Salt

Salt also known as sodium chloride (DW: Four to Doomsday) was a mineral that was used on Earth as a preservative and flavour enhancer.

Due to its unique chemical properties, disrupting the osmotic balance and electrical conductivity of their cells, table salt could act as a defence against the Fendahl. The Fourth Doctor could kill Fendahleen by firing table salt out of a rifle. The power of salt to ward off evil, then, entered into human folklore in the form of throwing salt over one's shoulder for good luck. (DW: Image of the Fendahl) The Hervoken were also similarly vulnerable to salt. (NSA: Forever Autumn)

Around 6100 BC Osirians utilised salt-based spacecraft. (BFA: The Bride of Peladon)

After rejecting pure salt for being "too salty," the Tenth Doctor ate anchovies containing salt in order to eject cyanide from his body. (DW: The Unicorn and the Wasp)

Adric annoyed Tegan by requesting that she pass him "the sodium chloride". At first, she did not know what he meant. (DW: Four to Doomsday)

Clyde Langer suggested that the salt in his cold chip sandwich was what the Slitheen were allergic to. Luke Smith noted that if the Slitheen were made mostly of water it would dehydrate them like slugs. However, that theory debunked as salt was contained in the bread and water of the sandwich another Slitheen ate. It was soon realised that the Slitheen were calcium-based and allergic to vinegar. (SJA: Revenge of the Slitheen)

Salt was the currency of the Sontila. (ST: The Doctor's Coat)