A person cooks a meal in a 30-cm-diameter pot that is covered with a well-fitting lid and lets the food cool to the room temperature of 20°C. The total mass of the food and the pot is 8 kg. Now the person tries to open the pan by lifting the lid up. Assuming no air has leaked into the pan during cooling, determine if the lid will open or the pan will move up together with the lid.
The Attempt at a Solution
Ignore the values, my problem is with the concepts in the question. I'm going to try and explain how I think it works and then how the book solves it.
I believe that the process involved would be something along the lines of:
Sate 1: Pot at 20 C, water, food and air. Pressure roughly 1 atm.
First the water will go to its boiling temperature (more or less 99.8C) then it will start boiling and water vapor will force all air INSIDE the pot to go out. The pressure will not change.
State 2: Pot at 100 C, water (liq and vapor) and food. Pressure still 1 atm.
Then it will cool down, the vapor inside will condense and become water again, but since the air went out a vacuum will be formed inside, and the pressure will increase.
State 3: Pot at 20 C, water (liq only? liq and vapor? Can I determine that?) and food. Increased pressure.
So, my only problem was I didn't understand how to determine the new pressure inside. I don't know how to quantify the "generated vacuum" so to speak.
The book then takes the saturation pressure of water at 20 C and uses that (2.34 kPa) to calculate the rest the of the problem, it doesn't really explains why it uses this pressure, though.
- In State 3, is there only liquid water, only vapor water or both?
- How do I quantify the "vacuum"?