Is pressure and temperature the same

2.1 Macroscopic observation

2.1.1 Law of Avogadro

It has been observed that, in contrast to solids and liquids, gases expand and, to a certain extent, can also be compressed. The density of gases at atmospheric pressure is about three orders of magnitude lower than in the solid and liquid phase.

Macroscopically observable phenomena show the relationship between the state variables of a gas. In experiments, one of the state variables temperature T, Print p and volume V. be kept constant.

Experiment: Boyle-Mariotte's Law

Isotherms, isobars, isochors and adiabats in a p (V) diagram

temperature T constant (isotherm):

Boyle Mariotte Law

print p constant (isobars):

Gay Lussac Law

volume V. constant (isochore):

Qualitative V.(T) Diagram for different gases at constant pressure. Extrapolation leads to absolute zero and thus to the definition of the Kelvin scale.

Overall, one obtains Avogadro's law:

Equal volumes of gas at the same pressure and temperature contain the same number of molecules regardless of their chemical composition.

The constant is proportional to the "amount of gas" or generally to the amount of substance .


Law of Avogadro

Here designated R. the general gas constant and the amount of substance of the gas. The law applies to ideal gases. Real gases do the better, the higher the temperature and the lower the pressure.




© University of Würzburg, Tilo Hemmert 2000