 In this video, we examine the historical and present-day significance of the law of conservation of mass. In the 18th century, scientists thought that when things burn, a substance called phlogiston came out of them. Then, experiments in closed vessels where substances could be accurately weighed began to help early scientists, such as Lavoisier, understand that when things burn, oxygen is added. He realized that matter could be changed but not destroyed. In 1789, he established the law of conservation of mass. In chemical reactions, no matter is lost or gained. These experiments on combining weights also showed that elements always combined in fixed ratios, the law of definite proportions and constant composition. And they led Dalton to publish his atomic theory in 1808. Modern chemistry had started. By mid-19th century, only half the elements had been discovered but enough to enable Mendeleev to produce his famous periodic table. That was in 1869. Chemical equations began to be written as we now know them from about this time. For example, the burning of magnesium. Ah, this needs balancing. When we balance an equation, we are simply obeying the law of conservation of mass. All the atoms in the reactants must be accounted for in the products. It's easy to see this if we draw out the atoms, rearranging themselves to form the products. You can see clearly that no atoms are gained or lost during a reaction, so the mass is conserved. Here is another example. A reaction you'll see every time you use a gas cooker which uses natural gas. But is this reaction balanced? Pause the video and count the carbon, hydrogen and oxygen atoms on each side. It is easy to count if we draw out the molecules as models. Carbon, one each side. Oxygen, two to start with, but now three. Hydrogen, four to start with, but now only two. To balance the hydrogens, we need to add two more hydrogens on the right. To do this, we add another water molecule. Pause and count. Is it balanced now? Well, the carbon and the hydrogen balance, but we have four oxygens in the product, but only two in the reactants. So we add another oxygen molecule to the reactants. Now we have conserved mass. In summary, the law of conservation of mass is simply saying that during chemical change there is no loss or gain of atoms. It is for this reason that we always balance chemical equations. Until you are really familiar with using formulae, it's easier to draw out the molecules as models to enable you to account for all the atoms.