Acids are generally liquids that, when dissolved in water, give off a solution of hydrogen ion activity greater than pure water. Another method to classify acids are if they donate protons. However, borane can not fit, and so the Lewis definition is the broadest and makes it accept electrons, opposed to its proton donor behavior. There are many acids used for different purposes such as vinegar (which contains acetate) and lemon juice for flavor.

Acidic solutions have a pH of less than 7.

There are generally two types of acids: Strong acids and weak acids.

Strong acids, such as HCl and HBr, completely deionize when dissolved in an aqueous solution. In other words, every single molecule of the strong acid will break up into its constituent ions when dissolved in an aqueous solution due to the dipole-ion interactions between the polar solvent and the dipole molecule.

Weak acids, such as carbonic acid and acetic or ethanoic acid, do not dissolve in water as easily, as their ions are rather tightly bound to their central configuration. This configuration usually has carbon in it in weak acids, and single atoms from groups 14 to 17, period 2, in strong acids. Strong acids usually have multiple ions.