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Reaction
The most impressive dehydration reaction is the one between sugar and sulfuric acid, which turns the sugar into a solid mound of frothy black carbon. The normal formula for table sugar is C12H22O11; the reaction removes the H22O11 as water molecules (11 H2O) and leaves behind pure carbon. Like all dehydration reactions, it removes water molecules or the equivalent (two hydrogens and an oxygen). The acid is not actually consumed in the reaction.
Reverse Reaction
Sulfuric acid can also cause the reverse reaction by adding the equivalent of water to an alkene (double-bonded carbon) to create an alcohol. This reverse process is called hydration because it adds water, while dehydration takes it away, and the sequence of steps by which it takes place is the exact reverse of the dehydration process. Whether dehydration or hydration predominates depends on the reaction conditions and the concentration of water in the reaction mixture.
Conditions
By choosing the right temperature and acid concentration, you can turn alkenes into alcohols, or alcohols back into alkenes. If you conduct the reaction at low temperature in dilute solution (i.e. lots of water), you favor hydration and alcohol formation. If you carry it out at high temperature with concentrated acid (i.e. low water concentration), you favor dehydration and alkene formation. Ultimately, the relationship between the alkene and the alcohol is an equilibrium you can push either way.
Considerations
The acid is just a catalyst; it speeds up both the forward and reverse processes. Both alkenes and alcohols are ordinarily stable in water if the acid were is not present. Sulfuric acid is the most commonly used for this purpose, because concentrated hydrochloric acid can actually cause a different kind of reaction, where the chlorine takes the place of the alcohol group in the organic compound. Depending on what you're trying to make, however, this reaction may be desirable too.