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At any given temperature, all substances exist in one of the

three states of matter.

What are the three states of matter?

solid liquid gas


Changes of state

Each change of state has a special name:

melting evaporation

freezing condensation

ice water steam


Sublimation

Some substances sublime. This means that they go straight

from solid to gas without passing through the liquid state.

solid gas

sublimation

Substances that sublime include:

iodine

carbon dioxide (dry ice)

plug-in air fresheners

ink in printers and photocopiers

comets as they approach the Sun.


Melting point and boiling point

The temperature at which a substance

changes from a solid to a liquid is called

its melting point (or freezing point).

What state is water at 25°C, 300°C and 100°C?

What is the melting point of pure water?

The temperature at which a

substance changes from a liquid

to a gas is called its boiling point.

What is the boiling point of pure water?


Reducing the freezing point of water

Salt reduces the freezing point of water. This means that the

temperature has to be much colder than 0°C for it to freeze.

Can you explain why people

put salt on roads and paths

during icy weather?

In what other situations would it

be useful to reduce the freezing

point of water? How about in

making ice-cream?


Particles

The differences between solids, liquids and gases can be

explained by looking at the particles.

All substances are made up

of particles.

The particles are attracted to

each other. Some particles

are attracted strongly to each

other, and others weakly.

The particles move around. They are

described as having kinetic energy.

The kinetic energy of the particles

increases with temperature.


What are the properties of solids?

Solids:

have a high density, as the particles are packed very

closely together

cannot be compressed because there is very little empty

space between particles

have a fixed shape

because the particles

are held tightly together

cannot diffuse because

the particles are not

able to move.


What are the properties of liquids?

Liquids:

have a fairly high density because the particles are close

together

cannot be compressed because there is very little empty

space between particles

take up the shape of its

container because the

particles can move

can diffuse because the

particles are able to

change places.


What are the properties of gases?

Gases:

have a low density because the particles are spaced far

apart

can be compressed because there is space between

particles

have no fixed shape

because the particles

move about rapidly in

all directions

can diffuse because

the particles are able to

move in all directions.


How do smells spread out?


What is diffusion?

Diffusion is the random movement of particles. When particles

diffuse, they spread out and mix with other particles.

For example, the

smell of aftershave

or perfume can be

detected by people

on the other side of

a room because the

perfume particles

diffuse in the air.

Can you think of some more everyday examples of diffusion?


Diffusion in solids

The particles in liquids and gases diffuse easily because they

are free to move around. However, it is possible for solids to

diffuse as well.

Scientists have found that if they put two metals

next to each other and leave them for a long

time, eventually a few particles of one metal

will diffuse into the other.

This is because the particles in a solid vibrate,

and some of them have enough energy to

move around a little bit as well. Eventually, a

few particles move around enough to diffuse

into the other metal.


Diffusion in liquids and gases

Which will diffuse faster, a gas or a liquid?

The particles of a gas will diffuse faster. Why?

The gas particles are moving around faster than the particles

in a liquid because they have more energy. This means that

the gas particles will spread out more quickly.

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