The origin of saturated vapour pressure
The evaporation of a liquid
The average energy of the particles in a liquid is governed by
the temperature. The higher the temperature, the higher the average
energy. But within that average, some particles have energies higher
than the average, and others have energies lower than the average.
Some of the more energetic particles on the surface of the liquid can
be moving fast enough to escape from the attractive forces holding the
liquid together. They evaporate.
The diagram shows a small region of a liquid near its surface.
Notice that evaporation only takes place on the surface of the
liquid. That's quite different from boiling which happens when there is
enough energy to disrupt the attractive forces throughout the liquid.
That's why, if you look at boiling water, you see bubbles of gas being
formed all the way through the liquid.
If you look at water which is just evaporating in the sun, you don't
see any bubbles. Water molecules are simply breaking away from the
surface layer.
Eventually, the water will all evaporate in this way. The energy
which is lost as the particles evaporate is replaced from the
surroundings. As the molecules in the water jostle with each other, new
molecules will gain enough energy to escape from the surface.
The evaporation of a liquid in a closed container
Now imagine what happens if the liquid is in a closed container.
Common sense tells you that water in a sealed bottle doesn't seem to
evaporate - or at least, it doesn't disappear over time.
But there is constant evaporation from the surface. Particles
continue to break away from the surface of the liquid - but this time
they are trapped in the space above the liquid.
As the gaseous particles bounce around, some of them will hit the
surface of the liquid again, and be trapped there. There will rapidly
be an equilibrium set up in which the number of particles leaving the
surface is exactly balanced by the number rejoining it.
In this equilibrium, there will be a fixed number of the gaseous particles in the space above the liquid.
When these particles hit the walls of the container, they exert a pressure. This pressure is called the saturated vapour pressure (also known as saturation vapour pressure) of the liquid.
Measuring the saturated vapour pressure
It isn't difficult to show the existence of this saturated vapour
pressure (and to measure it) using a simple piece of apparatus.
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If you have a mercury barometer tube in a trough of mercury, at 1
atmosphere pressure the column will be 760 mm tall. 1 atmosphere is
sometimes quoted as 760 mmHg ("millimetres of mercury").
If you squirt a few drops of liquid into the tube, it will rise to
form a thin layer floating on top of the mercury. Some of the liquid
will evaporate and you will get the equilibrium we've just been talking
about - provided there is still some liquid on top of the mercury. It
is only an equilibrium if both liquid and vapour are present.
The saturated vapour pressure of the liquid will force the mercury
level down a bit. You can measure the drop - and this gives a value for
the saturated vapour pressure of the liquid at this temperature. In
this case, the mercury has been forced down by a distance of 760 - 630
mm. The saturated vapour pressure of this liquid at the temperature of
the experiment is 130 mmHg.
You could convert this into proper SI units (pascals) if you wanted to. 760 mmHg is equivalent to 101325 Pa.
A value of 130 mmHg is quite a high vapour pressure if we are
talking about room temperature. Water's saturated vapour pressure is
about 20 mmHg at this temperature. A high vapour pressure means that
the liquid must be volatile - molecules escape from its surface
relatively easily, and aren't very good at sticking back on again
either.
That will result in larger numbers of them in the gas state once equilibrium is reached.
The liquid in the example must have significantly weaker intermolecular forces than water.
The variation of saturated vapour pressure with temperature
The effect of temperature on the equilibrium between liquid and vapour
You can look at this in two ways.
There is a common sense way. If you increase the temperature, you
are increasing the average energy of the particles present. That means
that more of them are likely to have enough energy to escape from the
surface of the liquid. That will tend to increase the saturated vapour
pressure. |
