A chef is making pasta and brings water in a 18cm radius pot

A chef is making pasta and brings water in a 18cm radius pot to a boil. Then, he places a lid on it and allows it to cool to room temperature, which is 25 degree C. Assume that no air leaks into the pan as it cools. The total mass of the pot and its contents is 3.5kg. Now the chef wants to lift the lid off the pot. Will he be able to, or will the pot remain fixed to the lid and both be lifted together?

Solution

Just before placing the lid, there will be a considerable amount of water vapour which will be forming at the surface. When the lid is placed, there will be formation of vapour and the vapour forces the lid to open slightly and escapes into atmosphere. Once the pot is cool enough that the formation of vapour stops, then the lid sits snug on the pot. Once the pot is near the ambient temperature, the water vapour inside the pot condenses and creates a considerable vacuum inside the pot. The pressure will be less than the atmospheric pressure. This pressure difference is sufficient for supporting the weight of the pan. Thus the pot will remain fixed to the lid

Quantification

Force due to vacuum = vacuum pressure at 25degree c * Pot Area

Fv= 3559.707*pi*0.18^2= 362.33 N

Max Mass of Pot and Contents that can be supported M= Fv/g = 362.33/9.81

M= 36.935 kg

Since the mass of pot and its contents is only 3.5kg which is less than M The lid remains fixed to the pot.