Heating water in a sealed bottle

redargon

Say I want to put a bottle of water in the microwave and heat it up. How do I determine the pressure in the bottle as a relation to the temperature of the water? Assuming that there is a headspace of air between the liquid and the bottle cap. Is it dependant on the volume of the air space? ie. would a half filled bottle experince a greater pressure at the same temperature than a bottle that is 90% filled?

I'm trying to use partial pressures, but I'm battling to find out how much of the water is turned into vapour at certain temperatures.

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lalbatros

As long as the headspace is large enough,
as long as the amount of water is large enough,
the amount of headpace will not influence the pressure​
otherwise
all the water could be varporised​
otherwise
the the dilatation of water could make the pressure increase much faster with temperature​
end

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redargon

As long as the headspace is large enough,
as long as the amount of water is large enough,
the amount of headpace will not influence the pressure​
otherwise
all the water could be varporised​
otherwise
the the dilatation of water could make the pressure increase much faster with temperature​
end
ummm... I said a bottle of water in a microwave. Think, standard sealed bottle of water. The rest of your post wasn't helpful, but thanks anyway.

russ_watters

Mentor
Boiling steam at atmospheric pressure has a partial pressure equal to atmospheric pressure. Use the ideal gas law to calculate the pressure of the air and add the two together. lalbatros is right that the volume of air is irrelevant....unless you intend to heat it past boiling, which I wouldn't recommend. But you can find the associated pressure with a steam table: http://www.efunda.com/materials/water/steamtable_sat.cfm

redargon

So I found out that the vapour pressure of water is about 0.198bar at 60°C (using steam tables as suggested). Now what density do I use for the ideal gas law for calculating the pressure of the air? 1.28kg/m³ (STP) or the value for air at 60°C or the density of the air/water avpour mixture. I'm running myself in circles here because I can't frikkin remember my basic chem... :(

redargon

Also, if I'm using ideal gas laws, couldn't I just use P1/T1=P2/T2. As in, the pressure of gas in the volume of the sealed bottle is atmospheric at 21°C, I stick it in the microwave and the temperature rises to 60°C, so the pressure rises accordingly.

redargon

d'oh. if the volume and the mass stay the same (as they are in the closed bottle), then the density must be constant. Sorry, I think I had a brain fart there.

Lok

Id say that the headspace air does quite a lot.

Consider two cases

One where there is very little air in the bottle. Heating it in the microwave will dilate the water the bottle and the air. Assuming the bottle dilates less then water then pressure will go up and it will compress the air some, plus the dilation of the air.

And one where the there is very little water. So dilation of the water adds very little to bottle pressure but the heating of the air ( by contact to the water or anyway) will be the sole significant contributor.

I'm sure that 1 cm3 of extra air in the headspace doesn't do much but it will be different.

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