Glossary

Absolute zero

The temperature at which all substances have no thermal energy; 0 K or –273.15°C.

Absorption spectrum

The series of discrete lines at characteristic frequencies representing the energy required to make
an atom undergo a transition to a higher energy state.

Acid

A species that donates hydrogen ions and/or accepts electrons. See Acidic solution; Arrhenius acid;
Brønsted-Lowry acid; Lewis acid.

Acid dissociation constant (Ka)

The equilibrium constant that measures the degree of dissociation for an acid under specific
conditions. For an acid HA:

Acidic anhydride

An oxide that dissolves in water to form an acidic solution.

Acidic solution

An aqueous solution that contains more H+ ions than OH− ions. The pH of an acidic solution is less
than 7 at 25°C.