# Homework Help: Determining pressure in the pot.

1. Nov 30, 2008

### Micko

1. The problem statement, all variables and given/known data
I'm trying to figure out if and how pressure in the closed pot, half full of water is going to change if pot is placed on the cooker and continuously heated.

2. Relevant equations

p*V=R*T for ideal gas

3. The attempt at a solution

I think the following will occur:
First there would be an increase in water temperature until boiling point 100C (I'm assuming outside pressure is 1 atm). After water starts boiling temperature will not change until all water vaporize. Also during heating water will slightly exapand, but pressure inside the closed pot will reamain the same.
Now, after all water is turned to vapor, further heating will cause temperature to start rising (superheated vapor). Vapor will expand and pressure is still the same. But since pot is closed further adding of heat will cause pressure inside the pot to start rising (pot's volume is constant)?
Am I right here?

Can you explain me at which point pressure inside the pot will start rising?

2. Nov 30, 2008

### mgb_phys

The pressure will rise continually.
As the water boils more gas is created, the volume of the water goes down slightly, but the extra gas will create more pressure. As the water is boiling the temperature of the stream won't increase very much because most of the energy is going into the water.
Once all the water is boiled then it becomes a gas law problem the volume is fixed, so if you keep heating the steam pressure wil rise.

If you need a more detailed answer you will have to look at steam tables for the vapour pressure at each point.

3. Nov 30, 2008

### Micko

Thanks, but want to be sure about one thing, and that is about point where pressure will start to increase. I think that pressure will start rising only after water starts boiling. Am I right there?
Is it OK to look at equation of ideal gas, because water vapor can be approximated by ideal gas in some cases?

4. Nov 30, 2008

### glueball8

P1V1/T1=P2V2/T2

Volume is constant so P1/T1=P2/T2

5. Nov 30, 2008

### mgb_phys

Even before it boils there will be vapour produced.