How to To Demonstrate That the Boiling Point of a Liquid Depends on the Pressure

A light layer of vacuum grease is applied to the rim of the belljar. Water at room temperature is placed inside and the vacuum pump is then used to evacuate the vessel. When the air pressure is reduced to the vapour pressure of water at room temperature the water will begin to boil.

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    9 Discussions


    6 years ago on Introduction

    when I was in Jr high we had all of this in our science class. because of costs of insurance and lawyers fees they removed most of the equipment. and after a while the teachers saw no reason for any real science . It is no wonder our schools are failing. Your project and others like it are what can inspire others to greatness. if we can by pass those who are afraid of lawyers and accountants. Good job.

    I've had another theory about the pressure is lowered the water boils thing, that the water has oxygen in it, so that oxygen is being sucked out by the vacuum.

    4 replies

    This is actually what happens if the pump pulls some vacuum but not enough to cause the water to boil.  So a lot of people think they have caused water to boil but only removed dissolved gases.

    However, if you have a good enough pump, you will eventually reduce the pressure to the point where water actually boils.

    Brake bleeder hand pumps pull enough vacuum for the former, but not the latter.

    It appears that the video was made using high end equipment capable of actually boiling water though.

    wupmeCallum Snowden

    Reply 9 years ago on Introduction

    No its actually the oposite of a pressure cooker.
    While the pressure cookier raises the boiling point with overpressur, the vakuum lowers it with underpressur.

    What you see there ain't oxygen, it is realy water vaporizing because it reached its boiling point.
    A nice proof of this is a restaurant i know in the Alps (Muechner Haus on the Zugspitze).
    They have a lower air pressure because they are in about 3kilometers height. Because of that the kitchen HAS to use presure cooking technique to prepare food. Otherwise the water would boil at a way to low tempreature and food, like potatoes, would take ages to cook.


    9 years ago on Introduction

    If you leep "raising" the vacuum at some point the water will stop to boil and freeze because the boiling took so much Energy in form of heat.


    11 years ago on Introduction

    Smart, could use this in school and stuff, this could help many teachers on Instructables, they could show this to children in class, nice work!