| Amorphous solid | a solid with considerable disorder in its structure. |
| Amphoteric substance | a substance that can behave either as an acid or as a base. |
| Anion | a negative ion. |
| Anode | the electrode in a galvanic cell at which oxidation occurs. |
| Aqueous solution | a solution in which water is the dissolving medium or solvent. |
| Aromatic hydrocarbon | one of a special class of cyclic unsaturated hydrocarbons, the simplest of which is benzene. |
| Arrhenius concept | a concept postulating that acids produce hydrogen ions in aqueous solution, while bases produce hydroxide ions. |
| Arrhenius equation | the equation representing the rate constant as k 5 Ae2Ea /RT , where A represents the product of the collision frequency and the steric factor, and e2Ea /RT is the fraction of collisions with sufficient energy to produce a reaction. |
| Atmosphere | the mixture of gases that surrounds the earth’s surface. |
| Atomic number | the number of protons in the nucleus of an atom |
| Atomic radius | half the distance between the nuclei in a molecule consisting of identical atoms. |
| Atomic weight | the weighted average mass of the atoms in a naturally occurring element. |
| Accuracy | the agreement of a particular value with the true value. |
| Acid | a substance that produces hydrogen ions in solution; a proton donor |
| Acid–base indicator | a substance that marks the end point of an acid–base titration by changing color. |
