The change of @[email protected] caused by addition of a given species to a base solution. When an area of liquid covered with a spread substance is separated from a clean area of surface by a mechanical barrier, the force acting on unit length of the barrier is called the surface pressure, \(\pi \) or \(\pi _{\text{s}}\), and is equal to \(\gamma ^{0} - \gamma \) where \(\gamma ^{0}\) is the @[email protected] of the @[email protected] and \(\gamma \) that of the covered surface.