Naked Science Forum
General Science => General Science => Topic started by: yovav on 22/03/2021 09:25:57
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Will the release of pressure in the thermos affect the temperature of the water inside it?
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Nice question. I'd say yes, a bit.
The headspace gas above the hot liquid will be under pressure because heat from the fluid and the walls of the flask will heat the gas. When you open the lid, you release the pressure and the gas expands. This will drop the temperature of the gas and hence also rob some energy from the liquid because the gas has surface contact with the interface. But the effect will be very small.
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Hi all,
Yes is the short answer, you can calculate it using the ideal gas laws assuming just the drop in pressure of the steam, using
pv= nRT
https://en.m.wikipedia.org/wiki/Ideal_gas_law
Rearrange the equation to
T =pv/nR
P is pressure in pascals
V is volume in m3
n is number of moles
R is the gas constant = 8.314 J K-1 mol-1
T is temperature in K
Is the units I was taught but I believe there are others you can use also
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Nope, gas cylinders are the same temperature no matter what the pressure, only changing when the gas is released, similar to the temperature achieved in boiling point under differing pressures, the pressure inside a pressure vessel affects the boiling point only when heated or during pressure release. 1 bar (2 bar in absolute terms) equals 120C boiling point
https://en.wikipedia.org/wiki/Pressure_cooking
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Hi all,
Petro I’m not sure that’s correct, I once saw a co,2 bottle blow a seal because it had been left in direct sunlight which would fit with the link to pressure and temperature as per the gas laws.
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n is number of moles
Where?
It's an interesting fact that (assuming ideal gas laws) heating a rom doesn't increase the energy in the air in the room.
The increase (per mole) is exactly compensated by the expansion forcing some air out of the room.
Due to ono ideal behaviour the gas in the thermos will cool slightly.
But the big (in energy terms) effect is likely to be the evaporation of from the drink to make up for that lost .
Whether that's the dominant effect in terms ot temperature might depend on how much liquid is present etc.
Nope, gas cylinders are the same temperature no matter what the pressure,
Except on a hot day (or a cold one).
I think you were aiming for this, and missing.
https://en.wikipedia.org/wiki/Joule%E2%80%93Thomson_effect
An ideal gas won't cool as it expands, but a real gas will.
Water vapour is a real gas.
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Hi all
BC could you expand on your question regarding (n is the number of moles ) as I’m not sure what you mean.
Here is a link to some examples similar to the sort of calculations I remember doing.
https://www.chemicool.com/examples/gas-law-calculations.html
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Hi all,
Petro I’m not sure that’s correct, I once saw a co,2 bottle blow a seal because it had been left in direct sunlight which would fit with the link to pressure and temperature as per the gas laws.
Gas cylinders do have warnings about sunlight or heating even to ambient room temperatures for this very reason, do not store them in a sauna! Co2 is a black metal cylinder I believe? This is not a good combination for temperature maintenance, metal's very good at changing its temperature, cars get frost in the night as they emmit their energy quickly even though they are painted shiny and smooth.
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Hi all
BC could you expand on your question regarding (n is the number of moles ) as I’m not sure what you mean.
Here is a link to some examples similar to the sort of calculations I remember doing.
https://www.chemicool.com/examples/gas-law-calculations.html
If n is the number of moles, and you let some of them out of the flask...
You can't readily use that formula because n changes.
Co2 is a black metal cylinder I believe?
No, it is two cobalt atoms stuck together.
For what it's worth most of the CO2 in a tank is actually present as a liquid.
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Hi all,
So BC a little ray of sunshine as always, 8)
If n is the number of moles, and you let some of them out of the flask...
You can't readily use that formula because n changes
I believe I covered that in my first post
Yes is the short answer, you can calculate it using the ideal gas laws assuming just the drop in pressure of the steam, using
pv= nRT
I believe it would be reasonable to be able to calculate n and delta n before and after unscrewing the lid, in a controlled experiment to the system which is the flask.
Also are you sure this is correct BC, regarding carbon dioxide ?
quote] No, it is two cobalt atoms stuck together./quote]
don't you end up with an extra c
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There are 2/Two parts to this OP.
1) Does Pressure in a Thermos Flask affect the Temperature of the Water inside?
In a closed system where volume is held constant, there is a direct relationship between Pressure and Temperature.
For example, when the pressure increases then the temperature also increases. When the pressure decreases, then the temperature decreases.
But the point to be Noted is...
Water is essentially incompressible, especially under normal conditions.
Compressing water customarily heats it. But under Extreme Compression.
Hence, No the Pressure build up due to Heated Water inside of a Thermos is Not sufficient enough to heat it further or to bring it to a boil.
Will the release of pressure in the thermos affect the temperature of the water inside it?
Yes, most definitely as Energy escapes once the lid is opened up.
P.S. - Fun Fact!
Water boils under normal atmospheric pressure at 212°F (100°C).
But if Water is placed at room temperature into a vacuum chamber and the Air is removed.
Eventually, the boiling temperature will fall below the water temperature and Boiling will begin without Heating.
✌️
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I was wondering if there's a way to introduce a water heater coil without interrupting the vacuum or are there better ways to boil water in a flask with a let on?
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Will the release of pressure in the thermos affect the temperature of the water inside it?
Pressure will change the boiling point of water, of course. So, for example, in your car radiator pure water will boil at about 100°C. Pressurize it and it will boil at a higher temperature. Ignoring antifreeze which will impact it further.
If you heat water up from say 30°C to 100°C, it will also increase volume due to thermal expansion, and cause an increase in pressure in a sealed container.
Even if your container of water isn't experiencing a rigorous boil, it will exert a certain amount of vapor pressure. Without pressure a seal won't make much difference. But, with pressure the vapor pressure should be higher. Lowering that vapor pressure will cause more evaporation, and thus cooling of the liquid. Replacing the moist air above the liquid with dry air will also cause more evaporation and cooling.
I was wondering if there's a way to introduce a water heater coil without interrupting the vacuum or are there better ways to boil water in a flask with a let on?
I think I've seen travel mugs (sealed?) with 12V heating elements.
However, if you have a non-metallic container, then you could try a microwave. There are some cases where metal can also be introduced to a microwave to improve heat transfer. I'm not sure the limits, but years ago I worked a job that used a RF curing oven with a metalized glue. Of course, metal without sharp edges can also be used as a shield in a microwave.