A substance that yields hydroxyl (OH-) ions when added to a solution. Alkalis typically have a caustic effect on many substances. The alkalinity of a solution is described in terms of pH, a scale running from 1:14, where 7 is defined as neutral and alkalinity increases with increasing pH. Alkalis turn litmus paper blue and neutralize acids, substances occupying the opposite end of the pH scale. An alkali is also known as a base. A substance capable of functioning both as an acid an an alkali is referred to as amphoteric. See pH.