CO₂ Density Demonstration
Summary
Carbon dioxide is heavier than air, so you can “pour” the invisible gas over a row of candles and snuff them from the bottom up. You’ll be able to explain why a gas’s density follows its molar mass, why CO₂ smothers flame, and how the same property turned deadly at Lake Nyos.
History
Joseph Black1’s discovery of “fixed air” (carbon dioxide) in 1756 included the observation that it was denser than ordinary air. This property fascinated natural philosophers - an invisible gas that could be “poured” like water.
The density of CO₂ has real consequences. In 1986, Lake Nyos in Cameroon released a massive cloud of CO₂ that flowed downhill, suffocating 1,700 people and thousands of animals. The gas had accumulated in the lake bottom from volcanic activity.
On a smaller scale, this property makes CO₂ an excellent fire extinguisher. Since it’s denser than air and doesn’t support combustion, it smothers fires by displacing oxygen. Modern CO₂ fire extinguishers use this same principle.
Hazards & preparation
PPE: safety glasses; an adult should handle the lit candles.
- Open flame. Keep hair, sleeves, and paper away from the candles, and have a way to extinguish them at hand.
- Ventilation. CO₂ is an asphyxiant in high concentration — do this in a well-ventilated room. Demonstration amounts are safe.
Disposal: the spent baking-soda-and-vinegar liquid is harmless down the drain. See the Safety page.
Materials
- Baking soda - 30g (2 tablespoons)
- Vinegar - 250mL (1 cup)
- Tall beaker or jar - 1L
- Large pitcher or container to generate/collect the gas
- Candles (tea lights) - 3-4 at different heights
Procedure
- Arrange and light the candles at different heights inside the tall beaker.
- In the pitcher, combine 30g baking soda with 250mL vinegar — it will fizz vigorously, generating CO₂.
- Wait 10-15 seconds for the fizzing to slow so mostly gas (not liquid) remains.
- Slowly tip the pitcher and “pour” the invisible CO₂ over the candles — pour the gas, not the liquid.
- Watch the candles go out from the bottom up as CO₂ fills the beaker.
What you should see
Nothing visible leaves the pitcher, yet the lowest candle flickers and dies first, then the next, then the top one — the invisible gas is filling the beaker from the bottom like water.
| Symptom | Likely cause | Fix |
|---|---|---|
| Candles don’t go out | Poured too slowly, or gas already dispersed | Make a fresh, vigorous batch and pour promptly |
| A candle relights / flickers back | Not enough CO₂ reached it | Generate more gas; pour steadily to keep it filling |
| Liquid splashes onto candles | Poured too soon/too far | Wait for the froth to subside; pour only the gas above it |
The reactions
\[\ce{NaHCO3 + CH3COOH -> CH3COONa + H2O + CO2}\]
baking soda + vinegar → sodium acetate + water + carbon dioxide gas
The Science
CO₂ is ~1.5× denser than air (1.98 g/L vs 1.29 g/L at STP). This extra density comes from the heavier carbon and oxygen atoms compared to the nitrogen and oxygen in air.
Being denser, CO₂ sinks and can be “poured” like water. It flows over the rim of the container and down into the beaker.
CO₂ doesn’t support combustion because fire requires oxygen. As CO₂ displaces air from the bottom up, candles at lower levels extinguish first, then higher ones.
Why you can’t see it: CO₂ is a colorless gas. We only “see” it flowing by observing its effect on the flames.
Questions to Explore
Why is CO₂ denser than air? Air is mostly nitrogen (N₂, molar mass 28) and oxygen (O₂, molar mass 32). CO₂ has a molar mass of 44. How does molar mass translate into density for a gas at the same temperature and pressure?
Hint / answer
At the same temperature and pressure, equal volumes of any gas hold the same number of molecules. So a gas made of heavier molecules weighs more per litre: CO₂ (44) is about 1.5× the average mass of air (~29), and therefore about 1.5× as dense.
Why does CO₂ put out fire? It’s not that CO₂ is cold — it extinguishes flame at room temperature. What does fire actually need, and how does CO₂ deprive it of that requirement?
Hint / answer
Fire needs oxygen to keep burning. Dense CO₂ sinks around the flame and pushes the oxygen-containing air away, so the flame is smothered — no oxygen, no combustion, regardless of temperature.
Why can’t you see it flowing? The gas is invisible, yet it behaves like a liquid as you pour it. If you added water vapor or dry-ice fog to the demonstration, what would that let you see — and why would that be more visible?
Hint / answer
CO₂ gas is colourless, so you only infer its flow from the dying flames. Dry ice chills the air and condenses water into a visible fog that rides along with the CO₂, letting you actually watch the heavy gas pour and pool.
The Lake Nyos disaster. In 1986 a volcanic CO₂ release from a lake in Cameroon killed 1,700 people and all the nearby animals, but birds flying overhead were unharmed. Based on CO₂ density, explain why altitude determined who survived.
Hint / answer
The heavy CO₂ hugged the ground and flowed downhill, displacing breathable air at low levels — fatal for people and animals in the valleys. Birds in flight were above the layer of dense gas, in normal air, so they were spared.
Limits of the demonstration. The CO₂ you produce doesn’t stay in the beaker forever — eventually it mixes with the surrounding air. What physical process causes this, and why does it take longer in still air than in a breeze?
Hint / answer
Diffusion slowly mixes the CO₂ molecules into the surrounding air until the concentration evens out. In still air that mixing is gradual; a breeze stirs the gases together mechanically, so the CO₂ disperses much faster.
Going further
- Prove it’s CO₂. Pour some of the gas into a glass of clear limewater — it should turn milky, confirming the invisible gas really is carbon dioxide.
- Make it visible. Repeat with a chunk of dry ice in warm water to get a fog-laden heavy gas you can watch pour and pool.
Footnotes
Joseph Black — Scottish chemist (1728–1799) who discovered carbon dioxide (‘fixed air’).↩︎