CHEMISTRY              LAWS OF CHEMICAL COMBINATION     Ever wonder how and why substances react? The substances react on the basis of these laws. • THE LAW OF CONSERVATION OF MASS This law states that mass can neither be created nor be destroyed in any ordinary chemical reaction. While considering a chemical change this would mean that the total mass of all the reactants in a chemical reaction is equal to the total mass of the products in it. E.g. Q- Hydrogen + Oxygen = Water         4gms + ? = 36gms     Ans- using the law of conservation of mass we find that the total mass of water is equal to the mass of the reactants i. e. the sum of the mass of hydrogen and mass of oxygen. Therefore,     Mass of oxygen = 36gms – 4gms = 32gms Thus, 32gms of oxygen was used.     • THE LAW OF CONSTANT PROPORTIONS The law states that every pure substance always contains the same elements combined in the same proportions by weight. This also means that any compound always contains the same elements combined in the same proportions by weight. This is evident from the fact that water taken from anywhere will always contain hydrogen and oxygen in the ratio of 1:8 by mass.     • THE LAW OF MULTIPLE PROPORTIONS The law states that if two elements combine to form two or more than two compounds then the ratio of the masse of the second element which combines with the fixed mass of the first element will be a ratio of small whole numbers. Let us suppose that two elements; say element A and element B combine to form two or more than two compounds and different masses of element B always combine with a fixed mass of the element A to form them. In every case, the ratio in which element B combines with element A will be a ratio of a small whole number.