You may have been learnt that all particles of matter are always in constant motion. The average speed of particles depends on which state that the object exists. Under the same conditions, gas molecules are faster than that of liquids. Atoms in solids are in constant vibration but cannot move freely within the bulk.

When a substance is heated up, its partcles start to move more rapidly (increase the average kinetic energy) provided that no change in the state of matter (solid to liquid, or liquid to gas) takes place. If there is change in state of matter, the heat supplied to the substance breaks the bond between molecules or atoms by then increasing their potential energy. You can see that in both cases, heating a substance is adding kinetic or potential energy to it. This shows that heat is a form of energy, and it is therefore measured in joules (J).

Heating a substance, which does not undergo a phase change, increases its temperature If two objects at different temperatures are brought in contact, energy flows from the hotter object to the cooler one. This energy in transit is called heat. Thus, heat is defined as a form of energy flowing between two objects, which are in thermal contact, due to the difference in their temperatures.

Key Terms

• Thermal contact Two objects are said to be in thermal contact if they can exchange heat energy, provided that they may or may not be in physical contact.