Dehydration, in medical terms, means loss of excessive water or fluids from the body or an organ, which results into restlessness, fatigue and even death in severe cases.
In chemical reactions, however, dehydration means removal/loss of water molecules (H2O) from a substance or compound.
For example, the conversion of alcohols into ethers is a dehydration reaction:
2 C2H5-OH → C2H5-O-C2H5 + H2O
Ehananol Diethyl ether Water
Hydrolysis, on the other hand, means addition of water molecules (H2O) to a substance or compound that results into splitting up of chemical bonds (Hydrolysis= Hydro meaning water and Lysis meaning breaking down). On the ionic level, the H2O molecule breaks down into –OH and H3O ions.
An example of Hydrolysis is when an ester breaks down into carboxylic acid and an alcohol by the addition of water molecules.
CH3COOC2H5 + H2O → CH3COOH + C2H5OH
Ethyl acetate Water Acetic acid Ethanol