The Surprising Science of Boiling: Fresh vs. Salty Water in a Vacuum! In a vacuum chamber, both fresh and salty water show that boiling depends on pressure as well as temperature. When air is removed, the pressure drops, letting water molecules escape into vapor even at low temperatures. Fresh water boils first because salt ions in salty water hold onto water molecules, making it harder for them to escape. This shows how pressure and dissolved substances affect when a liquid boils. #science #lea