~:text=When%20you%20cook%20in%20a,C%20(250%C2%B0F).
https://www.hippressurecooking.com/pressure-cooker-psi-faq-the-stuff-you-didnt-think-to-ask/
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