Water Boiling In A Vacuum

Atoms or molecules that make upwards a solid or a liquid are very close together. If you lower the pressure (in other words, make a vacuum) this means that in that location will be very few gas atoms or molecules near the surface of the solid (or liquid). This, in turn, makes information technology very unlikely for whatsoever molecule that leaves the surface to be replaced. So as you make the pressure lower, the 'stuff' you're dealing with will have a harder time coming together to make a solid (or a liquid).

Actually, beneath a certain pressure, you can't become a liquid at all. Beneath that pressure, if you cool a gas down, it volition do something called 'crystal lattice formation' and go straight from a gas to a solid. (The opposite of this, going from a solid to a gas, is chosen 'sublimation.')

The boiling bespeak for a liquid will as well drop at lower pressures. In fact, you lot can actually get a liquid to eddy at room temperature if you lot accept a vacuum. If you want to run into this for yourself, hither'due south an experiment you tin can try: go to a doc or veterinary's office and enquire if you can borrow a syringe (yous don't need a needle). Suck a bit of water into the syringe. Now comprehend the hole on the syringe with your finger (or a plastic cap if you take one) and pull back on the plunger equally difficult and equally fast every bit you can. This volition create a fractional vacuum inside the syringe, and with some luck you should be able to come across the water in it boiling.

To see the relationship betwixt the freezing/melting points and pressure, yous tin expect at a stage diagram:

(from http://invsee.asu.edu/ed/stage/phasefeat.htm#elevation)

In this picture, the line between the solid and liquid phases is the graph of the freezing point, and the line between the liquid and vapor phases is the graph of the boiling bespeak. Between the solid and vapor phases is the graph of the sublimation point, like I was describing before. ('nbp' and 'nfp' stand up for 'normal boiling point' and 'normal freezing point,' respectively. 'tp' stands for 'triple betoken,' and 'cp' stands for 'critical bespeak,' above which the liquid and gas phases sort of blur together.)

This graph is a general one, and the verbal location of each line is dissimilar for every chemical compound. Water is ane well-known exception, where the freezing point really goes up equally pressure decreases. This is because for water, the solid form (ice) actually takes up more space than the liquid (water). Only once you become beneath the betoken where you tin have a liquid, the sublimation betoken does get lower as the pressure drops (since the solid takes upwardly less space than the gas). Hither's a film of that graph:

(from http://www.sou.edu/chem/ch202/ch11/h2ophase.htm)