Knowledge database: Basic laws of chemistry: Law of combining volumes

The law of combining volumes says that the volumes of gases that react with each other or are formed by a chemical reaction, are in a ratio of small whole numbers when the measurements are performed at a constant pressure and temperature.

For instance, one liter of oxygen reacts with exactly two liters of hydrogen, giving two liters of water, i.e. water vapor. Similarly 1 liter of oxygen reacts with 2 liters of carbon monoxide, giving 2 liters of carbon dioxide.

After the discovery of this law, some scientists have proposed that there must be a simple relationship between the number of atoms present in the same volume of different gases at the same conditions.

Initially it was thought that all gases of equal volume at the same conditions contain the same number of atoms. However this has been proven wrong. More about this can be found in the chapter concerning Avogadro's law.