Avogadro’s law is a fundamental law of gases proposed in 1811.
It states that equal volumes of different gases contain equal number of molecules under the same temperature and pressure. For example, one litre of Nitrogen gas and one litre of Chlorine gas contain the same number of molecules at Standard Temperature and Pressure (STP). STP is the standard condition often used for measuring gas density and volume, where air is at 0 degree Celsius.
In the 1800s, scientists were still learning the difference between atoms and molecules. Therefore, discovering that the volume of a gas was directly proportional to the number of particles it contained, was revolutionary. It was crucial in establishing the formulas for simple molecules. Over time, it was understood that gases show small deviations from the ideal behaviour and the law holds only approximately.
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