Why does a pot...

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The_Unbeliever

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...that's simmering with a lid on, but the lid is slightly off to the one side so the steam can escape, won't cook over, but a pot with a lid on, will cook over?

Both simmering at the same temperature.

Quite interesting.
 
my guess would be the heat escaping :p

so the other indeed gets hotter as it traps heat
 
Something to do with pressure. Higher pressure causes quicker boiling (or something to that effect)
 
The_Librarian said:
Both simmering at the same temperature.
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The stove might be set to the same temp but the pot contents are not. Take it to the extreme: Open pot vs closed pot. Open pot...all the heat escapes.

The pressure difference is negligible & def won't cause a temp increase in itself...you need lots of pressure to heat something.
 
Temperature, volume and pressure are related.

The volume (space available inside the pot) can't change because the pot doesn't change shape.

In any system with constant volume pressure is directly proportional to temperature. It was covered in highschool science.
 
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porchrat said:
Temperature, volume and pressure are related.

The volume (space available inside the pot) can't change because the pot doesn't change shape.

In any system with constant volume pressure is directly proportional to temperature. It was covered in highschool science.
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Of course. The pressure is limited to the weight of the lid though...beyond that the lid simply pops off releasing the pressure. Unless the lid is sealed like a pressure cooker the effect on temperature is negligible because pressure can't build to anything near what is required to raise the temp meaningfully...
 
Interesting.

So this also explain why a pot of oats will just boil over and make a fine mess on the oven (should you be stupid enough to keep the lid on) :D:D:D
 
Freshy-ZN said:
Something to do with pressure. Higher pressure causes quicker boiling (or something to that effect)
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Wrong way round. :p Take a car's water bottle - if the cap is on (high pressure) the water goes beyond 100 degrees C without boiling. Take the cap off, and the water will boil.

I suspect Havoc is right. With the lid on you have a closed system where all the heat is trapped inside, so the temperature goes higher without boiling. The pressure inside increases, eventually pushing off the lid. At that point the pressure is diminished, pushing the boiling point of water back to the standard 100 degrees C, and the water must boil (the temperature is already beyond boiling point).

HavocXphere said:
Of course. The pressure is limited to the weight of the lid though...beyond that the lid simply pops off releasing the pressure. Unless the lid is sealed like a pressure cooker the effect on temperature is negligible because pressure can't build to anything near what is required to raise the temp meaningfully...
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Yep, in this (OP) case the pressure builds because the temp increased (in a closed system).
 
HavocXphere said:
Of course. The pressure is limited to the weight of the lid though...beyond that the lid simply pops off releasing the pressure. Unless the lid is sealed like a pressure cooker the effect on temperature is negligible because pressure can't build to anything near what is required to raise the temp meaningfully...
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Of course but he was asking why it boiled over. It is because of that build up in pressure that the contents are able to lift that lid to escape. To not mention the pressure would be a big mistake. It is combination of the increased temperature and associated increased pressure that leads to the contents boiling over.

That pressure builds because the volume remains constant and yet temperature continues to rise as more and more water attempts to evaporate and enter that limited space between the water and the lid. Eventually when the pressure builds to apply enough force to the lid to overwhelm the downward force of gravity on the lid the lid moves and the volume is increased thus relieving the pressure and reducing the temperature.

EDIT As a side note Librarian this is often how liquids are sterilised. Massive pressure is applied to the liquid. When pressure is really high boiling points of liquids rise. You can get water well beyond 100 degrees C without boiling if you apply enough pressure.
 
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An "I can do science me" thread! Too bad I can't. *Slowly walks away while looking at the ground, with a sad face*
 
porchrat said:
EDIT As a side note Librarian this is often how liquids are sterilised. Massive pressure is applied to the liquid. When pressure is really high boiling points of liquids rise. You can get water well beyond 100 degrees C without boiling if you apply enough pressure.
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Not exactly helping your own argument there... ;)

Haven't done this stuff since highschool so forgive mistakes...

1kg lid (heavy) & 1 l of water. 1kg = ~10 Newton = 10 Joule. 10/4.2 = 2.38 small cal. Which raises the temp of the 1 l water by 0.00238 degrees. And that is why I think the pressure isn't relevant...there simply isn't enough of it to make any difference whatsoever.

porchrat said:
Of course but he was asking why it boiled over.
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yes. Temperate can't escape, more bubbles result & the water boils over.

No idea why you guys are so keen on the pressure/volume angle...the lid simply restricts easy escape of hot air.

I bet if you cover the pot with something that will allows pressure to escape but not temp then it'll still boil over. Like a balloon or something...

Gotta go study now...
 
HavocXphere said:
Not exactly helping your own argument there... ;)

Haven't done this stuff since highschool so forgive mistakes...

1kg lid (heavy) & 1 l of water. 1kg = ~10 Newton = 10 Joule. 10/4.2 = 2.38 small cal. Which raises the temp of the 1 l water by 0.00238 degrees. And that is why I think the pressure isn't relevant...there simply isn't enough of it to make any difference whatsoever.
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Did you miss this sentence in that paragraph: "As a side note Librarian this is often how liquids are sterilised."

In case you need me to explicitly state it I was talking about things like autoclaves. I didn't think I needed to actually mention that I was no longer talking about the pot... that is what I meant by "As a side note".


yes. Temperate can't escape, more bubbles result & the water boils over.
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Water can't escape with or without bubbles unless pressure lifts that lid.


No idea why you guys are so keen on the pressure/volume angle...the lid simply restricts easy escape of hot air.

I bet if you cover the pot with something that will allows pressure to escape but not temp then it'll still boil over.
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but that wasn't what was asked... we're dealing with a pot and a lid. Pressure is key.
 
ponder said:
and your blood starts boild at high altitude/space if you get a leak in your suite.
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Yeah, I remember a cool experiment where we had a flask half filled with water at room temperature and 99% of the air was removed with a vacuum pump, after which the flask was sealed. You could then boil the water in the flask simply by touching the palm of your hand to it. Amazing to see.
 
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