The color change volcano chemistry demonstration is a noteworthy chemical volcano because its “lava” changes color from purple to orange. It’s a great demonstration for the chemistry classroom or even home school lab because the chemicals are readily available and it’s safe washing the waste down the drain. The color change volcano illustrates a chemical change, an acid-base reaction, and the use of an acid-base indicator. Here’s how to perform the demonstration.
Color Change Volcano Materials
The color change volcano uses an acid, a base, and a pH indicator. Get the best eruption by performing the reaction in a container with a narrow neck, like a model volcano or flask.
- Model volcano or flask
- Muriatic acid (concentrated hydrochloric acid, HCl)
- Baking soda (sodium bicarbonate, NaHCO3)
- Bromocresol purple
Get muriatic acid from a hardware store or hydrochloric acid from chemical supply store. It’s the same chemical, either way. Baking soda comes from the supermarket. Find bromocresol purple online. It’s sold both as a powder and as a pre-mixed solution. If you have the powder, prepare an indicator solution of 50 mg bromocresol purple in 100 ml ethanol.
Make the Chemical Volcano Erupt
- Dissolve about 10 grams of baking soda in 200 ml of water in the model volcano or flask.
- Place the volcano in a glass or plastic pan or tub. This captures the liquid from the eruption and makes clean-up easier.
- Add about 20 drops of bromocresol indicator solution to the volcano. The original solution is orange, but it turns purple in the basic baking soda solution.
- When you are ready for the eruption, pour 50 ml of hydrochloric acid into the volcano. The purple lava in the flask turns orange as the pH goes from alkaline to acidic. Orange lava overflows the container into the tub.
- Sprinkle baking soda on the orange lava and watch it turn purple as it neutralizes the acid. Carefully applying the baking soda gives you a mixture of orange (acidic) and purple (neutral) lava.
After the eruption, add baking soda and fully neutralize the solution (purple). Pour the neutralized liquid down the drain.
How the Color Change Chemical Volcano Works
The volcano’s “lava” changes color in response to the pH or acidity of the solution. Bromocresol purple is purple above pH 6.8. So, the lava is purple when the liquid is neutral or alkaline. The indicator changes from purple to amber below pH 5.2, when the liquid is acidic.
The volcano erupts due to the acid-base reaction between the acid (muriatic acid) and base (baking soda), forming carbon dioxide and water. The lava results from carbon dioxide gas bubbling out of the liquid.
HCO3– + H+ ↔ H2CO3 ↔ H2O + CO2
The acid-base reaction is exothermic, so the volcano produces heat, too.
- Wear gloves, goggles, and other safety gear appropriate for a chemistry demonstration.
- Neutralize the reaction (or any accidental spills) with baking soda. The acid-base reaction makes salt and water, which is safely washed down the drain.
Color Change Chemical Volcano for Kids
Replacing muriatic acid with vinegar (acetic acid) makes the color change volcano safe for kids to perform under adult supervision. Children should not touch or play with the lava because bromocresol purple is an irritant. However, if you replace bromocresol purple with phenolphthalein, you get a reaction safe to touch that changes lava colors from white (acidic) to pink (basic).
Basically, this is the classic chemical volcano demonstration, except including a pH indicator for color-changing lava. Acetic acid is a weak acid, so it doesn’t fully dissociate in water. So, you’ll use more of it to change pH and color.
- National Library of Medicine. “Bromocresol purple.” NCBI PubChem.
- O’Neil, M.J. (ed.) (2006). The Merck Index – An Encyclopedia of Chemicals, Drugs, and Biologicals. Whitehouse Station, NJ: Merck and Co., Inc.
- Summerlin, L. R. (1988). Chemical Demonstrations : A Sourcebook for Teachers (2nd ed.). Volume 1. Washington, DC: American Chemical Society. ISBN 978-0841215351.