Acid was a type of chemical compound that was caustic in nature.
Acid, sometimes in large bodies, could be found on many planets. The seas of Marinus (TV: "The Sea of Death"), 16 Alpha Leonis One (PROSE: Eternity Weeps) and Endpoint were all composed of acid. (PROSE: Hope) The planet Vortis also had pools of acid on its surface. (TV: The Web Planet) Skaro had a river of acid, where the Dalek Emperor attempted to kill Zeg. (COMIC: Duel of the Daleks)
Some species used acid as a method of killing. This could include regular weaponry or an impromptu defence.
- The First Doctor used a syringe of prussic acid to kill a Zilgan on the Sense Sphere. (PROSE: The Monsters from Earth)
- A special acid that could destroy silicon-based life was developed and used on Kastria in the form of traps. After being returned to Kastria, Eldrad fell victim to one of these traps. (TV: The Hand of Fear)
- The calcium-based bodies of Raxacoricofallapatorians were vulnerable to acetic acid. On Raxacoricofallapatorius, criminals were executed by being dissolved in a vat of acetic acid while still alive. (TV: Aliens of London, World War Three, Boom Town)
- The Second Doctor defeated the Krotons by using sulphuric acid against them. (TV: The Krotons)
- The Second Doctor used a bottle of acid to drive off a panther that was threatening him. (COMIC: The Monsters from the Past)
- On Jaconda, Mestor's body was destroyed with a beaker of acid. (TV: The Twin Dilemma)
- On Earth, coronic acid was used against the Sontarans Stike and Varl. (TV: The Two Doctors)
- Cassandra O'Brien's surgeons used moisturisers filled with acid to defend her. (TV: The End of the World)
- Acid was used in acid jars, a power source reliant upon wind to produce usable energy. (TV: Frontios)
- The humans of Varos used acid as a method of body disposal. The Sixth Doctor escaped from being dissolved in an acid bath. (TV: Vengeance on Varos)
- The Mantis leader, in his attempts to learn the nature of the Doctor's TARDIS, had it submerged in a beaker of acid in his ship's lab. The acid had little effect on the TARDIS. (COMIC: Size Control)