In chemistry, water(s) of crystallization or water(s) of hydration are watermolecules that are present inside crystals. Water is often incorporated in the formation of crystals from aqueous solutions.[1] In some contexts, water of crystallization is the total mass of water in a substance at a given temperature and is mostly present in a definite (stoichiometric) ratio. Classically, "water of crystallization" refers to water that is found in the crystalline framework of a metal complex or a salt, which is not directly bonded to the metal cation.
Upon crystallization from water, or water-containing solvents, many compounds incorporate water molecules in their crystalline frameworks. Water of crystallization can generally be removed by heating a sample but the crystalline properties are often lost.
Compared to inorganic salts, proteins crystallize with large amounts of water in the crystal lattice. A water content of 50% is not uncommon for proteins.
Knowledge of hydration is essential for calculating the masses for many compounds. The reactivity of many salt-like solids is sensitive to the presence of water. The hydration and dehydration of salts is central to the use of phase-change materials for energy storage.[2]

A salt with associated water of crystallization is known as a hydrate. The structure of hydrates can be quite elaborate, because of the existence of hydrogen bonds that define polymeric structures.[3][4] Historically, the structures of many hydrates were unknown, and the dot in the formula of a hydrate was employed to specify the composition without indicating how the water is bound. Per IUPAC's recommendations, the middle dot is not surrounded by spaces when indicating a chemical adduct.[5] Examples:
For many salts, the exact bonding of the water is unimportant because the water molecules are made labile upon dissolution. For example, an aqueous solution prepared from CuSO4·5H2O and anhydrous CuSO4 behave identically. Therefore, knowledge of the degree of hydration is important only for determining the equivalent weight: one mole of CuSO4·5H2O weighs more than one mole of CuSO4. In some cases, the degree of hydration can be critical to the resulting chemical properties. For example, anhydrous RhCl3 is not soluble in water and is relatively useless in organometallic chemistry whereas RhCl3·3H2O is versatile. Similarly, hydrated AlCl3 is a poor Lewis acid and thus inactive as a catalyst for Friedel-Crafts reactions. Samples of AlCl3 must therefore be protected from atmospheric moisture to preclude the formation of hydrates.

Crystals of hydrated copper(II) sulfate consist of [Cu(H2O)4]2+ centers linked to SO2−4 ions. Copper is surrounded by six oxygen atoms, provided by two different sulfate groups and four molecules of water. A fifth water resides elsewhere in the framework but does not bind directly to copper.[6] The cobalt chloride mentioned above occurs as [Co(H2O)6]2+ and Cl−. In tin chloride, each Sn(II) center is pyramidal (mean O/Cl−Sn−O/Cl angle is 83°) being bound to two chloride ions and one water. The second water in the formula unit is hydrogen-bonded to the chloride and to the coordinated water molecule. Water of crystallization is stabilized by electrostatic attractions, consequently hydrates are common for salts that contain +2 and +3 cations as well as −2 anions. In some cases, the majority of the weight of a compound arises from water. Glauber's salt, Na2SO4(H2O)10, is a white crystalline solid with greater than 50% water by weight.
Consider the case of nickel(II) chloride hexahydrate. This species has the formula NiCl2(H2O)6. Crystallographic analysis reveals that the solid consists of [trans-NiCl2(H2O)4] subunits that are hydrogen bonded to each other as well as two additional molecules of H2O. Thus one third of the water molecules in the crystal are not directly bonded to Ni2+, and these might be termed "water of crystallization".
The water content of most compounds can be determined with a knowledge of its formula. An unknown sample can be determined through thermogravimetric analysis (TGA) where the sample is heated strongly, and the accurate weight of a sample is plotted against the temperature. The amount of water driven off is then divided by the molar mass of water to obtain the number of molecules of water bound to the salt.
