2. Observations on the three states of matter and
how they change can be explained by considering that
matter is made of particles. This is called the ‘particle
theory of matter’.

3. In solids, strong forces hold the particles together
in a three-dimensional structure. In many solids, the
particles form an orderly arrangement called a lattice.
The particles in all solids move a little. They do not
change position but vibrate to and fro about one
position.

4. In liquids, the forces that hold the particles in a
liquid can change position by sliding over each other.
When it is in container, it takes up the shape of the
container. It also vibrates a little bit faster then the
particles in solids.

5. Gas
In gases, the forces of attraction
between the particles are very small
and the particles can move away from
each other and travel in all directions.
When they hit each other or the
surface of their container they bounce
and changes direction.

6. Changes in Solid
When solid is heated it expands and when it cools
it contract. For example, when it is cool a metal bar fits
inside a metal gauge. Its end will also fit through the
hole in the gauge or in the hole. When the bar cools
down it will fit in the gauge and hole again.

7. Changes in Liquid
When liquids are heated they generally expand
much more than solids for a given temperature rise.
They also contract to their original volume when they
are cooled. The expansion and contraction of a liquid
can be demonstrated by setting up the apparatus.
When the water gets hot, it expands and rises up the
glass tube. When the water is allowed to cool the level
of water in the tube falls.

8. Changes in Gases
Like most liquids and solids, gases expand when
they are heated and contract when they are cooled. If a
gas and a liquid undergo the same temperature rise the
gas expands about ten times more than the liquid.