Fountain Experiment — Solubility of HCl Gas

The classic demonstration that HCl gas is extremely soluble in water and acidic in solution. A dry flask of HCl is fitted with a jet tube dipping into blue litmus solution and a dropper of water; squeezing the dropper dissolves the gas almost completely, creating a partial vacuum, so the outside atmospheric pressure drives the litmus up the jet tube where it bursts out as a red fountain. Students vary the water squeezed in and the flask volume and switch the gas between HCl, NH₃, CO₂ and O₂, watching the pressure inside, Δp and the water column that Δp can support (1 atm ≡ 1033 cm) decide whether a fountain happens at all. The gas comparison separates the two conclusions students routinely conflate: the fountain proves high solubility, while the colour change proves acidity — NH₃ is just as soluble and gives just as strong a fountain, but the litmus stays blue.