Chemistry Reference
In-Depth Information
All gases diffuse to fi ll the space available. In
Figure 1.13, after a day the brown-red fumes of
gaseous bromine have spread evenly throughout
both gas jars from the liquid present in the lower
gas jar.
thermometer
melting point
tube
rubber band
oil
solid
heat
Figure 1.12
Apparatus shown here if heated slowly can be used
to fi nd the melting point of a substance such as the solid in the melting
point tube.
In the same way, if you want to boil a liquid such
as water you have to give it some extra energy. This
can be seen on the graph (Figure 1.11) where the
curve levels out at 100 °C - the boiling point of
water.
Solids and liquids can be identifi ed from their
characteristic melting and boiling points.
The reverse processes of condensing and freezing
occur on cooling. This time, however, energy is given
out when the gas condenses to the liquid and the
liquid freezes to give the solid.
Figure 1.13
After 24 hours the bromine fumes have diffused throughout
both gas jars.
Gases diffuse at different rates. If one piece of
cotton wool is soaked in concentrated ammonia
solution and another is soaked in concentrated
hydrochloric acid and these are put at opposite
ends of a dry glass tube, then after a few minutes
a white cloud of ammonium chloride appears
(Figure 1.14). This shows the position at which
the two gases meet and react. The white cloud
forms in the position shown because the ammonia
particles are lighter and have a smaller relative
molecular mass (Chapter 4, p. 62) than the
hydrogen chloride particles (released from the
hydrochloric acid) and so move faster.
Questions
1
Write down as many uses as you can for liquid crystals.
2
Why do gases expand more than solids for the same
increase in temperature?
3
Ice on a car windscreen will disappear as you drive
along, even without the heater on. Explain why this
happens.
4
When salt is placed on ice the ice melts. Explain why.
5
Draw and label the graph you would expect to produce if
water at 100 °C was allowed to cool to −5 °C.
Diffusion also takes place in liquids (Figure 1.15)
but it is a much slower process than in gases. This
is because the particles of a liquid move much more
slowly.
When diffusion takes place between a liquid and a gas
it is known as
intimate mixing
. The kinetic theory can
be used to explain this process. It states that collisions
are taking place randomly between particles in a liquid
or a gas and that there is suffi cient space between the
particles of one substance for the particles of the other
substance to move into.
●
Diffusion - evidence for
moving particles
When you walk past a cosmetics counter in a
department store you can usually smell the perfumes.
For this to happen gas particles must be leaving open
perfume bottles and be spreading out through the
air in the store. This spreading out of a gas is called
diffusion
and it takes place in a haphazard and
random way.
