A vial of liquid is a quest item used in Recruitment Drive during the Miss Cheevers challenge. It is first used on a metal spade head after adding cupric sulphate in order to heat it so that it sticks in a hole in a door. Afterwards, the spade head can be used as a door handle to open the door. The vial of liquid is then used again in the second part of the challenge to create a plaster cast of the bronze key. The liquid should be used with the tin (gypsum), which will create the tin (hardening).
Technically, the chemical name for water is Hydrogen Hydroxide, due to the bonding of a Hydrogen ion to a Hydroxide ion. However Dihydrogen Monoxide is a sometimes used name for water, so it is essentially a Vial of water.