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A "p-V diagram" is a graph showing changes in the pressure and volume of a sample of gas.
It is useful to be able to recognise various types of change of the state of a gas from a p-V diagram. Four examples are given below.
In practice, changes of state do not quite follow any of these ideal paths. However, approximate isothermal, adiabatic etc changes can occur.
for an isovolumetric change, heat the gas in a fixed volume container (one made of a material having a low thermal expansivity)
for an isobaric change, trap a small quantity of the gas in a tube using a thread of mercury (or other liquid, as in experiment 6TP) and heat it slowly
for an isothermal change, compress (or expand) the gas slowly in a container of high thermal conductivity
for an adiabatic change, compress (or expand) the gas rapidly in a container of low thermal conductivity.