<P> Most ionic compounds are very brittle . Once they reach the limit of their strength, they cannot deform malleably, because the strict alignment of positive and negative ions must be maintained . Instead the material undergoes fracture via cleavage . As the temperature is elevated (usually close to the melting point) a ductile--brittle transition occurs, and plastic flow becomes possible by the motion of dislocations . </P> <P> The compressibility of an ionic compound is strongly determined by its structure, and in particular the coordination number . For example, halides with the caesium chloride structure (coordination number 8) are less compressible than those with the sodium chloride structure (coordination number 6), and less again than those with a coordination number of 4 . </P> <P> When ionic compounds dissolve, the individual ions dissociate and are solvated by the solvent and dispersed throughout the resulting solution . Because the ions are released into solution when dissolved, and can conduct charge, soluble ionic compounds are the most common class of strong electrolytes, and their solutions have a high electrical conductivity . </P> <P> The solubility is highest in polar solvents (such as water) or ionic liquids, but tends to be low in nonpolar solvents (such as petrol / gasoline). This is principally because the resulting ion--dipole interactions are significantly stronger than ion - induced dipole interactions, so the heat of solution is higher . When the oppositely charged ions in the solid ionic lattice are surrounded by the opposite pole of a polar molecule, the solid ions are pulled out of the lattice and into the liquid . If the solvation energy exceeds the lattice energy, the negative net enthalpy change of solution provides a thermodynamic drive to remove ions from their positions in the crystal and dissolve in the liquid . In addition, the entropy change of solution is usually positive for most solid solutes like ionic compounds, which means that their solubility increases when the temperature increases . There are some unusual ionic compounds such as cerium (III) sulfate, where this entropy change is negative, due to extra order induced in the water upon solution, and the solubility decreases with temperature . </P>

If an aqueous solution of a soluble ionic com- pound is made it will usually
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