Vacuum Pressure Vs. Boiling Point

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When chemists want to distill mixtures of compounds that might break down at higher temperatures, they make use of a technique called vacuum distillation because the boiling point of liquids decreases when you decrease the pressure by placing the liquid in a vacuum. At even lower pressures, materials will sublime rather than boil.

Vacuum

  • Liquids boil when their vapor pressure equals the pressure exerted on them by the surrounding atmosphere. This concept becomes easier to understand if you think about it in a different way. When a liquid boils, bubbles of vapor form and escape from the liquid, which can only take place if the bubbles push harder than the surrounding atmosphere pushes back on them. Consequently, when you decrease the pressure on the liquid, you decrease its boiling point.

Demonstration

  • With enough vacuum, you can actually cause water to boil at room temperature. This phenomenon is easier to demonstrate in a lab, but reproducing it at home is possible with luck and a hypodermic syringe. Verify that the syringe doesn't have a needle in it, then use it to suck up a little water and cap the tip. If you pull the plunger out rapidly, you can decrease the pressure to such an extent that the water will boil briefly.

Triple Point

  • If you take a liquid in a container and continue to decrease the pressure and temperature, eventually you'll reach what chemists call the triple point, which is a pressure and temperature at which liquid, solid and vapor all coexist in the same flask. For most compounds, the triple point is the minimum pressure at which the liquid form of the compound can exist. Decreasing the pressure even further will enable the compound to sublime or go directly from solid to gas without melting.

Water in Space

  • It wouldn't be an easy experiment to run, but the ultimate way to observe how the boiling point responds to decreasing pressure would be to open a bottle of water in space. The water would quickly begin to boil because the pressure is so low. Evaporating water absorbs heat, however, and space is extremely cold, so the remaining water would quickly begin to freeze. Ultimately, you would end up with a block of ice plus some drifting water vapor.

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