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In this question, I am only considering conditions fairly close to STP and moderately but not extremely high accuracy. E.g. a quite well equipped school laboratory.

For gases, the density will be proportional to the molecular weight but this will not be so for liquids and solids. Many factors will affect the density but if we take two isotopes of a single element e.g. $^{118}Sn$ and $^{120}Sn$ or $^{79}Br$ and $^{81}Br$, will the density be proportional to the atomic mass?

For a more complex example, how about varieties of water with different hydrogen and oxygen isotopes.

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  • $\begingroup$ The answer is yes. Why would it not be? $\endgroup$ – Ben Crowell Sep 2 at 14:54
  • $\begingroup$ I could not see a reason but I wanted to confirm that. With the elements, it seemed very likely. With compounds, e.g. the varieties of water, slightly less so. $\endgroup$ – badjohn Sep 2 at 14:57
  • $\begingroup$ Couldn't adding neutrons to the nucleus alter the valence electron orbital bonding distance? Say, diamond form of Carbon. If the bond length is increased by just 2%, the density would drop by nearly 10% which should be measurable under the conditions you described. $\endgroup$ – Joseph Hirsch Sep 2 at 15:05
  • $\begingroup$ @JosephHirsch That's the sort of consideration I wanted to check. $\endgroup$ – badjohn Sep 2 at 15:07
  • $\begingroup$ Hydrogen gas (H2) has a density of 0.0899 g/mL, while the pure isotope version, D2, is about twice as dense, with a value of 0.169 g/mL. $\endgroup$ – Farcher Sep 2 at 15:09

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