This document describes changes of state between solids, liquids, and gases. It explains that: - Solids melt when heated and particles gain enough energy to overcome bonding forces, becoming a liquid. Different substances have different melting points depending on bonding strength. - Gases condense when cooled, as slower particle motion allows intermolecular forces to attract particles together into a liquid. - Liquids evaporate when heated, as faster particle motion overcomes bonding forces and particles escape as a gas. Evaporation leads to boiling when it occurs throughout the liquid. - The boiling point of a liquid depends on surrounding air pressure, with lower pressure resulting in a lower boiling point.

1. 3.10 All change
30 January 2015
Why can’t I get a
good cup of tea
up here on
Everest?

2. Aims
• Describe what happens when solids melt and
liquids evaporate
• Explain what happens when gases condense
and liquids freeze

3. Changes of state
• Almost all substances
can exist as solids,
liquids and gases at
different temperatures
• We live in England
where water is usually
a liquid but if we lived
in Antarctica it would
usually be a solid. It
can also be a gas.

4. Melting
• Strong forces join the
particles in a solid together
• The particles can only
vibrate and they cannot
move around
• When a solid is heated up,
its particles vibrate so
much that some of the
forces are overcome
• The particles can now
move around each other
• Melting has happened to
turn the solid into a liquid

7. Melting point
• The temperature at
which something changes
from a solid to a liquid is
called its melting point
• Different substances have
different melting points
• Ice melts at 0oC but iron
melts at 1583oC
• The stronger forces
holding the particles
together, the higher the
melting point

8. Condensing
• When a gas is cooled,
its particles move
around more slowly
• If the gas gets cold
enough, its particles
move slowly enough
for forces to attract
them together again
• Condensing has
happened to turn the
gas into a liquid

11. Evaporating
• When a liquid is heated
up, its particles move
around more quickly
• Some particles move
around so much that
almost all of the forces
between them are
overcome
• The separate particles
escape from the surface
of the liquid
• Evaporating has
happened to turn some
of the liquid into gas

12. • The more the liquid is
heated, the faster
evaporating happens
• Boiling happens when
evaporating takes place
throughout the liquid
• The temperature at
which something boils is
called its boiling point
• The boiling point of a
liquid depends on the
pressure of the air
around it
• The lower the pressure,
the lower the boiling
point

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Questions
1. The melting point of ice is 0°C. In what
state is it:
a. at 25°C?
b. at –10°C?
2. Explain what these words mean: melting,
evaporating, condensing, freezing.
3. Air contains water vapour. Water vapour is
separate particles of water. Why do droplets
of water form on cold windows?
(Level 3)
(Level 3)
(Level 4)
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Questions
4. If you put wet washing outside on a washing
line, it soon dries. Martin thought that this
was because the heat from the Sun boiled the
water in the clothes. Was he right or wrong?
Explain your answer.
5. The boiling point of a substance is always
higher than its melting point. Explain this
observation using the idea of particles and
the forces between them.
6. Iodine is unusual. It turns directly from a
solid into a gas when warmed up. Suggest
why this happens. Use the idea of particles
and the forces between them in your answer.
(Level 5)
(Level 6)
(Level 7)
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Questions
7. Instant coffee is made by ‘freeze drying’ it
at the factory. Instead of heating the coffee
to boil off the water, it is put under very low
air pressure and the water rapidly escapes.
a. Explain why the water escapes rapidly
from the coffee even though it is not at
100°C.
b. Suggest why the coffee may actually freeze
during this process.
(Level 7)
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For your notes
 Forces between particles are overcome
when something melts or boils.
 The stronger the forces, the higher the
melting point and the boiling point.
 Liquids boil at lower temperatures when
under lower pressure.
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