Ionic structures and bonding

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1. Why do salts which look crystalline not contain water of crystallisation?
2. Why do only some salts crystallise with water of crystallisation?
091008

Corrie writes .....
 
1. A crystalline appearance of a solid is an indication of a structure in which there is regular arrangement (a lattice) of the particles (atoms, molecules or ions) of which the solid consists. There are many, many crystalline substances which do not contain any water at all, including metals themselves (if you look closely) and many salts and organic compounds.
 
2.The water molecules in substances containing water of crystallisation, such as metal salts, are usually attached to the metal ion. Water molecules are polar, and the negative end of the molecule is attracted by the positively charged metal ion. The strength of the attraction will depend mainly on the charge and the size (radius) of the metal ion. Small metal ions with a high charge, such as Mg²⁺, have a high charge density, and attract water molecules strongly. Larger, singly charged metal ions, like Na⁺, attract water molecules much less strongly.
 
Ionic crystals are held together by the attraction of the oppositely charged ions. This attraction will be affected by whether the metal ion is surrounded by water molecules or not. Whether the water molecules that surround a metal ion in solution are taken into the solid when it crystallises depends how strongly these warter molecules are held compared to the forces involved in holding the lattice together.
 
For NaCl, as an example, it is energetically more favourable to leave the water molcules behind in solution and form crystals without any water of crystallisation, whereas for magnesium chloride a stable crystal lattice can be formed with each Mg²⁺ ion surrounded by 6 water molecules, giving the salt MgCl₂.6H₂O.
 
As a result of these two balancing factors, some salts made with no water of crystallisation will absorb water from the air. Conversely, some crystalline salts containing water of crystallisation which is only weakly held will lose water when exposed to air and become powdery.

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updated: 10 October 2008