Avogadro's law

Avogadro's law states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. For a fixed amount of gas, volume is proportional to the amount of substance when temperature and pressure are held constant.

Ideal gas law: The equation of state relating a gas's pressure, volume, temperature, and amount of substance. Its amount term makes Avogadro's proportionality explicit: at fixed pressure and temperature, volume tracks moles.
Kinetic theory of gases: A model that explains gas properties through the motion and collisions of microscopic particles. Its molecular picture gives a physical framework for comparing gas samples by particle number.
Amedeo Avogadro: An Italian scientist who proposed in 1811 that equal gas volumes at equal temperature and pressure contain equal numbers of molecules. He formulated the molecular hypothesis later known as Avogadro's law.
Molar volume: The volume occupied by one mole of a substance under specified conditions. For gases at fixed temperature and pressure, it follows from volume being proportional to amount.
Real gas: A gas whose behavior departs from the ideal-gas model because its molecules interact and occupy finite volume. Real gases can depart from the law, especially at high pressure or low temperature.
Mole (unit): The SI unit for amount of substance, defined by a fixed number of specified elementary entities. The law counts molecules through the amount of gas measured in moles.
Temperature: A physical quantity that characterizes thermal state and determines the direction of heat transfer. Equal temperature is essential because gas volume changes with temperature.
Gay-Lussac's law of combining volumes: An empirical law stating that reacting gas volumes combine in simple whole-number ratios at equal temperature and pressure. Its volume ratios helped motivate Avogadro's interpretation in terms of molecules.
Standard conditions for temperature and pressure: Agreed reference values of temperature and pressure used to report and compare gas properties. A stated reference condition makes gas-volume and molar-volume comparisons meaningful.
Van der Waals equation: An equation of state that corrects the ideal gas law for molecular attraction and finite molecular volume. Its corrections account for conditions where ideal Avogadro proportionality is only approximate.

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