Illustrate the higher density of carbon dioxide relative to air by pouring it over a lighted candle in this demonstration

In this experiment, students watch as a flask is filled with carbon dioxide from a cylinder or generator. They then observe what happens as the gas is poured over a lighted candle, extinguishing the flame and demonstrating that carbon dioxide is denser than air.

This is a quick, simple, teacher demonstration taking about two minutes. Although it could be done as a class experiment, it is probably not worth the extra time that it would take. However, pupil assistance with the demonstration could enhance the impact.



  • Eye protection (safety spectacles, or goggles if handling dry ice)
  • Thermal protection gloves and handling tongs (if using dry ice)
  • Beakers, 100 cm3, x2
  • Conical flask, 250 cm3, with cork or bung to fit
  • Wooden splints


  • Candles (short 1–2 cm pieces), x2 (see note 3 below)
  • Carbon dioxide cylinder with regulator (see note 4)
  • Solid carbon dioxide (dry ice), a few small pieces (see note 5)

Health, safety and technical notes

  • Read our standard health and safety guidance.
  • Wear eye protection throughout and use thermal (insulated) gloves.
  • Nightlights or tea-lights may be used in place of short pieces of candle.
  • Carbon dioxide cylinder – see CLEAPSS Hazcard HC020a and also Laboratory Handbook Section 9.9 about the safe storage and use of gas cylinders.
  • If using solid carbon dioxide (dry ice), this should be obtained within 24 hours of the demonstration in substantially larger quantity than required for the experiment, and stored in a vented insulated container until required. All handling must be done using thermal protection gloves and handling tongs – see CLEAPSS Hazcard HC020a.
  • If neither a carbon dioxide cylinder nor a supply of dry ice is available, carbon dioxide gas may be generated chemically – see these standard techniques for generating, collecting and testing gases. Replace the thistle funnel with a tap funnel or unstoppered separating funnel. Add the hydrochloric acid (100 cm3, 2 M – IRRITANT) a few cm3 at a time to the marble chips (10 g) to generate a steady stream of carbon dioxide, and allow the heavier gas to displace the air from the collection flask. This can be checked by sampling the gas emerging from the neck of the flask using a dropping pipette to suck up a sample of gas, then bubbling it through fresh limewater in a test tube. Immediate and dense milkiness of the lime water should indicate the flask is full of carbon dioxide, which may then be securely corked until required for the demonstration.
  • The 250 cm3 conical flask may be filled prior to the lesson, either with gas from the cylinder or by adding a few lumps of dry ice, and allowing these to evaporate in the flask. The flask should be lightly corked and clearly labelled so the class can read the label, but the teacher may prefer to prepare the flask when required during the lesson. 


  1. Place the two beakers side by side on the bench and put a short length of candle in each.
  2. Light the candles with a splint. They will continue to burn.
  3. Pour carbon dioxide from the flask into one of the beakers and the candle will go out while the candle in the other beaker continues to burn.
  4. Attempt to relight the first candle with a splint. This will fail and the splint will go out.
  5. Now pour the carbon dioxide out of the beaker, and try again to relight the candle. This should now succeed.

Teaching notes

Point out the use of carbon dioxide in some types of fire extinguisher and the reasons for its use.

Carbon dioxide (relative molecular mass 44) is about one and a half times denser than air (average relative molecular mass about 29), and mixing of gases of differing densities by diffusion can be a slow process unless the mixture is deliberately stirred. Hence gases can be effectively poured from one vessel to another, and also of course be collected in a flask or gas jar by displacement of air.

As an additional demonstration the presence of a layer of dense gas in the beaker containing carbon dioxide can be shown by blowing a small soap bubble (using a commercial bubble mixture) and ‘catching’ it in the beaker (a larger beaker will make this easier!). With luck the bubble will descend into the beaker without bursting and then proceed to ‘float’ on the carbon dioxide layer.

Carbon dioxide can also be poured into a test tube containing a little limewater. On shaking the limewater will turn milky.