Matter is made of tiny particles (atoms or molecules) that are in random and continuous motion.
Effects of Temperature on Molecular Motion
When temperature increases, the average kinetic energy of the molecules increases. This leads to increase in the average speed of the molecules, resulting in
the particles collide with the wall/other particles (if they are present) more vigorously and frequently.
the particles move faster and change direction of their motion more frequently.
In the case of temperature decreases, everything goes in reversal mode.
Shape and volume
For gas, it has no fixed shape and no fixed volume. Two factors are responsible for that. Molecules in gas have very weak inter-molecular bonds between them and they move about randomly at very high speed.
