More languages
More actions
Main article: Water of crystallization.
Crystal hydrates - crystals of substances containing water molecules as a part of the crystal lattice. These crystals are formed from water solution. In cases of solvents other than water, they are called solvates instead of hydrates.
To form a crystal hydrate, a substance should dissociate to cations and anions, these ions should attract the molecules of solvent with electostatic force, and solvent should bond to cations or anions better than cations and anions are bound together. In such a case, a crystal formed from this solution will contain water molecules, embedded into the crystal lattice.
For example, dissolving copper(II) sulfate CuSO4 in water produces copper(II) cation Cu2+ and sulfate anion [SO4]2-. But water molecules are electrostatically attracted to copper ions, and wrap the latter, producing a hexaaquacopper(II) ion [Cu(6H2O)]2+. Growing a crystal from this solution leads water molecules bound to copper(II) ions to become a part of the crystal, forming a copper(II) sulfate pentahydrate CuSO4 · 5H2O
If a substance does not dissociate into ions, or bond between water and substance molecules is not strong enough, then crystals are formed without water being a part of crystal lattice. These crystals are anhydrous.
Hydrates
Hydrates are a term describing substances which form a crystal with n molecules of water per 1 molecule of substance, (X · nH2O). The number of water molecules is mentioned in substance formula as well as the hydrate name prefix:
- monohydrate (X · H2O)
- sesquihydrate (X · 1.5H2O)
- dihydrate (X · 2H2O)
- trihydrate (X · 3H2O)
- tetrahydrate (X · 4H2O)
- pentahydrate (X · 5H2O)
- hexahydrate (X · 6H2O)
- heptahydrate (X · 7H2O)
- octahydrate (X · 8H2O)
- nonahydrate (X · 9H2O)
- decahydrate (X · 10H2O)
- undecahydrate (X · 11H2O)
- dodecahydrate (X · 12H2O)
- and so on
Conditions
Substances may form multiple different hydrates depending on some conditions. Usually this is caused by the temperature of a solution - for example, at room temperature magnesium sulfate crystallizes as heptahydrate MgSO4 · 7H2O, at 48.4°C it crystallizes as hexahydrate MgSO4 · 6H2O, and at -2°C it crystallizes as undecahydrate MgSO4 · 11H2O. Another case is adding some other substances into solution - for example, under normal conditions nickel sulfate crystallizes as heptahydrate NiSO4 · 7H2O , but addition of large amounts of sulfuric acid leads to crystallization of hexahydrate NiSO4 · 6H2O. This is because sulfuric acid bonds to water molecules, keeping them in the solution and not allowing them to become a part of a crystal.
-
Heptahydrate. Grown within a monthNickel sulfate, Egor Kazakov, VKontakteVKontakte
-
Hexahydrate. Small addition of sulfuric acid (~7 ml of concentrated acid per 100 ml of solution)Nickel sulfate, Vladimir Semikin, VKontakteVKontakte
Physical properties
Different hydrates have different crystal lattices. This is because the number of water molecules influences the size of complex aqua cations, so they align differently inside a crystal lattice. This also influences a shape of the final crystal - for example, the shape of manganese(II) sulfate pentahydrate and tetrahydrate are very much different:
-
Growing using slow evaporation method at room temperatureManganese(II) sulfate, Ury Vashkovich
-
Crystals of tetrahydrate, grown at 26-29 °C. Recrystallization of compound bought in chem store.Manganese(II) sulfate, NeptuneQuartz, redditReddit
Also the different number of water molecules inside complex aqua ions leads to different colors of crystals and solution. Most of anhydrous crystals are while of very pale, but complex aqua ions have very bright color, like blue for hexaaquacopper(II) ion [Cu(6H2O)]2+ or deep green of hexaaquanickel(II) ion [Ni(6H2O)]2+, and so on.
Chemical properties
From chemical point of view, crystal hydrates are just a mixture of original substance and water with different proportions. Dissolving both a hydrate crystal and anhydrous one will produce exactly the same solution and same ions.
But this addition of water makes solution more diluted. So if you need to calculate a mass of the reagents for some equation, consider that crystal hydrates actually less concentrated than anhydrous substance, so you'll need more of it. For example, 100g of copper(II) sulfate pentahydrate CuSO4 · 5H2O actually contains only 63.92 of copper(II) sulfate CuSO4, and all other mass is just a water.
Dehydration
Reference data: Compound/Stability.
Most of crystal hydrates are not very stable in the open air, and slowly dehydrates. This is caused by evaporating water molecules from crystal lattice, so the lattice has cavities and then falls apart, converting crystals to an anhydrous powder.
-
Heptahydrate. Growth during 1 monthMagnesium sulfate, Ludmila Samokhina, VKontakteVKontakte
-
Dehydration of pentahydrate and tetrahydrate crystalsManganese(II) sulfate, DrWim, redditReddit
The dehydration speed is different for different compounds and different hydrates. Usually hydrates with less number of water molecules (dihydrate, tetrahydrate) are more stable than other hydrates (pentahydrate, decahydrate). Increasing the temperature also increases the speed of dehydration, and some hydrates that were initially stable may start to dehydrate.
There are multiple ways of preventing crystal dehydration. Most of they are based on reducing the rate of water evaporation by "sealing" it inside a crystal. More details in crystals storage.
Recommended
Theory | |
---|---|