I learnt from my textbook and the question/answer - Why is active mass of a pure solid or liquid always taken as unity? that active mass of a solid or liquid is taken to be unity as:

$$[A]=\frac n V=\frac{m}{M\times V}=\frac{\rho}{M}=k$$

where $[A]$ is the active mass, $n$ is the amount of substance, $V$ is the volume, $m$ is the mass, $M$ is the molar mass, $\rho$ is the density and $k$ is a constant.

This is concisely stated in the linked question by:

Molar concentration is directly proportional to density. Since density of solid or liquid always remains constant, the active mass is taken as 1 .

The answer to the linked question explains that, in the course of a reaction mass and volume of solid and liquid substances change but their ratio i.e., density remains constant at a fixed temperature.

As far as I understood, the above explanation holds true for all solids and liquids. So, is the active mass of every solid and liquid taken to be unity? Or are there any exceptions to this fact? If so, how do such substances vary from others for which the active mass is taken to be unity?



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