Chemical reactions involve the breaking and forming of chemical bonds between atoms. During a chemical reaction, the total mass of the reactants is always equal to the total mass of the products. This is known as the law of conservation of mass. For example, when hydrogen gas (H2) reacts with oxygen gas (O2) to form water (H2O), the total mass of the hydrogen and oxygen atoms in the reactants is equal to the total mass of the water molecules in the products. This means that no atoms are lost or gained during the reaction. Chemical reactions can be represented using chemical equations, where reactants are written on the left side of the arrow and products are written on the right side. Coefficients are used to balance the equation and ensure that the law of conservation of mass is obeyed. Understanding chemical reactions and the law of conservation of mass is essential in chemistry, as it allows us to predict the outcome of reactions and calculate the amount of products formed. By applying these principles, chemists are able to design new materials, drugs, and technologies that benefit society.