Water is a particularly common solvent to be found in crystals because it is small and polar. But many other solvents can be hosted in crystals, known as solvates.[7][8] Water is noteworthy because it is reactive, whereas other solvents such as benzene are considered to be chemically innocuous. Occasionally more than one solvent is found in a crystal, and often the stoichiometry is variable, reflected in the crystallographic concept of "partial occupancy". It is common and conventional for a chemist to "dry" a sample with a combination of vacuum and heat "to constant weight".
For other solvents of crystallization, analysis is conveniently accomplished by dissolving the sample in a deuterated solvent and analyzing the sample for solvent signals by NMR spectroscopy. Single crystal X-ray crystallography is often able to detect the presence of these solvents of crystallization as well. Other methods may be currently available.
In the table below are indicated the number of molecules of water per metal in various salts.[9][10]
| Hydrated metal halidesand their formulas | Coordination sphereof the metal | Equivalents of water of crystallization that are not bound to M | Remarks |
|---|---|---|---|
| Calcium chlorideCaCl2(H2O)6 | [Ca(μ-H2O)6(H2O)3]2+ | 0 | example of water as a bridging ligand[11] |
| Calcium bromideCaBr2(H2O)9 | [Ca(H2O)8]2+ | 1 | the most hydrated calcium halide[12] |
| Calcium iodideCaI2(H2O)7 | [Ca(H2O)8]2+[12] | 0 | bridging water ligands |
| Calcium iodideCaI2(H2O)8 | [Ca(H2O)8]2+[12] | 0 | |
| Calcium iodideCaI2(H2O)6.5 | [Ca(H2O)8]2+[12] | 0 | bridging water ligands |
| Titanium(III) chlorideTiCl3(H2O)6 | trans-[TiCl2(H2O)4]+[13] | 2 | isomorphous with VCl3(H2O)6 |
| Titanium(III) chlorideTiCl3(H2O)6 | [Ti(H2O)6]3+[13] | none | isomeric with [TiCl2(H2O)4]Cl.2H2O[14] |
| Zirconium(IV) fluorideZrF4(H2O)3 | (μ−F)2[ZrF3(H2O)3]2 | none | rare case where Hf and Zr differ[15] |
| Hafnium tetrafluorideHfF4(H2O)3 | (μ−F)2[HfF2(H2O)2]n(H2O)n | 1 | rare case where Hf and Zr differ[15] |
| Vanadium(III) chlorideVCl3(H2O)6 | trans-[VCl2(H2O)4]+[13] | 2 | |
| Vanadium(III) bromideVBr3(H2O)6 | trans-[VBr2(H2O)4]+[13] | 2 | |
| Vanadium(III) iodideVI3(H2O)6 | [V(H2O)6]3+ | none | relative to Cl− and Br−, I− competes poorly with water as a ligand for V(III) |
| Nb6Cl14(H2O)8 | [Nb6Cl14(H2O)2] | 4 | |
| Chromium(III) chlorideCrCl3(H2O)6 | trans-[CrCl2(H2O)4]+ | 2 | dark green isomer, aka "Bjerrums's salt" |
| Chromium(III) chlorideCrCl3(H2O)6 | [CrCl(H2O)5]2+ | 1 | blue-green isomer |
| Chromium(II) chlorideCrCl2(H2O)4 | trans-[CrCl2(H2O)4] | none | square planar/tetragonal distortion |
