Relative activity
The relative activity a A of a substance A in a liquid or solid mixture is the dimensionless quantity defined by:
a A = l A / l A*
where l A is the absolute activity of A in the mixture and l A* is the absolute activity of pure substance A at the same temperature and the same pressure.
Relative activity can be obtained from the relationship:
RT ln a A = m A - m A*
where m A is chemical potential of A in the mixture and m A* is the chemical potential of pure substance A at the same temperature and the same pressure.
It results from this definition that the activity a A of a substance A tends towards 1 when its molar fraction cA tends towards 1.
A mixture for which the approximation a A = a A for all the components is called an ideal mixture.