##### 1.6 Real system and Raoult's Law

In reality, a liquid-vapor system does not follow the linear correlation expressed by the Raoult law, but more likely, the pressure-composition correlations are represented by curves (green in the here below picture).

The Raoult law can be modify to taken in consideration the real gas behaviour by introducing a coefficient, called activity coefficient, as shown here below:

P_{A} = g_{A} x_{A} P_{A}^{v}

P_{B} = g_{B} x_{B} P_{B}^{v}

where g_{A} and g_{B} depend on temperature, pressure and composition:

g_{i}= g_{i} (T, P, x_{i})
For g_{i} > 1 the deviation is **positive** and the curves will look like in the above figure with their bending to the top;

For g_{i} < 1 the deviation is **negative** and the curves will have their bending to the bottom:

A system with g_{i} = 1 is an **ideal** system.

Actually it is always possible to use the linear version of the Raoult's law when working at higher range of composition. In this case, as it is shown in the graph, the Raoult line coincides with the tangent to the curve of the real system.