Consider a quantity of gas in a container of variable volume. If we change the temperature of the gas and allow it to change volume at constant pressure, the results are as shown by the graph below. (See experiment 6TP.)

Making the same extrapolation as in the previous experiment, we find the same "magic" figure; -273°C.

This suggests that the volume occupied by the gas because of the motion of its molecules would be zero at -273°C. Again, this suggests that this is the lowest temperature possible.

Note that the absolute zero of temperature has not been attained in practice; it is a theoretical prediction found by extrapolation of experimental results.

This conclusion of this experiment is expressed in The Pressure Law, stated as follows.

These results do not depend on the type of gas.

The conclusion of this experiment is expressed in Charles’ Law, stated as follows.

If a fixed mass of gas has initial (absolute) temperature T₁ and initial volume V₁ and final (absolute) temperature and volume T₂ and V₂ respectively, then we can write

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