What is meant by a chemical reaction?

A chemical reaction is process by which one set of chemical substances is transformed to another. There are different types of chemical reactions such as:
Acid-base Reactions
Precipitation Reactions
Synthesis Reactions
Oxidation-Reduction Reactions
Displacement Reactions
Decomposition Reactions
Substances which chemically react are called reactants and the newly formed substances are called products.
A French chemist, Antoine Lavoisier, who is known as the father of modern chemistry, changed chemistry from a qualitative to a quantitative science. He proved that the mass of the products in a chemical reaction is equal to the mass of the reactants. There are no more atoms at the end of the chemical reaction than there were at the beginning.
The Law of Conservation of Mass states that matter can neither be created nor destroyed in a chemical reaction.


Consider the formation of water molecule. Hydrogen combines with oxygen to form a water molecule.