The Law of Conservation of Mass was established as a basic rule of chemistry from Antoine Lavoisier’s work on combustion. The Law of Conservation of Mass says that matter may be changed from one form to another but cannot be created or destroyed.
Lavoisier heated mercury to create mercuric oxide. He proved that oxygen is an element and he proved the Law of Conservation of Mass with this experiment. He determined that the loss of weight from the air during the reaction was equal to the weight gained by the mercury. Heating the mercuric oxide formed resulted in the separation of the mercury and the oxygen. Again the weight of the mercuric oxide was equal to the weight of the mercury and the oxygen gas. Lavoisier came to the same conclusion with every substance he burned; he burned charcoal, sulfur, phosphorus, and many metals.
The snacks we eat vanish but then they become extra pounds on our frame. As the food we eat is digested, molecules are broken down into sugars and fats providing energy and new compounds to the body. Gasoline burns in the car's cylinders producing energy to move the vehicle forward. Internal combustion uses the carbon and hydrogen in the gasoline to react with oxygen in the air to form water, carbon dioxide, and carbon monoxide. The product gases are dispersed into the atmosphere. In both these cases the atoms in fuels have been rearranged into new molecules, but matter was not destroyed.
Sometimes substances can be reclaimed or recycled. Aluminum and paper are recycled to make new aluminum cans and new reams of paper. If the individual atoms are counted before and after the chemical reaction, the number of atoms remains the same. Balanced chemical equations illustrate this principle. Scientists interested in global warming can calculate how much water, carbon dioxide, and carbon monoxide are dispersed into the air with each gallon of gasoline burned. They do the same thing when analyzing how many tons of particulates are put into the air when coal is burned to create electricity. Even though the coal is burned up, the mass can be accounted for in air pollution.
When solutions of potassium iodide and copper nitrate are combined, a solution of potassium nitrate and a precipitate of copper iodide are formed. If the original amounts of potassium iodide and copper nitrate are weighed before they are dissolved in water, and compared to the weight of the dry copper iodide and the evaporated potassium nitrate, the weights will be the same. The atoms have been rearranged, but the same number of atoms are present. Matter has been changed, but matter has not been destroyed.