Is this purely a historical artifact, a consequence of how the original formulators of the three laws were thinking and experimenting at the time? Or is there a compelling scientific/logical reason why we ought to not include mass conservation as a fourth (or zeroth, or... negative first) law?

Note that I'm talking specifically about non-relativistic thermodynamics. So any answer invoking mass-energy equivalence, in order to show how mass conservation follows from mass-energy conservation at low speeds, isn't really what I'm looking for.


closed as unclear what you're asking by ACuriousMind, John Rennie, user36790, knzhou, Gert Jul 19 '16 at 2:11

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ Well, there is already a zeroth law of thermodynamics...see this website. (Just saying.) $\endgroup$ – heather Jul 17 '16 at 21:53
  • 4
    $\begingroup$ I'm not sure what you mean. When do we need conservation of mass? $\endgroup$ – knzhou Jul 17 '16 at 21:55
  • $\begingroup$ @knzhou Well first of all, in terms of the conceptual structure of thermodynamics, it seems to me like the experimental discovery of mass conservation is on par logically with the discovery of energy conservation, and so it makes sense to me that we ought to afford both laws a similar status when formalizing basic thermodynamic principles. And in terms of "needing" mass conservation experimentally, I mean we assume it for basically every thermodynamic experiment right? Otherwise how could we know that the gas molecules in our canisters aren't just disappearing or re-materializing at random? $\endgroup$ – Wade Hodson Jul 17 '16 at 22:26
  • 3
    $\begingroup$ @WadeHodson Mass conservation is indeed extremely important, but it doesn't fit into the mathematical description of thermodynamics. One of thermo's great strengths is that it's independent of the detailed dynamics of the system it's considering -- i.e. it would still work if gas molecules were disappearing and reappearing! For that matter, thermodynamics predated the discovery of molecules themselves, and it works for, say, nuclear reactions, where mass is not conserved. $\endgroup$ – knzhou Jul 17 '16 at 22:32
  • $\begingroup$ @knzhou hmm that's a good point! i'll give that some thought. $\endgroup$ – Wade Hodson Jul 17 '16 at 22:45