Table of Contents

The Science of Gases

The Vapour Pressure of Water

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If liquid water is left in an open container it will generally undergo a process
called evaporation.

This is because water molecules exhibit a range of molecular speeds, some of which are capable of leaving the liquid's surface and existing in the gas state (vapour).

The water molecules exert a gas pressure throughout the liquid, known as vapour pressure. Generally, only the molecules near the surface of the liquid will escape into the surrounding air.

The vapour pressure within a liquid depends upon the temperature of the liquid.

The graph above shows this relationship for water.

Notice from the graph that at 100°C the vapour pressure of water is 101.3 kPa, exactly equal to standard atmospheric pressure. At this temperature the vapour pressure of molecules (within the water) causes the rapid and often violent formation of bubbles within the water. This is the boiling process.

Boiling occurs whenever the vapour pressure of the molecules within the water equals (or exceeds) the local atmospheric pressure.

Normal body temperature for humans is about 37°C. One's blood is essentially salty water held at this temperature by regulatory metabolic processes. At this temperature the vapour pressure of water is about 6 kPa.

If an unprotected astronaut were to be suddenly exposed to a very low-pressure environment, below 6 kPa, the astronaut's blood would boil! The catastrophic formation of bubbles in the blood stream would lead to a rapid and painful death!

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