How many molecules of water does it take to dissociate HCl? It’s complicated

Experiments in ultracold water droplets have revealed new insights into how covalent hydrogen chloride turns into hydrochloric acid as water molecules are added one by one. The work could potentially provide insight into other proton transfer reactions such as base chemistry or even DNA mutation.

Hydrochloric acid is perhaps the best known acid in chemistry. Like most acids, it is a covalent compound when anhydrous. ‘When water molecules interact with the HCl molecule, they form hydrogen bonds,’ explains spectroscopist Fan Xie at DESY in Hamburg. ‘During this process the hydrogen atom leaves the chlorine atom, forming ions, then the hydrogen ion, or proton, combines with water to form a hydronium ion… We know that without water it does not dissociate, with one or two waters it just forms hydrogen bonds, but in solution – with billions and billions of water molecules, it dissociates. So, the question is how many water molecules does it take to dissociate one hydrochloric acid?’