Avogadro's law states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. In equation form, it says that gas volume is directly proportional to the amount of gas when temperature and pressure remain constant.
The law is named after Italian scientist Amedeo Avogadro, who proposed the idea in 1811. His insight helped distinguish atoms from molecules and supported the emerging atomic theory of the nineteenth century. For example, two balloons of different gases can occupy the same volume and contain the same number of molecules if their temperature and pressure are identical, even though the gases have different masses.
Avogadro's law is often confused with Avogadro's number, which is the fixed number of particles in one mole: approximately 6.022 × 10²³. The law describes a relationship between volume and amount of gas; the number describes how many particles make up a mole. Real gases deviate slightly from ideal behavior, especially at high pressure or low temperature, but the law is an excellent approximation under ordinary conditions.