Chemistry Reference
In-Depth Information
●
Changes of state
The kinetic theory model can be used to explain
how a substance changes from one state to
another. If a solid is heated the particles vibrate
faster as they gain energy. This makes them 'push'
their neighbouring particles further away from
themselves. This causes an increase in the volume
of the solid, and the solid expands. Expansion has
taken place.
Eventually, the heat energy causes the forces
of attraction to weaken. The regular pattern of
the structure breaks down. The particles can now
move around each other. The solid has melted.
The temperature at which this takes place is
called the
melting point
of the substance. The
temperature of a pure melting solid will not rise
until it has all melted. When the substance has
become a liquid there are still very significant
forces of attraction between the particles, which is
why it is a liquid and not a gas.
Solids which have high melting points have
stronger forces of attraction between their particles
than those which have low melting points. A list of
some substances with their corresponding melting
and boiling points is shown in Table 1.1.
called the
boiling point
of the substance. At the
boiling point the pressure of the gas created above
the liquid equals that in the air -
atmospheric
pressure
.
Liquids with high boiling points have stronger
forces between their particles than liquids with low
boiling points.
When a gas is cooled the average energy of the
particles decreases and the particles move closer
together. The forces of attraction between the
particles now become significant and cause the gas
to
condense
into a liquid. When a liquid is cooled
it
freezes
to form a solid. In each of these changes
energy is given out.
Changes of state are examples of
physical changes
.
Whenever a physical change of state occurs, the
temperature remains constant during the change
(see Heating and cooling curves, p. 5). During a
physical change no new substance is formed.
An unusual state of matter
Liquid crystals
are an unusual state of matter
(Figure 1.7). These substances look like liquids
and flow like liquids but have some order in the
arrangement of the particles, and so in some ways
they behave like crystals.
Table 1.1
Substance
Melting point/°C
Boiling point/°C
Aluminium
661
2467
Ethanol
−117
79
Magnesium oxide
827
3627
Mercury
−30
357
Methane
−182
−164
Oxygen
−218
−183
Sodium chloride
801
1413
Sulfur
113
445
Water
0
100
If the liquid is heated the particles will move around
even faster as their average energy increases. Some
particles at the surface of the liquid have enough
energy to overcome the forces of attraction between
themselves and the other particles in the liquid and
they escape to form a gas. The liquid begins to
evaporate
as a gas is formed.
Eventually, a temperature is reached at which
the particles are trying to escape from the liquid so
quickly that bubbles of gas actually start to form
inside the bulk of the liquid. This temperature is
Figure 1.7
A polarised light micrograph of liquid crystals.
Liquid crystals are now part of our everyday
life. They are widely used in displays for digital
watches, calculators and lap-top computers, and
in televisions (Figure 1.8). They are also useful
in thermometers because liquid crystals change
colour as the temperature rises and falls.
