I understand the statement of 'Law of mass action', but I don't understand clearly what 'mass action' means.

I investigated it and found the following descriptions.

  • The law was originally suggested as a law about 'active mass'. [from Wikipedia]
  • The term 'active mass' is a historical term that professional chemists do not use anymore. 'Active mass' meant 'amount per unit volume'. [from Chemistry Stack Exchange #58141]

From these explanations, I understood that 'active mass' corresponds to 'concentration' in modern words, which is usually expressed in the unit 'mole per liter' today.

However, a question still remains for me. What does the term 'mass' intend to originally? I am not a native English speaker, so I cannot understand the nuance well.

The word 'mass' has some different meanings.

  • a physical quantity expressed by the unit kilogram,
  • a large amount of a substance,
  • a lot of people, etc.

Of all these meanings, which do native English speakers understand the word 'mass' as?

Background of this question

In Japan, 'Law of mass action' has been translated as '質量作用の法則' (shitsuryou-sayou-no-housoku). '質量' corresponds to 'mass'. Nowadays, however, this is accused as a mistranslation because the law states the nature of concentration and it does not relate to mass. On the other hand, if native English speakers also understand the word 'mass' as 'a physical quantity expressed by the unit kilogram', the translation is not a 'mistranslation' but just a historical word originally. So I would like to consult those who are familiar with both chemistry and English.


"Mass" in this context refers to the large number of particles and consequent behavior in a stochastic manner, as opposed to a small set of particles, where Maxwell's demon might have been able to influence outcomes.Of course, Maxwell didn't know know of Heisenberg's uncertainty principle.


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