| Chromium(III) chlorideCrCl3(H2O)6 | [Cr(H2O)6]3+ | none | violet isomer. isostructural with aluminium compound[16] |
| Manganese(II) chlorideMnCl2(H2O)6 | trans-[MnCl2(H2O)4] | 2 | |
| Manganese(II) chlorideMnCl2(H2O)4 | cis-[MnCl2(H2O)4] | none | cis molecular, the unstable trans isomer has also been detected[17] |
| Manganese(II) bromideMnBr2(H2O)4 | cis-[MnBr2(H2O)4] | none | cis, molecular |
| Manganese(II) iodideMnI2(H2O)4 | trans-[MnI2(H2O)4] | none | molecular, isostructural with FeCl2(H2O)4.[18] |
| Manganese(II) chlorideMnCl2(H2O)2 | trans-[MnCl4(H2O)2] | none | polymeric with bridging chloride |
| Manganese(II) bromideMnBr2(H2O)2 | trans-[MnBr4(H2O)2] | none | polymeric with bridging bromide |
| Rhenium(III) chlorideRe3Cl9(H2O)4 | triangulo-[Re3Cl9(H2O)3] | none | heavy early metals form M-M bonds[19] |
| Iron(II) chlorideFeCl2(H2O)6 | trans-[FeCl2(H2O)4] | two | |
| Iron(II) chlorideFeCl2(H2O)4 | trans-[FeCl2(H2O)4] | none | molecular |
| Iron(II) bromideFeBr2(H2O)4 | trans-[FeBr2(H2O)4] | none | molecular,[20] hydrates of FeI2 are not known |
| Iron(II) chlorideFeCl2(H2O)2 | trans-[FeCl4(H2O)2] | none | polymeric with bridging chloride |
| Iron(III) chlorideFeCl3(H2O)6 | trans-[FeCl2(H2O)4]+ | two | one of four hydrates of ferric chloride,[21] isostructural with Cr analogue |
| Iron(III) chlorideFeCl3(H2O)2.5 | cis-[FeCl2(H2O)4]+ | two | the dihydrate has a similar structure, both contain FeCl−4 anions.[21] |
| Cobalt(II) chlorideCoCl2(H2O)6 | trans-[CoCl2(H2O)4] | two | |
| Cobalt(II) bromideCoBr2(H2O)6 | trans-[CoBr2(H2O)4] | two | |
| Cobalt(II) iodideCoI2(H2O)6 | [Co(H2O)6]2+ | none[22] | iodide competes poorly with water |
| Cobalt(II) bromideCoBr2(H2O)4 | trans-[CoBr2(H2O)4] | none | molecular[20] |
| Cobalt(II) chlorideCoCl2(H2O)4 | cis-[CoCl2(H2O)4] | none | note: cis molecular |
| Cobalt(II) chlorideCoCl2(H2O)2 | trans-[CoCl4(H2O)2] | none | polymeric with bridging chloride |
| Cobalt(II) bromideCoBr2(H2O)2 | trans-[CoBr4(H2O)2] | none | polymeric with bridging bromide |
| Nickel(II) chlorideNiCl2(H2O)6 | trans-[NiCl2(H2O)4] | two | |
| Nickel(II) chlorideNiCl2(H2O)4 | cis-[NiCl2(H2O)4] | none | note: cis molecular[20] |
| Nickel(II) bromideNiBr2(H2O)6 | trans-[NiBr2(H2O)4] | two | |
| Nickel(II) iodideNiI2(H2O)6 | [Ni(H2O)6]2+ | none[22] | iodide competes poorly with water |
| Nickel(II) chlorideNiCl2(H2O)2 | trans-[NiCl4(H2O)2] | none | polymeric with bridging chloride |
| Platinum(IV) chloride[Pt(H2O)2Cl4](H2O)3[23] | trans-[PtCl4(H2O)2] | 3 | octahedral Pt centers; rare example of non-first row chloride-aquo complex |
| Platinum(IV) chloride[Pt(H2O)3Cl3]Cl(H2O)0.5[24] | fac-[PtCl3(H2O)3]+ | 0.5 | octahedral Pt centers; rare example of non-first row chloride-aquo complex |
