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If the boiling point of water is at 100 °C, why does a hydrate need to be heated higher than that to evaporate water from the crystals? Does it have anything to do with their intermolecular forces?

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    $\begingroup$ For the same reason that some hydrates lose their water below 100°C: They don't know what liquid water is, and they also don't resemble it in any way. $\endgroup$
    – Karl
    Oct 25, 2022 at 18:44

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Hydrates are substances that have absorbed water into their crystal structure. Much of this attraction is caused by interparticle forces (notice I didn't say 'intermolecular'). Since water has a very strong dipole, it is quite often attracted to not only other molecules, but ions inside ionic crystals.

In some cases, water can easily be attracted into the crystal structure so much that it makes the substance deliquescent. Other times, crystals may only absorb some water, nor others. This is because of the competition in attractions between an ionic crystals ion-ion attractions (ionic bond) and the ion-dipole attractions to water (interparticle force).

To finally discuss why we have to go above the boiling point of water $100^\circ C$: If the attraction between the dipoles of water and ions of the salt are stronger than the standard interparticle forces of water, it will take more energy to break those forces and transition them to gaseous state.

Likewise, if a substance has quite weak ion-dipole forces, it would more readily release the water and dehydrate without much or no added heat.

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    $\begingroup$ I did indeed notice you didn't say "intermolecular". ... I have no clue why you felt it was critically important to scrupulously avoid using that term here, but (due in no small part to the presence of the vaguely worded parenthetical) I did notice. $\endgroup$
    – R.M.
    Oct 26, 2022 at 2:56
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    $\begingroup$ @R.M. While water indeed is made out of molecules, I suppose the term intermolecular was avoided to avoid calling ions in a lattice molecules. $\endgroup$
    – JoonasD6
    Oct 26, 2022 at 3:08
  • $\begingroup$ @R.M. It is probably a bit dogmatic of me to use that term and to be quite opinionated about it. Teaching students forces of attraction reveals the blurry lines of bonds/not bonds or intermolecular/interparticle attractions tends to be a learning blocker. Using more generic words, especially in cases like this question where the reader might be learning these topics for the first time helps them to understand attractions can occur between different materials/compounds and we shouldn't box in specifically this idea into "only molecules". $\endgroup$
    – Avogadro
    Oct 26, 2022 at 9:56
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    $\begingroup$ While it may be a useful simplification, treating water in crystal just as separate molecules isn't really correct. Sometimes it's indeed hydrogen bonded water, but usually it's coordinated to metal. $\endgroup$
    – Mithoron
    Oct 27, 2022 at 21:36
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    $\begingroup$ And in either case, the crystalline substance and it's hydrate are two different chemical compounds. Chladrathe compounds are a bit different, as they don't even have a loosely related base form of the matrix substance. $\endgroup$
    – Karl
    Oct 28, 2022 at 21:54

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