The Acid Fountain
Demonstrate extreme solubility of HCl in water
What this lesson covers
Why it matters
A jet of coloured water shoots up into a dry flask like a miniature geyser. This dramatic fountain is driven not by heat, but by the sudden disappearance of a gas.
The idea in plain words
Map what happens as HCl meets water: each HCl molecule hands its proton to a water molecule, forming a hydronium ion (H₃O⁺) and a chloride ion (Cl⁻).
Predict first
Why does the water rush up into the flask containing HCl gas?
Hydrogen chloride dissolves rapidly in water, creating a partial vacuum that sucks the liquid up.
- HCl gas is highly soluble, collapsing the pressure inside — correct
- HCl reacts with water to produce a new gas that pushes the water up
- The water is heated by the reaction, creating steam pressure
- HCl gas is heavier than air and sinks into the water
What you do
Send a jet of water into the flask of dry HCl gas. The gas dissolves the instant it touches water — the pressure drops and water rushes up as a red fountain.
Check yourself
Why does the coloured water jet up into the flask of HCl gas?
HCl is extraordinarily soluble — a little water dissolves a whole flask of gas almost instantly. The sudden pressure drop lets atmospheric pressure drive the water up as a fountain.
The fountain experiment proves that HCl gas is:
The fountain works only because HCl is extremely soluble in water.
The fountain water carries an indicator. What colour does it turn, and why?
HCl dissolves and ionises (HCl + H₂O → H₃O⁺ + Cl⁻), making the solution strongly acidic, so the indicator turns red.
- HCl dissolves so fast that the pressure inside drops, and outside air pressure pushes the water up — correct
- The HCl gas repels the water upward
- HCl is lighter than air, so it lifts the water
- The water boils and rises as steam
- Highly soluble in water — correct
- Insoluble in water
- Less dense than water
- A strong oxidising agent
- Red — the dissolved HCl forms acidic hydrochloric acid (H₃O⁺ ions) — correct
- Blue — the solution becomes basic
- It stays colourless — dissolving is not a chemical change
- Green — HCl is neutral