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Dissolution reactions are normally endothermic, and crystallization reactions are normally exothermic, but there are some exceptions -- such as the dissolution of calcium chloride. Why is this so?

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  • $\begingroup$ Could you add a bit about what your level of understanding of the topic is? And what you've already looked into for this answer? $\endgroup$ – ManishEarth Nov 8 '12 at 8:17
  • $\begingroup$ Many dissolutions (especially of acids and bases into water) are exothermic. I agree with @Manishearth. Could you tell us more about what you know on this topic? $\endgroup$ – Ben Norris Nov 8 '12 at 12:07
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The endothermicity (taking up heat) or exothermicity (giving off heat) depends on a balance of factors including the entropy change, the degree with which the ions become associated with water, and the possibility of actual chemical reactions.

The most notorious example is sulfuric acid, which reacts so strongly with water that it can actually boil the water, and spatter the acid, which is why one always adds the acid to the water(water as solvent) and precautionary measures (gloves and proper clothing) are needed to handle the reaction.

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