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Author Topic: Why the increased pressure over certain liquids?  (Read 3088 times)

Offline Atomic-S

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Why the increased pressure over certain liquids?
« on: 04/08/2011 04:53:51 »
After placing an object into methylene chloride to dissolve epoxy, in a sealed jar, and then later opening the jar, it was found that the pressure inside had increased noticeably above atmospheric. My first thought that this was because the vapor pressure of methylene chloride at room temperature is higher than atmospheric. But if that were so, methylene chloride ought to spontaneously boil in any open container, which it does not. Any idea what is happening?

I have had similar experiences with acetone, straight out of the source can with no additional objects present in it.


 

Offline Supercryptid

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Why the increased pressure over certain liquids?
« Reply #1 on: 04/08/2011 08:01:14 »
Perhaps the volume of the liquid changed. I've heard that mixing ethanol with water will result in a mixture that is smaller in volume than the volumes of either chemical alone. Such a mixture in a sealed container would reduce the pressure inside of it. Perhaps other mixtures have an increase in volume instead, and therefore increase the pressure.

Alternatively, maybe there was a chemical reaction that liberated small amounts of gas.
 

Offline CliffordK

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Why the increased pressure over certain liquids?
« Reply #2 on: 04/08/2011 08:32:36 »
Bored Chemist?

I'm wondering if it has to do with the air mixture.

I was thinking that the air might get dissolved in the methylene chloride.  But, perhaps not.

It has been a long time since I've studied partial pressures...  but, here is a guess.

Say you have a mixture:
liquid methylene chloride with a mixture of 1% Methylene chloride + 99% air (oxygen/nitrogen) over the top of it.

But, the partial pressure of the liquid is high enough that you should get 10% methylene chloride, and 90% air.

It wouldn't necessarily boil, but in an open system, you would get significant evaporation.
In a closed system, you would get enough evaporating to change your proportions from 1%/99% to 10%/90%.  Assuming the actual amount of air changes very little, you might get a slight pressure increase.

Now.
If you repeated the experiment with PURE water.
You should get a small portion of the Oxygen and Nitrogen dissolving into the water, especially at very low temperatures.  And, thus you might actually get a pressure decrease.
 

Offline lightarrow

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Why the increased pressure over certain liquids?
« Reply #3 on: 04/08/2011 16:54:15 »
After placing an object into methylene chloride to dissolve epoxy, in a sealed jar, and then later opening the jar, it was found that the pressure inside had increased noticeably above atmospheric. My first thought that this was because the vapor pressure of methylene chloride at room temperature is higher than atmospheric. But if that were so, methylene chloride ought to spontaneously boil in any open container, which it does not. Any idea what is happening?

I have had similar experiences with acetone, straight out of the source can with no additional objects present in it.
methylene chloride boils at 39.6C, so if the reaction with epoxy is just a bit exothermic...
 

Online Bored chemist

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Why the increased pressure over certain liquids?
« Reply #4 on: 04/08/2011 19:51:52 »
Clifford has got the basic point.
The liquid has some vapour pressure- quite a lot with dichloromethane.
You put the liquid in the container, and the container has an atmosphere worth of air pressure in already. You close the lid quickly. The solvent evaporates and the pressure of the vapour adds to the pressure of the air.
 

Offline lightarrow

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Why the increased pressure over certain liquids?
« Reply #5 on: 07/08/2011 19:19:09 »
So the same should happen with, let's say, 10 drops of CH2Cl2 inside a 1L bottle which is immediately closed with its top? Interesting.
I don't have that solvent, maybe the OP could try.
 

Offline Atomic-S

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Why the increased pressure over certain liquids?
« Reply #6 on: 30/08/2011 05:10:17 »
I would suppose so. The explanation of vapor pressure that develops after the lid is closed sounds reasonable. I neglected to consider that a liquid, immediately after being poured into a container, is not necessarily in equilibrium with its environment.

As for observing this in acetone upon opening the source container: I guess I should have mentioned that the temperature at which this occurred was not determined, nor was the temperature at the time of the previous lid closing. I guess with anything as volatile as acetone, that needs to be taken into consideration. I imagine that the vapor pressure of liquids of this sort varies rapidly with temperature.
 

Offline Atomic-S

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Why the increased pressure over certain liquids?
« Reply #7 on: 30/08/2011 05:12:24 »
So the phenomenon in question must pertain to liquids whose vapor pressure at room temperature is less than atmospheric (otherwise it would be boiling), but still high enough to be a significant fraction of atmospheric.
 

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Why the increased pressure over certain liquids?
« Reply #7 on: 30/08/2011 05:12:24 »

 

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