 In this video we're going to look at how to balance chemical equations, and while we're doing that we'll discuss one of the primary laws of chemistry, and indeed the universe, the law of conservation of mass. The law of conservation of mass states that matter can neither be created nor destroyed. It can be changed from one form into another, and mixtures can be separated or made, and pure substances can be decomposed into other pure substances, but the total amount of mass remains constant. This means that whenever a chemical reaction occurs the total mass of the reactants must equal the total mass of the products, since mass cannot be gained or lost. And the reason the mass remains constant is that the atoms cannot be created or destroyed, only rearranged. So another way of looking at this, a way that helps us in balancing equations, is that the number and type of atoms in the reactants, which are written on the left of an equation, must be identical to the number and type of atoms in the products, which are written on the right of the equation. All that can happen is that the atoms are rearranged. This law came about from the work of many early chemists, but it was the Frenchman Antoine Lavoisier, along with his wife Marianne, who most conclusively showed it to be true in the late 1700s. And it was only by some excruciatingly precise and careful lab work and the use of the best technology of the day, in particular accurate balances, that they were able to prove this law conclusively.