The change of @I03088@ 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 @S06192@ of the @C01099@ and \(\gamma \) that of the covered surface.