Question

What determines the state of matter of a substance at a given temperature?

Answer 1

1. We know that matter exists in three states-solid, liquid and gas. These are also called the three phases of matter. The most common substance that can exist in all three forms is water. The three states are ice-solid; water-liquid; and water vapour or steam-gas.
2. The combined effect of molecular motion and the force of molecular attraction determines the state of a substance at a particular temperature.
3. When the temperature is raised, molecular motion increases. This can overcome the molecular attraction.
4. Heating solid increases the vibration of molecules. At a certain temperature, the molecules start moving from their fixed positions.
5. The distance between them increases and they begin to flow. When this happens, the solid is changed into a liquid state.
6. On further heating, the molecules become more mobile and overcome the forces of molecular attraction completely.
7. They fly off from the surface in all directions. Now they are in the vapour form or gaseous state. When the vapour is cooled, the changes are reversed. Thus, heat causes a change of state in a substance.

Hence, the combined effect of molecular motion and the force of molecular attraction determines the state of a substance at a particular temperature