vacuum

[dibble1]

I don't remember being taught this but apparently liquids boil in a vacuum. Does this generate heat? If so why can't we harness this heat?

[Clanad]

The "boiling" you describe by the liquid in a vacuum is just that... not boiling in the usual sense of a liquid being heated to it's boiling point (which, in itself, is depenent on the surrounding air pressure) but only the air coming out of solution because of the reduced air pressure... which maintained it in solution in the first place. There is no heat produced... in fact there's probably a slight cooling effect due to the energy release... but that's just a guess...

[vascop]

Also you would need to PROVIDE energy to create the vacuum in the first place!

[chakka35]

Clanad, surely it isn't just the air coming out of solution; it is the liquid itself evaporating, which, as you rightly say, happens at lower temperatures the lower the pressure.

Don't they say that you can't make a decent cup of tea (British style) on a high mountain because the water boils at too low a temperature to infuse the tea properly? (You Americans have no problem because you like your tea weak, having just hung a teabag in barely-hot water for a few seconds.)

[beso]

Thie boiling deos not cause just a slight cooling. An enormous cooling. Boiling a liquid under reduced pressure is exactly how refrigeration works.

Much the same heat is absorbed by the liquid to be come a gas as the amount of heat required to boil it at room temperature. It represents the energy embodiied in the intermolecular forces caused by the electrostatic attraction between molecules. These bonds must be torn apart to turn the liquid into a gas.

This energy is known as the latent heat of vaporisation of the substance. A similar concept applies to melting and is known as the latent heat of fusion.

Water has strongly polarised molecules and they stick toegther like the electrostatic version of a pile of magnets. Other substances such as some hydrocarbons and ammonia have a very large latent heat of vaporisation which is why they are used as refrigerants.

[Old_Geezer]

Boyle's law.

Just because you get a substance trying to turn into a gas doesn't mean you have gained extra energy.

What energy does take to create the vacuum in the first place ?

[beso]

I tried tea in Ecuador at 2800 metres and it is definitely impossible to make a good cuppa there. And I like it weak. The problem is that it was always cold by the time it was poured in the cup because it never was hot in the jug.

[beso]

The process of boiling in iteself It isn't an application of Boyle's Law despite the guy's name.

Boyle's Law refers to the ideal gas rather than change of state.
Pressure by Volume is proportional to Temperature. The vacuum pump reduces the the pressure of the air above the water and temperature drops according to Boyle's Law but this energy is tiny compared to the process of boiling the liquid itself.

It takes five times as much energy to boil water as is required to raise its temperature from zero to 100 degress Celcius. The energy involved in changing the temperature of a litre of air by the same temperature is another five thousand times smaller.

[Old_Geezer]

Ah I thought my memory of physics lessons wasn't too far out. You reduce the pressure the volume increases, so things turn into a gas. without putting in or removing energy; or something like that. Given a bit of slack or so. :-)

[dibble1]

You're all great. Thanks.

[beso]

When a liquid turns into a gas it always involves energy as those intermolecular bonds break.

The reason the liquid turns to gas under a vacuum is due to the molecules no longer being confined closely together by the pressure. Allowed to move apart they can escape from each other.