Careful measurements of mass and gas volume showed five regular patterns in how elements combine. Dalton explained them in 1808 with the idea that matter is made of atoms that combine in whole-number ratios.
Around 1800, nobody could observe individual atoms. Chemists could weigh substances before and after a reaction, and measure the volumes of gases that reacted. Those measurements showed regular patterns. Five of them are called the laws of chemical combination. Each is an observation about masses or volumes.
Antoine Lavoisier weighed reactions carried out in closed vessels, so that no gas could escape or enter. The total mass after the reaction was equal to the total mass before it.
The law of conservation of mass: in a chemical reaction, the total mass of the products equals the total mass of the reactants. This is why chemical equations must balance.
Joseph Proust analysed copper carbonate from natural ores and copper carbonate made in the laboratory. Every sample contained copper, carbon and oxygen in the same ratio by mass.
The law of definite proportions: a given compound always contains the same elements combined in the same ratio by mass, whatever its source. Water always contains hydrogen and oxygen in the mass ratio 1 : 8.
Pure water is obtained from a river, from distilled seawater, and from burning hydrogen in a laboratory. What does the law of definite proportions predict about their composition?
All three samples contain hydrogen and oxygen in the same mass ratio, 1 : 8.
The composition of a compound does not depend on where it came from. A sample with a different ratio would not be pure water. It would be a mixture, or a different compound such as hydrogen peroxide.
Some pairs of elements form more than one compound. Hydrogen and oxygen form water and also hydrogen peroxide.
Fix the mass of hydrogen at 2 g. In water, it is combined with 16 g of oxygen. In hydrogen peroxide, it is combined with 32 g of oxygen. The oxygen masses are in the ratio 16 : 32 = 1 : 2.
The law of multiple proportions, stated by John Dalton: 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 ratio of small whole numbers.
Two compounds of the same two elements. For a fixed 2 g of hydrogen, the masses of oxygen are in the ratio 1 : 2. The law states that such ratios are always small whole numbers.
In 1808, Joseph Gay-Lussac measured the volumes of reacting gases, all at the same temperature and pressure. They were in simple whole-number ratios. 2 volumes of hydrogen react with 1 volume of oxygen to give 2 volumes of steam. This is the law of gaseous volumes.
In 1811, Amedeo Avogadro proposed an explanation: equal volumes of all gases, at the same temperature and pressure, contain equal numbers of molecules. This is Avogadro’s law. A ratio of gas volumes is then a ratio of numbers of molecules. That idea leads to the mole, in the next session.
Each law is an observed pattern. In 1808 John Dalton proposed a model that explains them. Its main points:
Some parts of this model were later revised. Atoms are made of smaller particles: electrons, protons and neutrons. Atoms of one element can have different masses; these are called isotopes. But the parts that explain the laws of combination still hold: atoms are not created, destroyed or split in chemical reactions, and they combine in whole-number ratios.
How does “atoms combine in fixed whole-number ratios” explain both the law of definite proportions and the law of multiple proportions?
Atoms combine as whole units. A molecule contains 1, 2 or 3 atoms of an element, never 1.6.
Definite proportions: each water molecule has 2 hydrogen atoms and 1 oxygen atom, and each atom of an element has a fixed mass. So every sample of water has the same mass ratio.
Multiple proportions: hydrogen peroxide has 2 hydrogen atoms and 2 oxygen atoms per molecule. For the same number of hydrogen atoms, it has twice as many oxygen atoms as water. So the oxygen masses are in the ratio 1 : 2.
One assumption explains both laws. That is the evidence that made the atomic model accepted, before atoms could be observed directly.
1The law of conservation of mass states that in a chemical reaction…
2The law of definite proportions states that a given compound…
3For 2 g of hydrogen, water contains 16 g of oxygen and hydrogen peroxide contains 32 g. This 1 : 2 ratio illustrates the law of…
4Avogadro’s law states that equal volumes of all gases, at the same temperature and pressure, contain equal numbers of ____.
5Which is a point of Dalton’s atomic theory?
6Which part of Dalton’s theory was later revised?
Next: the mole, a way to count atoms by weighing.
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