Dalton's law applies to a mixture of gases provided the gases:

1. The law assumes each gas behaves as if the others were absent. 2. Unless they chemically react with each other, the individual gases in a mixture of gases do not affect each other's pressure. 3. So each contributes its own partial pressure independently. 4. A reaction would consume molecules and change the count, breaking that independence. _Source: OpenStax Chemistry 2e (CC BY 4.0), Ch 9 'Gases', sections 9.1-9.6_