| Copper(II) chlorideCuCl2(H2O)2 | [CuCl4(H2O)2]2 | none | tetragonally distorted two long Cu-Cl distances |
| Copper(II) bromideCuBr2(H2O)4 | [CuBr4(H2O)2]n | two | tetragonally distorted two long Cu-Br distances[20] |
| Zinc(II) chlorideZnCl2(H2O)1.33[25] | 2 ZnCl2 + ZnCl2(H2O)4 | none | coordination polymer with both tetrahedral and octahedral Zn centers |
| Zinc(II) chlorideZnCl2(H2O)2.5[26][27] | Cl3Zn(μ-Cl)Zn(H2O)5 | none | tetrahedral and octahedral Zn centers |
| Zinc(II) chlorideZnCl2(H2O)3[25][26] | [ZnCl4]2− & [Zn(H2O)6]2+ | none | tetrahedral and octahedral Zn centers |
| Zinc(II) chlorideZnCl2(H2O)4.5 | [ZnCl4]2− & [Zn(H2O)6]2+ | three | tetrahedral and octahedral Zn centers[26][25] |
| Cadmium chlorideCdCl2·H2O[28] | none | water of crystallization is rare for heavy metal halides | |
| Cadmium chlorideCdCl2·2.5H2O[29] | CdCl5(H2O) & CdCl4(H2O)2 | none | |
| Cadmium chlorideCdCl2·4H2O[30] | CdCl4(H2O)4 | two | octahedral, doubly bridging chlorides |
| Cadmium bromideCdBr2(H2O)4[31] | [CdBr4(H2O)2 | two | octahedral Cd centers |
| Aluminum trichlorideAlCl3(H2O)6 | [Al(H2O)6]3+ | none | isostructural with the Cr(III) compound |
| Aluminum triiodideAlI3(H2O)6 | [Al(H2O)6]3+ | none | other hydrates are known[32] |
| Aluminum triiodideAlI3(H2O)15 | [Al(H2O)6]3+ | 9 | other hydrates are known[32] |
| Aluminum triiodideAlI3(H2O)17 | [Al(H2O)6]3+ | 11 | the highest hydrate crystallized[32] |
Transition metal sulfates form a variety of hydrates, each of which crystallizes in only one form. The sulfate group often binds to the metal, especially for those salts with fewer than six aquo ligands. The heptahydrates, which are often the most common salts, crystallize as monoclinic and the less common orthorhombic forms. In the heptahydrates, one water is in the lattice and the other six are coordinated to the ferrous center.[33] Many of the metal sulfates occur in nature, being the result of weathering of mineral sulfides.[34][35] Many monohydrates are known.[36]
| Formula of hydrated metal ion sulfate | Coordinationsphere of the metal ion | Equivalents of water of crystallization that are not bound to M | mineral name | Remarks |
|---|---|---|---|---|
| MgSO4(H2O) | [Mn(μ-H2O)(μ4,-κ1-SO4)4][36] | none | kieserite | see Mn, Fe, Co, Ni, Zn analogues |
| MgSO4(H2O)4 | [Mg(H2O)4(κ′,κ1-SO4)]2 | none | sulfate is bridging ligand, 8-membered Mg2O4S2 rings[37] | |
| MgSO4(H2O)6 | [Mg(H2O)6] | none | hexahydrate | common motif[34] |
| MgSO4(H2O)7 | [Mg(H2O)6] | one | epsomite | common motif[34] |
| TiOSO4(H2O) | [Ti(μ-O)2(H2O)(κ1-SO4)3] | none | further hydration gives gels | |
| VSO4(H2O)6 | [V(H2O)6] | none | Adopts the hexahydrite motif[38] | |
