Study of Compounds · Hydrogen Chloride

Fountain Experiment

A dry flask of HCl gas, a squeeze of water from the dropper — and the gas vanishes into that water almost completely. The pressure inside collapses, so the ordinary outside atmosphere pushes blue litmus solution up the jet tube, where it bursts out as a red fountain. The fountain is the proof: HCl is enormously soluble in water, and its solution is acidic.

● gas molecules in flask ● blue litmus solution ● turned red (acidic) ready — squeeze the dropper

Run the experiment

  1. Water enters the flask from the dropper
  2. Gas dissolves — partial vacuum forms
  3. Outside pressure drives liquid up the jet tube
  4. Fountain sprays; litmus turns red

Gas in the flask

HCl dissolves to about 450 volumes of gas per volume of water. Its solution is acidic, so blue litmus turns red.

Conditions

Water squeezed in Vw 2.0 mL
Flask volume Vf 500 mL
Strong fountain
gas dissolves almost completely
0.04
pressure inside (atm)
0.96
Δp outside − inside
991
water column Δp lifts (cm)
100
% of gas dissolved
jet tube ≈ 40 cm 1 atm lifts 1033 cm of water

Try this

Switch to O₂ and squeeze. Nothing happens — the water cannot swallow the gas, so the pressure inside stays at 1 atm and there is nothing for the atmosphere to push against. Now try CO₂: the liquid creeps a little way up the tube but never reaches the jet. Only a very soluble gas empties the flask fast enough to make a fountain.