For an experiment, my teacher gave me containers of hydrates and their corresponding anhydrous compounds, but I have a feeling that for one of the compounds she switched the hydrate and anhydrous label by accident. How can I check if this is true?

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    $\begingroup$ Anhydrate is a word that does not exist. Anhydrides are existing, but it is not your question. $\endgroup$
    – Maurice
    Commented Nov 20, 2020 at 21:12
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    $\begingroup$ In a way, anhydrate is a word that should have existed. Pity it doesn't. $\endgroup$ Commented Nov 20, 2020 at 21:13
  • $\begingroup$ Most anhydrous materials are fairly hygroscopic, so put a bit of it on a sensitive scale. If it "gets heavier" sitting there it used to be anhydrous. $\endgroup$
    – Gwyn
    Commented Nov 20, 2020 at 21:32
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    $\begingroup$ Which compounds do you suspect were swapped? $\endgroup$ Commented Nov 21, 2020 at 3:06
  • $\begingroup$ There will be a difference in their appearance. $\endgroup$ Commented Nov 21, 2020 at 9:11

1 Answer 1


There is no general answer to this question.

Heating is probably the best approach. It is not the only one. For some compounds, heating the hydrated variety produces an emission of water vapor, and nothing else, like $\ce{CuSO4·5H2O}$ or $\ce{Na2CO3·10H2O}$. For others compounds, it may produce a total decomposition like $\ce{AlCl3·6H2O}$, which is transformed into $\ce{Al(OH)3 + HCl}$ at high temperature. Considering the corresponding anhydrous compounds, $\ce{CuSO4}$ is decomposed into $\ce{CuO + SO2 + O2}$ at $650$°C. $\ce{Na2CO3}$ resists decomposition up to $1200$°C. $\ce{AlCl3}$ sublimes at $177$°C. You see that there is no general rule.

You may also check the behaviour of your hydrated compound with substances that react violently with water like thionyl chloride $\ce{SOCl2}$. if there is a reaction, it is probably a hydrate. But it may also be an anhydrous compound that react with $\ce{SOCl2}$, a metal for example, or an organic acid.

If you want a better answer, please give us the names of the particular compounds you are interested in.


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