Acid

Acid was a type of chemical compound that was caustic in nature.

Natural occurrences
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)

Offensive purposes
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)

Eldrad developed an acid that neutralised the molecular bond in silicon-based lifeforms. When he returned to Kastria after his execution, Eldrad triggered a booby trap that fired a missile containing the acid upon him. Knowing there to be no antidote, he was forced to use the regenerator chamber. (TV: The Hand of Fear)

The calcium-based bodies of Raxacoricofallapatorians were vulnerable to acetic acid. (TV: World War Three) On Raxacoricofallapatorius, criminals were executed by being dissolved in a vat of acetic acid while still alive. (TV: 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)

Other uses
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)

Other information
Diplosians were allergic to citric acid. (TV: The Stones of Blood)

In the 22nd century, an acid factory was located at St John's Monastery. To protect themselves from its effects, the miners wore acid suits. (TV: The Rebel Flesh)