Citric Acid Volcano
Summary
This is a neutralization1 between an acid and a carbonate that releases carbon dioxide gas. After doing it you’ll be able to explain where the gas actually comes from, why dish soap turns a fizz into a foaming “eruption,” and how the fact that citric acid is triprotic2 sets the ratio of ingredients the recipe needs.
History
The reaction between acids and carbonates has been known for centuries. Joseph Black3’s 1756 discovery that carbonates release “fixed air” (carbon dioxide) when treated with acid was a landmark in chemistry, helping to overturn phlogiston theory.
The volcano demonstration became a staple of science education in the mid-20th century, popularized by science fairs and educational television. While usually done with vinegar, citric acid provides a more reliable and controllable reaction. The combination of acid-base chemistry with the visual drama of an “eruption” has introduced millions of children to chemistry.
Hazards & preparation
PPE: safety glasses — the foam can splash. Everything here is food-grade, so this is one of the safest experiments on the site.
- Citric acid is a mild eye and skin irritant in concentrated form; rinse any splashes with water.
- Expect overflow — work on a tray or in a sink to contain the mess, and keep the floor dry so no one slips.
Disposal: everything is food-safe; rinse it down the drain with plenty of water. See the Safety page for general practice.
Materials
- Baking soda - 30g (2 tablespoons)
- Citric acid - 15g (1 tablespoon)
- Warm water - 100mL (about ½ cup)
- Food coloring - 3-4 drops (optional)
- Dish soap - 1 teaspoon (5mL)
- Beaker, flask, or a tall glass/jar (250mL+)
This recipe deliberately uses about 50% more baking soda than the reaction needs (≈30 g, where the stoichiometry calls for only ~20 g against 15 g of citric acid). The excess makes the eruption more forgiving and longer-lived. See Going further for a stoichiometric comparison run.
Procedure
- Place 30g baking soda in the container.
- Add the dish soap and food coloring.
- Dissolve 15g citric acid in 100mL warm water.
- Pour the citric acid solution onto the baking soda.
- Watch vigorous CO₂ production create a foam volcano — it should erupt within a second or two and subside within a minute.
What you should see
Almost immediately the mixture fizzes hard, and within a couple of seconds a colored foam rises and spills over the rim. The eruption peaks in the first 10–15 seconds, then slows as the acid is used up, leaving a flat, soapy liquid after about a minute.
| Symptom | Likely cause | Fix |
|---|---|---|
| Weak or no fizz | Old baking soda, or citric acid not dissolved | Use fresh baking soda; make sure the citric acid is fully dissolved in warm water first |
| Fizzes but no foam | Too little dish soap, or soap added after the reaction | Stir the soap into the baking soda before pouring the acid |
| Overflows too fast to watch | Container too small or too much soap | Use a taller container, or cut the soap to a few drops |
The reactions
\[\ce{C6H8O7 + 3 NaHCO3 -> Na3C6H5O7 + 3 H2O + 3 CO2}\]
citric acid + baking soda → sodium citrate + water + carbon dioxide gas
The Science
Classic acid-base reaction producing carbon dioxide gas. The soap traps bubbles creating dramatic foam. Citric acid is triprotic (can donate three hydrogen ions), so it reacts with three molecules of baking soda.
Questions to Explore
Why citric acid and not vinegar? Both react with baking soda to produce CO₂, but the recipe calls for citric acid. Citric acid is triprotic — it can donate three hydrogen ions per molecule. How does this change the ratio of acid to baking soda you need, compared to acetic acid (vinegar) which is monoprotic?
Hint / answer
Because each citric acid molecule donates three H⁺, one mole of it reacts with three moles of baking soda. By mass, 15 g of citric acid (~0.078 mol) needs ~0.23 mol of NaHCO₃ ≈ 20 g. Acetic acid is monoprotic, so to release the same amount of CO₂ you’d need three times as many molecules of acid.
Where does the CO₂ come from? The carbon in the gas has to come from somewhere. Looking at the equation, which reactant contains the carbon that ends up as CO₂ — the acid or the baking soda?
Hint / answer
The carbon in the gas comes from the bicarbonate (NaHCO₃), not the acid. The acid’s job is to supply H⁺, which converts bicarbonate to carbonic acid (H₂CO₃); that immediately falls apart into water and CO₂.
What makes it foam rather than just bubble? Plain baking soda and vinegar in an open dish just fizzes. What does the dish soap do, and why does the structure of soap molecules make them good at trapping gas bubbles?
Hint / answer
Soap molecules have a water-loving end and an oil-loving end, so they crowd onto the surface of each bubble and lower the water’s surface tension. That lets the escaping CO₂ blow long-lasting bubble walls instead of popping instantly — the fizz becomes foam.
Can you slow it down? If you mixed citric acid and baking soda as dry powders, nothing would happen. Why does water matter for the reaction to proceed? What does this suggest about how the reaction actually works at the molecular level?
Hint / answer
Water lets both compounds dissolve into free-moving ions, so H⁺ and HCO₃⁻ can actually meet and react. As dry powders the ions are locked in their crystals and can’t rearrange — the reaction happens in solution, not in the solid.
What remains when it’s over? After the fizzing stops, the solution contains sodium citrate. Is it acidic, neutral, or basic? What would a pH indicator tell you — and why might that be surprising?
Hint / answer
Sodium citrate is the salt of a weak acid (citric) and a strong base (from the sodium), so the leftover solution is slightly basic — a mild surprise, given you started with an acid. An indicator would read just above neutral.
Going further
- Weigh the CO₂ you lost. Put the whole setup on a kitchen scale, note the mass, run the reaction, and weigh again once the fizzing stops. The difference is the mass of carbon dioxide that escaped into the air.
- Test the stoichiometry. Run it once with the full 30 g of baking soda and once with the stoichiometric ~20 g, keeping everything else the same, and compare the foam. Does the extra baking soda make a bigger eruption, or just leave more unreacted powder behind?
- Next in the Acid–Base track: capture the gas you just made and prove what it is with the Limewater CO₂ Test.
Footnotes
Neutralization — The reaction of an acid with a base to produce a salt and water, moving pH toward 7.↩︎
Monoprotic / triprotic — How many hydrogen ions an acid can donate — one (monoprotic), two (diprotic), or three (triprotic).↩︎
Joseph Black — Scottish chemist (1728–1799) who discovered carbon dioxide (‘fixed air’).↩︎