| VSO4(H2O)7 | [V(H2O)6] | one | hexaaquo[39] | |
| VOSO4(H2O)5 | [VO(H2O)4(κ1-SO4)4] | one | ||
| Cr(SO4)(H2O)3 | [Cr(H2O)3(κ1-SO4)] | none | resembles Cu(SO4)(H2O)3[40] | |
| Cr(SO4)(H2O)5 | [Cr(H2O)4(κ1-SO4)2] | one | resembles Cu(SO4)(H2O)5[41] | |
| Cr2(SO4)3(H2O)18 | [Cr(H2O)6] | six | One of several chromium(III) sulfates | |
| MnSO4(H2O) | [Mn(μ-H2O)(μ4,-κ1-SO4)4][36] | none | szmikite | see Fe, Co, Ni, Zn analogues |
| MnSO4(H2O)4 | [Mn(μ-SO4)2(H2O)4][42] | none | Ilesitepentahydrate is called jôkokuite; the hexahydrate, the most rare, is called chvaleticeite | with 8-membered ring Mn2(SO4)2 core |
| MnSO4(H2O)5 | ? | jôkokuite | ||
| MnSO4(H2O)6 | ? | Chvaleticeite | ||
| MnSO4(H2O)7 | [Mn(H2O)6] | one | mallardite[35] | see Mg analogue |
| FeSO4(H2O) | [Fe(μ-H2O)(μ4-κ1-SO4)4][36] | none | see Mn, Co, Ni, Zn analogues | |
| FeSO4(H2O)7 | [Fe(H2O)6] | one | melanterite[35] | see Mg analogue |
| FeSO4(H2O)4 | [Fe(H2O)4(κ′,κ1-SO4)]2 | none | sulfate is bridging ligand, 8-membered Fe2O4S2 rings[37] | |
| FeII(FeIII)2(SO4)4(H2O)14 | [FeII(H2O)6]2+[FeIII(H2O)4(κ1-SO4)2]−2 | none | sulfates are terminal ligands on Fe(III)[43] | |
| CoSO4(H2O) | [Co(μ-H2O)(μ4-κ1-SO4)4][36] | none | see Mn, Fe, Ni, Zn analogues | |
| CoSO4(H2O)6 | [Co(H2O)6] | none | moorhouseite | see Mg analogue |
| CoSO4(H2O)7 | [Co(H2O)6] | one | bieberite[35] | see Fe, Mg analogues |
| NiSO4(H2O) | [Ni(μ-H2O)(μ4-κ1-SO4)4][36] | none | see Mn, Fe, Co, Zn analogues | |
| NiSO4(H2O)6 | [Ni(H2O)6] | none | retgersite | One of several nickel sulfate hydrates[44] |
| NiSO4(H2O)7 | [Ni(H2O)6] | morenosite[35] | ||
| (NH4)2[Pt2(SO4)4(H2O)2] | [Pt2(SO4)4(H2O)2]2− | none | Pt-Pt bonded Chinese lantern structure[45] | |
| CuSO4(H2O)5 | [Cu(H2O)4(κ1-SO4)2] | one | chalcantite | sulfate is bridging ligand[46] |
| CuSO4(H2O)7 | [Cu(H2O)6] | one | boothite[35] | |
| ZnSO4(H2O) | [Zn(μ-H2O)(μ4-κ1-SO4)4][36] | none | see Mn, Fe, Co, Ni analogues | |
| ZnSO4(H2O)4 | [Zn(H2O)4(κ′,κ1-SO4)]2 | none | sulfate is bridging ligand, 8-membered Zn2O4S2 rings[37][47] | |
| ZnSO4(H2O)6 | [Zn(H2O)6] | none | see Mg analogue[48] | |
| ZnSO4(H2O)7 | [Zn(H2O)6] | one | goslarite[35] | see Mg analogue |
| CdSO4(H2O) | [Cd(μ-H2O)2(κ1-SO4)4] | none | bridging water ligand[49] |
Transition metal nitrates form a variety of hydrates. The nitrate anion often binds to the metal, especially for those salts with fewer than six aquo ligands. Nitrates are uncommon in nature, so few minerals are represented here. Hydrated ferrous nitrate has not been characterized crystallographically.
{{cite journal}}: CS1 maint: multiple names: authors list (link){{cite journal}}: CS1 maint: multiple names: authors list (link)