Download Molecules in gases

Molecules in gases
Solids
In a solid each molecule vibrates about a mean position. The hotter the solid the more
violently the molecules vibrate.
cold
hot
Gases
In a gas the molecules are all moving about randomly and at high speed. At room
temperature they travel at around 400 m/s. The molecules must be moving faster than
the speed of sound because it is just these moving molecules that transmit sound.
Heating the gas makes the molecules move around more violently while cooling the
gas makes the molecules slow down.
cold
hot
If you keep cooling the gas the molecules move slower and slower and have less and
less kinetic energy. In the end they would stop – zero kinetic energy and so zero
temperature. This temperature is called ABSOLUTE ZERO and it value is –273oC.
We do not believe that it is possible to reach absolute zero although scientists have
got close to this, down to 0.000 001 oC above that temperature.
As the molecules move around they collide with both each other and any solid object.
If the gas is in a container the molecules collide with the walls of the container at high
speed. It is these collisions that result in the pressure of the gas
If you compress the gas the molecules hit the walls of the container more often – this
means an increase in pressure.
If you heat the gas the molecules collide with the walls more violently – this also
increases the gas pressure.
The reverse is also true – expansion and cooling result in a decrease in gas pressure.
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Evidence for molecular motion
One of the best ways of showing that air
molecules are moving is known as
Brownian motion. Some smoke is blown
into a small glass pot containing air and
then viewed through a microscope. The air
molecules collide with much larger smoke
particles and if a strong light is shone on
the box the smoke particles can be seen.
They judder around as they are hit from all
sides by the much smaller and invisible air
molecules. The particles can be seen
because light is scattered from them and
they look like tiny stars.
Smoke particle
Collision with
an air molecule
(Compare with the polystyrene ball in the kinetic theory model apparatus using small
ball bearings to represent air molecules).
If some bromine is released into a tube of air the colour of the bromine gas can be
seen spreading slowly through the air – this is diffusion. The air molecules affect the
movement of the bromine molecules. If the experiment is repeated in an evacuated
tube the spread of colour is virtually instantaneous.
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