Color Change Chemical Volcano Demonstration

There are several chemical volcanoes that are suitable for use as a chemistry lab demonstration. This particular volcano is nice because the chemicals are readily available and may be safely disposed after the eruption. The volcano involves a color change of the 'lava' from purple to orange and back to purple. The chemical volcano can be used to illustrate an acid-base reaction and the use of an acid-base indicator.

Color Change Volcano Materials

• goggles, gloves, and a lab coat or apron
• 600 ml beaker
• tub large enough to accomodate the beaker
• 200 ml water
• 50 ml concentrated HCl (hydrochloric acid)
• 100 g sodium bicarbonate (NaHCO₃)
• bromocresol purple indicator (0.5 g bromocresol purple in 50 ml ethanol)

Make the Chemical Volcano Erupt

1. In the beaker, dissolve ~10 grams of sodium bicarbonate in 200 ml of water.

2. Set the beaker in the middle of of the tub, preferably inside a fume hood, since strong acid is used for this demonstration.

3. Add around 20 drops of indicator solution. Bromocresol purple indicator will be orange in the ethanol, but will turn purple when added to the basic sodium bicarbonate solution.

4. Add 50 ml concentrated hydrochloric acid to the purple solution. This will cause the 'eruption' in which the simulated lava turns orange and overflows the beaker.

5. Sprinkle some sodium bicarbonate on the now-acidic solution. The color of the lava will return to purple as the solution becomes more basic.

6. Enough sodium bicarbonate will neutralize the hydrochloric acid, but it is best to handle only the tub and not the beaker. When you are finished with the demonstration, wash the solution down the drain with plenty of water.

How the Volcano Works

The indicator solution changes color in response to changes in the pH or acidity of the 'lava'. When the solution is basic (sodium bicarbonate), then the indicator will be purple. When acid is added, the pH of the lava decreases (becomes more acidic) and the indicator changes color to orange. Sprinkling sodium bicarbonate on the erupting volcano will cause localized acid-base reactions so you can get purple and orange lava on different areas of the volcano. The volcano overflows the beaker because carbon dioxide gas is released when the sodium bicarbonate and hydrochloric acid react with each other.

HCO₃^- + H⁺ ↔ H₂CO₃ ↔ H₂O + CO₂