In the Van der Waals equation `(P + (n^(2)a)/(V^(2))) (V-nb) = nRT`, why is the term `n^(2)a//V^(2)` positive in sign.

Explanation

The Van der Waals equation is for real gases wheras the rocorded pressure P is for an ideal gas. While recording the pressure, it is believed that no intermolecular or intermolecular or interparticle force are present wherares they do exist actually. This means that the recorded pressure P lis less than the pressure for a real gas and some pressure correction `(n^(2)a//V^(2))` has to be added to account for the additional pressure. For more details consult section 5.19.