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Salt hydrates

Salt hydrates should also be discussed in connection with the molecular crystals. Many salts crystallize out of aqueous solution with water of crystallization. This may seem incomprehensible at first, because the incorporation of water molecules prevents the ions from approaching as much as is usual with ionic compounds. The special behavior of water is due to the strong dipole character and the small size of the molecule, i.e. water molecules can interact in the crystal with the cations, especially with complex anions and with other crystal water molecules.

Whether a salt crystallizes out of its solution anhydrous or with crystal water depends essentially on the size ratios. Alkali halides with relatively large cations crystallize anhydrous in the cesium chloride or sodium chloride lattice because high coordination numbers (8 or 6) are possible here. Small, more strongly charged cations bind their hydration shell more firmly even in solution and form so-called aqua complexes.

Aquacomplexes are much larger than the same compound without water entrapment, for example Al3+ with an ionic radius of 57 pm opposite [Al (H2O)6]3+ with 330 pm. You can also work with large anions such as sulfate (244pm) form tightly packed structures. The corresponding anhydrous salts are generally in a layered structure with lower coordination numbers.

Video: Properties of Solids. Elasticity. Hookes Law. Youngs Modulus. Stress. Strain 11. JEE. NEET (May 2022).