Deviation of real gases from ideal behaviour From the Amagat's curve we can see two types of result for real gases.


If we increase pressure,the value of $PV$ decreases initially and then the value starts increasing for the gases like- carbon dioxide, nitrogen, oxygen, methane, etc..
If we increase pressure,the value of $PV$ starts increasing initially for the gases like - hydrogen, helium, etc.


Now please explain what're the reasons behind this kinda behaviour of real gases?
The ideal gas law can be derived using kinetic molecular theory by making two very important assumptions that are not true for real gases:


There is no attractive force between gas molecules
The gas molecules have no volume


Think about how the PV curve would change when you remove one or both of these conditions. Then think about which types of molecules the assumption of "no attractive force" would be more important for.