In 1923, chemists Johannes Nicolaus Brønsted and Thomas Martin Lowry independently developed definitions of acids and bases based on the compounds' abilities to either donate or accept protons ( H+ ions). In this theory, acids are defined as proton donors; whereas bases are defined as proton acceptors. A compound that acts as both a Brønsted-Lowry acid and base together is called amphoteric.
The dissociation fraction, also called the degree of ionization, is the number of molecules that have dissociated. It is a function of total concentration of the species and its relevant equilibrium constant.
Water is both an acid and a base due to the autoionization, However, the amount of hydronium ions from water may be very small compared to the amount from an acid if the concentration of the acid is high. When calculating the pH of an acidic solution, approximation method or using the quadratic formula has been discussed in the modules on weak acids.
Indicators are substances whose solutions change color due to changes in pH. These are called acid-base indicators. They are usually weak acids or bases, but their conjugate base or acid forms have different colors due to differences in their absorption spectra.
Acids and bases are an important part of chemistry. One of the most applicable theories is the Lewis acid/base motif that extends the definition of an acid and base beyond H+ and OH- ions as described by Brønsted-Lowry acids and bases.
There are three major classifications of substances known as acids or bases. The Arrhenius definition states that an acid produces H+ in solution and a base produces OH-. This theory was developed by Svante Arrhenius in 1883. Later, two more sophisticated and general theories were proposed. These are the Brønsted-Lowry and the Lewis definitions of acids and bases. The Lewis theory is discussed elsewhere.
Thumbnail: Major structural changes accompany binding of the Lewis base to the coordinatively unsaturated, planar Lewis acid BF3. (Public Domain; Ben Mills ).