Let’s start our discussion of acids and bases by defining some terms that are essential to the topics that follow.

Arrhenius acids and bases are:

acid—a substance that increases the concentration of protons (H⁺) in water

base—a substance that increases the concentration of hydroxide ions in water (OH^-)

These definitions are limited to aqueous solutions.

Brønsted and Lowry acids and bases as:

acid—a substance that donates a proton to another substance

base—a substance that accepts a proton

These definitions can also apply to reactions that are not aqueous, so they are more accurate.

Lewis acids and bases are:

acid—a substance that accepts an electron pair

base—a substance that donates an electron pair

Here are some other terms that you’ll need to be familiar with:

hydronium (H₃O⁺)—H⁺ riding “piggyback” on a water molecule; water is polar, and the positive charge of the naked proton is greatly attracted to one of the negative electron pairs on adjacent oxygen

monoprotic—describes acids that can donate one H⁺

diprotic—describes acids that can donate two H⁺ ions

polyprotic—describes acids that can donate more than one H⁺ ion

amphiprotic—describes a substance that can act as either an acid or a base. This means it can either lose a proton or gain one. Water is amphiprotic: it can form either a hydroxide ion or a hydronium ion. Other examples of amphiprotic substances are , ,