Chemistry is the science of the composition, structure and properties of matters and the interaction between them. The study of the subject often involves understanding of the
material substances behavioral change at an atomic level. To be precise, it is the study of the interaction behavior of electrons at the outer part of the electronic orbital
of an atom.
A chemical reaction is the interaction between substances that result in a permanent change to the substances involved (reactants). The change and the subsequent product
obtained cannot be reversed by any physical means.
For example: mixing iron powder and sulfur powder at room temperature is
a physical process. The mixture can be separated into their original constituent without any behavioral change. This can be achieved by using a magnet and the iron will be attracted towards the magnet, leaving
the sulfur behind. However, a chemical reaction will take place when the mixture is burnt. A new product, iron sulfide, is formed and the iron component, which has lost its magnetic property in its original form,
no longer be able to separate by a magnet. Note that a new substance is formed as a result of chemical reaction. It is no longer being treated as a mixture.
Examples of chemical reactions and 'non-chemical' or physical processes in everyday life: