Molarity depends on temperature. Cause - Molarity is related to temperature. The volume of the solution changes with increasing and decreasing temperature. As a result molarity changes. As the temperature increases the volume increases so the molarity decreases i.e. the solution becomes lighter. As the temperature decreases, the volume decreases and the density increases. To measure molarity, the temperature has to be fixed. So it is difficult to use molarity for density measurement where there is more variation in temperature. So another is used to express the concentration of a single solution.