Polar solvents are distinguished by their large partial charges, as opposed to nonpolar solvents, which are more electrically neutral. Chemists recognize two groups of polar solvents, protic and aprotic, depending on whether or not the molecules of the solvent are capable of forming hydrogen bonds with the solute. Polar protic solvents include acetic acid, methanol and ethanol, while aprotic solvents include ethyl acetate and tetrahydrofuran.
Polar aprotic solvents, such as acetone and dimethyl sulfoxide, are highly reactive in dissolving charged solutes. Lacking the O-H or N-H structures of protic solvents, these compounds don't form hydrogen bonds with the solute and are therefore relatively free to act in solution.
Polar protic solvents also possess large partial charges, but unlike the aprotic solvents, they also contain O-H or N-H structures that are eager to form hydrogen bonds with solutes. Water (H2O) is the most common polar protic solvent, and it has even been called the "universal solvent" for its extremely high polarity and the ease with which it forms hydrogen bonds.
A third group of polar solvents exists, the "borderline" polar aprotic group. These solvents are considered borderline because, while they are technically polar, they have a low polarity compared with the "true" polar solvents. Borderline polar solvents, such as dichloromethane, ethyl acetate and tetrahydrofurane, all lack the O-H and N-H bonds of the protic group, making them good general-purpose solvents.