How to differentiate hydrous and anhydrous compound? [closed]

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?

• Anhydrate is a word that does not exist. Anhydrides are existing, but it is not your question. – Maurice Nov 20 '20 at 21:12
• In a way, anhydrate is a word that should have existed. Pity it doesn't. – Ivan Neretin Nov 20 '20 at 21:13
• 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. – Gwyn Nov 20 '20 at 21:32
• Which compounds do you suspect were swapped? – Nicolau Saker Neto Nov 21 '20 at 3:06
• There will be a difference in their appearance. – Nilay Ghosh Nov 21 '20 at 9:11

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.