Chemistry Reference
In-Depth Information
sublime
heat
(melt)
heat
(boil)
cool
(freeze)
cool
(condense)
solid
liquid
gas
Figure 1.10
Summary of the changes of state.
Heating and cooling curves
The graph shown in Figure 1.11 was drawn
by plotting the temperature of water as it was
heated steadily from −15 °C to 110 °C. You can
see from the curve that changes of state have
taken place. When the temperature was first
measured only ice was present. After a short time
the curve flattens, showing that even though
heat energy is being put in, the temperature
remains constant.
Figure 1.8
Liquid crystals are used in this TV screen.
An unusual change of state
There are a few substances that change directly from
a solid to a gas when they are heated without ever
becoming a liquid. This rapid spreading out of the
particles is called
sublimation
. Cooling causes a
change from a gas directly back to a solid. Examples
of substances that behave in this way are carbon
dioxide (Figure 1.9) and iodine.
110
100
liquid and
gas (liquid
water and
water
vapour)
all
gas
all
liquid
(liquid
water)
sublimation
0
15
all
solid
(ice)
solid and liquid
(ice and liquid
water)
solid
gas
sublimation
time/minutes
Figure 1.11
Graph of temperature against time for the change from ice
at −15 °C to water to steam.
In ice the particles of water are close together and are
attracted to one another. For ice to melt the particles
must obtain sufficient energy to overcome the forces
of attraction between the water particles to allow
relative movement to take place. This is where the
heat energy is going.
The temperature will begin to rise again only after
all the ice has melted. Generally, the heating curve
for a pure solid always stops rising at its melting
point and gives rise to a sharp melting point. A
sharp melting point indicates a pure sample. The
addition or presence of impurities lowers the
melting point. You can try to find the melting
point of a substance using the apparatus shown in
Figure 1.12.
Figure 1.9
Dry ice (solid carbon dioxide) sublimes on heating and can be
used to create special effects on stage.
Carbon dioxide is a white solid called dry ice at
temperatures below −78 °C. When heated to just
above −78 °C it changes into carbon dioxide gas. The
changes of state are summarised in Figure 1.10.
