Laws of chemical combination

Chemical combination follows five laws:

• Law of conservation of mass (Lavoisier, 1789): matter is neither created nor destroyed in any physical or chemical change. The total mass of the products equals the total mass of the reactants.
• Law of definite proportions (Proust): a compound always contains the same elements in the same proportion by mass, wherever it comes from.
• Law of multiple proportions (Dalton, 1803): when two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in a small whole-number ratio.
• Gay Lussac's law of gaseous volumes (1808): gases react, and gaseous products form, in simple whole-number ratios by volume, when measured at the same temperature and pressure.
• Avogadro's law (1811): equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules.

Definite proportions, in an everyday picture: three friends fetch water, one from an RO filter, one from the tap, and one from anywhere at all. The waters may taste different, but in every sample the pure water itself has hydrogen and oxygen in the mass ratio 1 : 8.

Multiple proportions, with carbon and oxygen: 12 g of carbon combines with 16 g of oxygen to form carbon monoxide, and with 32 g of oxygen to form carbon dioxide. Fixing the carbon at 12 g, the oxygen masses are in the ratio 16 : 32 = 1 : 2, a small whole-number ratio.

Gaseous volumes: in H₂ + Cl₂ → 2HCl, 1 volume of hydrogen reacts with 1 volume of chlorine to give 2 volumes of hydrogen chloride. The numbers in front of the formulas give the volume ratio directly.

Dalton's atomic theory (1808) explained most of these laws:

• Matter is made of tiny, indivisible atoms.
• All atoms of an element are identical, including in mass; atoms of different elements differ.
• Compounds form when atoms of different elements combine in a fixed ratio.
• Chemical reactions rearrange atoms; atoms are neither created nor destroyed.

But Dalton's theory could not explain the law of gaseous volumes.