Matter makes up all the things in the universe. It occupies space & has mass.Matter is made up of tiny particles called atoms.Usually,an atom does not exist independently.Therefore,atoms combine to form molecules.The atoms or molecules are tightly packed in solids & loosely packed in liquids & gases.These particles have random motion in gases.
Kinetic molecular theory of matter : The theory that enables us to understand the behaviour of molecules in solids,liquids & gases is called kinetic molecular theory.The main postulates of the kinetic molecular theory of matter are as follows:
1) Matter is composed of molecules.The molecules of a given substance are identical in all respects.For example, all the molecules of water are identical in terms of their chemical properties.
2) Molecules in matter constantly move.Due to this movement,molecules possess energy called kinetic energy.The kinetic energy of molecules of matter changes when it is heated or cooled.
3) There are empty spaces between molecules of matter.The empty spaces between different molecules are called inter-particle or intermolecular spaces.The empty spaces between molecules get larger as a sample of matter changes from the solid state to the liquid & gaseous state.For example,the intermolecular space between the molecules in solids is the least in comparison to liquid & gases.The intermolecular space between the molecules in gases is the largest.
4) There are attractive forces between the molecules of a substance.The force of attraction between molecules is called inter-particle or intermolecular force of attraction.The force of attraction becomes stronger as the molecules move closer to each other.As the intermolecular space between molecules decreases,the intermolecular force of attraction increases & vice versa.The intermolecular force is the strongest in solids as molecules in solids are closely packed.It is the weakest in gases as intermolecular force in gases is the largest.The force of attraction between the molecules of same kind is called cohesive force.For example,water molecules stick to each other to form a few drop because of cohesive force between them.The force of attraction between molecules of different kind is called adhesive force.For example,water drops cling to glassware because of adhesive force between molecules of water & glass.
5) The molecules of a substance collide with other.In the gaseous & liquid states ,the molecules also collide with the walls of the container in which the substance is kept.The collision between the molecules of the container exert pressure on the walls of the container.
6) The molecules of matter are in motion.In solids,they only vibrate.In liquids & gases they move randomly & thus possess kinetic energy.As the temperature of a substance increases,it’s molecules move faster & thus the kinetic energy of the molecules increases.As the temperature of a substance decreases,it’s molecules move slower & thus the kinetic energy of the molecule reduces.For example,when water is boiled,it’s molecules move faster & thus their kinetic energy increases. When water is cooled ,it’s molecules move slower & their kinetic energy decreases.