# Why does acetic acid mix with water?

Bothe formic and acetic acid are completely miscible in water. The carboxyl group dominates their solubility, and of course these acids are capable of polar interaction with the solvent. As the alkyl chain on the acid grows, the water solubility is substantially reduced. Valeric acid, ${C}_{4} {H}_{9} C {O}_{2} H$ has limited solubility in water, as the hydrophobic nature of the hydrocarbyl tail becomes dominant.