1. Thermal expansion of solid, liquid and gas
-Matter in general expands when heated. Solid Expansion: The expansion of a solid when heated is small. Its molecules vibrate faster about their fixed positions and move slightly further apart than when they are cold.

2. Liquids expansion: liquid particles are usually less tightly bound to each other molecules, they generally move further than solid particles when heated. The expansion in liquid is almost directly to the temperature rise.

3. Gas expansion: Gas molecules are far apart and weakly attracted to each other. Heat causes the molecules to move faster and the volume increases much more than solids and liquids. Gas do not have to expand if the volume is fixed.

4. When gas is heated, the molecules move faster
When gas is heated, the molecules move faster. The faster the molecules the more collisions with the container wall. These cause the pressure to increase which increases the volume too. Now, the collisions are less and pressure decreases. The volume stops expanding when the internal pressure equals to the original external pressure.