When two chemicals react with each other, they form new substances. The composition of substance changes during chemical change. This study these reactions and stated some laws of chemical combination. First one, law of conservation of matter. In a chemical reaction, the total weight of the reactants is same as the total weight of the products formed due to chemical reactions. Example, reactants are copper sulfate and sodium hydroxide. The total mass of the reactants is 60 gram and after completion of the reaction, copper hydroxide and sodium sulfate will form. The total mass of the products will be 60 gram. It means there is no loss or gain of mass during chemical reaction. Second, law of constant proportion. To understand this law, first we have to revise. How compound is formed? When two or more elements combine or react chemical with each other, new substance will be formed. Mostly, they have totally different properties than the constituent elements. For the formation of new compounds, we have to understand certain rules. Suppose we want to prepare carbon dioxide from carbon and oxygen. Then we will have to take 12 gram of carbon and 32 gram of oxygen. We cannot add two elements in any proportion we want. The proportion by weight of carbon to oxygen while making carbon dioxide should always be 3 is to 8. The law states that the proportion by weight of the constituent element in the various samples of the compound is fixed. This law was stated by friend scientist J. L. Proust. One more important thing which we have to understand is that though the source of the compound is different, the proportion of the constituent elements is always fixed. Suppose we are getting water from different sources as a product of different reactions. And then the proportion by weight of hydrogen to oxygen in water is always 1 is to 8.