CO₂ Density Demonstration
Difficulty: Easy | Time: 10 minutes | Visual Impact: High
Historical Context
Joseph Black’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.
Materials
- Baking soda - 60g (4 tablespoons)
- Vinegar - 250mL (1 cup)
- Tall beaker or jar - 1L
- Large pitcher or container for gas collection
- Candles (tea lights) - 3-4 at different heights
Procedure
- Arrange and light candles at different heights in the tall beaker
- In pitcher, combine 60g baking soda with 250mL vinegar - vigorous fizzing generates CO₂
- Wait 10-15 seconds for fizzing to slow
- Slowly “pour” invisible CO₂ gas over candles (tilt pitcher, gas flows out)
- Candles extinguish from bottom to top as CO₂ fills container!
Reaction
\[\ce{NaHCO3 + CH3COOH -> CH3COONa + H2O + CO2}\]
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.
Safety
Perform in well-ventilated area. CO₂ is an asphyxiant in high concentrations. Normal demonstration amounts are safe.