A02    Describe how matter and energy interact to form different states and structures, and how that interaction affects thermal expansion and stiffness of solids.

Matter can be found as gases, liquids and solids. In the solid state the atoms are either poorly organized (amorphous) or well organized (crystalline). By going from the gaseous state to the liquid state, and from the liquid state to the solid state, the interaction between the individual atoms increases. The stronger these interactions are, the more difficult it is to separate the atoms. As it becomes more difficult to separate the atoms, the stiffer the material becomes. At the same time, the thermal expansion decreases as more thermal energy is needed to separate the atoms a certain distance.