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 Keys of Marinus) 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) Lake Mono on the Gyre was composed of acid, where the humans put criminals to death by immersing them in it. (PROSE: Night of the Humans)

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) An Eldrad known as "the Watcher", who had been restored from the original's eye, was killed with an acid filled dart by the Eldrad who was restored from the original's hand. In turn, this Eldrad was killed with the same dart by Mulkris. (AUDIO: Eldrad Must Die!)

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 once 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
Acid was one of the few things which could prevent a Time Lord from regenerating. (PROSE: Night of the Humans)

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)

Strax, another Sontaran, expressed a fondness for warfare that involved acid. (TV: The Crimson Horror, Deep Breath)