A bottle holds dilute hydrochloric acid and a small tube with 1.00 g of sodium hydrogencarbonate. With the cap on, it weighs 85.42 g on an electronic balance.
Tipping the bottle mixes the two, and carbon dioxide bubbles out of the solution: \mathrm{NaHCO_3 + HCl \longrightarrow NaCl + H_2O + CO_2}. As long as the cap stays on, the balance still reads 85.42 g.
Taking the cap off lets the gas escape, and the reading falls by about 0.50 g, almost all of the 0.52 g of carbon dioxide formed. Nothing was lost; the gas simply left the bottle.
The law of conservation of mass states that the total mass of the substances does not change in a chemical reaction. It holds because a reaction only rearranges atoms, and every atom keeps its mass.
For the reaction in the bottle, 1 mol of each substance gives 84.0 g + 36.5 g = 120.5 g of reactants and 58.5 g + 18.0 g + 44.0 g = 120.5 g of products.
Both sides contain one sodium, two hydrogen, one carbon, three oxygen and one chlorine atom, so the masses must agree. A balanced equation is therefore also a statement that mass is conserved.
In 1774 Antoine Lavoisier heated tin in a sealed glass vessel. Part of the tin turned into a white powder, yet the sealed vessel weighed the same as before.
When he broke the seal, air rushed in and the vessel became heavier, by as much as the tin had gained. The tin had combined with part of the air inside, which was later named oxygen.
Careful weighing of closed systems led Lavoisier to the law of conservation of mass, and it made the balance one of the most important tools of chemistry.
Heating copper powder in air until its mass stops changing turns it into black copper(II) oxide. With 0.40 g of copper the oxide weighs 0.50 g, and with 2.00 g it weighs 2.50 g.
The oxygen taken up is 0.10 g and 0.50 g. On a graph both the oxide and the oxygen lie on straight lines through the origin, so copper and oxygen always combine in the mass ratio 4 : 1.
This is the law of constant composition, or definite proportions, found by Joseph Proust in 1799: a compound always contains the same elements in the same proportions by mass. For copper(II) oxide the ratio is 63.5 : 16.0, close to 4 : 1.
Carbon dioxide can be made by burning charcoal, by heating limestone or by adding hydrochloric acid to marble.
However it is made, it contains 27.3 % carbon and 72.7 % oxygen by mass, a ratio of 3 : 8 that matches the formula \mathrm{CO_2}, 12.0 : 32.0.
Because a compound has a fixed composition, its formula can be found from the masses of its elements, as in the article on empirical formulae. Explaining why compositions are fixed led to the atomic theory.