Acids and water
An acid will always be in a water solution.
Hydrogen chloride (HCl) is a colourless gas which is not an acid. It does however, form hydrochloric acid (HCl) when it comes into contact with water.
Further example (but this is getting way, way above what you need)
Upon contact, H2O and HCl combine to form hydronium cations (H3O+) and chlorine anions (Cl-)
HCl + H2O → H3O+ + Cl-
The resulting solution is a very strong acid called hydrochloric acid. The acid dissociation is large which means HCl dissociates (ionises) almost completely in water.