Constant proportions and Dalton's theory

Take water from a river, a borewell, a tap or the sea, and purify it. Break it down, and you always get hydrogen and oxygen in the same ratio by mass: 1 : 8. 9 g of water always gives 1 g of hydrogen and 8 g of oxygen.

Joseph Proust found this is true of every compound: its elements are always in a fixed ratio by mass, wherever it came from. This is the law of constant proportions (also called the law of definite proportions, or Proust's law). Proust showed it for copper carbonate, which always has copper, carbon and oxygen in the same proportions.

*An Indian example: cinnabar (hingula).* This red pigment is a compound of mercury and sulfur, always about 86.22% mercury and 13.78% sulfur by mass.

"Does this work for mixtures?" No. You can mix sand and salt in any amounts you like. The law holds only for compounds.

In 1808, John Dalton explained both laws with his postulates (starting ideas he proposed):

• All matter is made of tiny atoms.
• Atoms cannot be divided, and are neither created nor destroyed in reactions.
• All atoms of one element are identical in mass and properties.
• Atoms of different elements are different.
• Atoms join in simple whole-number ratios.
• A given compound always has the same numbers and kinds of atoms.

So in a reaction, atoms are just rearranged, like the same beads made into a different necklace. Hydrogen and oxygen atoms regroup to make water; magnesium and oxygen atoms join to make magnesium oxide. No atoms are lost, so mass is conserved; and each compound has a fixed recipe of atoms, so its proportions are constant.