Liquid was a state of matter.
On Svartos, liquid nitrogen was known as frostburn. (TV: Dragonfire)
Anti-plastic was a blue liquid that caused plastic to disintegrate. (TV: Rose, PROSE: Autonomy)
Historians who had been allowed access to the "Monster Vaults" of the databanks in the Doctor's TARDIS observed that the body of an Abzorbaloffs was highly unstable and required an artificially generated limitation field to prevent it from reverting to a liquiform state (PROSE: The Monster Vault) as had happened to Victor Kennedy when his cane which created the limitation field was broken, (TV: Love & Monsters) causing him to melt into a puddle. (WC: The Genuine Article)
The space probe Phoenix was sent to Mars by NASA on 31 July 2008. It found liquid water on the planet. (TV: The Waters of Mars)
The Encyclopedia Gallifreya was a series of books, in the form of purple liquid in bottles, with at least eleven volumes. (TV: Journey to the Centre of the TARDIS)
Dark water was a liquid through which only organic matter could be seen. (TV: Dark Water)
The Nyathoggoth was a vampiric creature with a liquid form. (PROSE: Project: Valhalla)
The Esturians were made entirely of liquid. (PROSE: The Terror of the Thames)
The Gabrielideans were a liquid-based lifeform. (PROSE: Alien Bodies)
Arcateenians were made mostly of liquid, but adopted a humanoid form. (TV: Cyberwoman)