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Following on from our ‘lava lamp‘ we did a variation of the chemical reaction involved, that shows how the carbon dioxide (CO₂) is heavier than air. The density is approx. 1.98 kg/m³, whereas air is only 1.2 kg/m³.

We placed a small cup in a deep dish and put a short burning candle in next to it. In the cup we put about 2 tsps each of

• sodium bicarbonate, aka baking soda (NaHCO₃)
• citric acid (C₆H₈O₇)

Then we added some water (H₂O).

The chemical equation for this is

3 NaHCO₃ + C₆H₈O₇ –> 3 CO₂ + 3 H₂O + Na₃C₆H₅O₇

You see the mixture bubbling from the production of CO₂, but you can’t see the CO₂ itself. However, because it is dense, the CO₂ spills over the top of the cup and collects in the dish below. When it is deep enough, it puts out the candle.
We also put several lighted matches down into the space at the bottom of the dish, and they went out too. This was quite an impressive effect for quite small amounts of chemical.

Here is how it looked:

Another noticeable effect is that the reaction is endothermic, it absorbs energy (heat), and therefore cools down. So the mixture left in the cup is much colder than what we started with. It was very noticeable to the touch, and you could see condensation on the cup as well. So, clearly we need to obtain a thermometer suitable for this kind of investigation.

Although mixing baking soda and vinegar produce CO₂ as well, the mixture tends to foam. That means that the CO₂ is released into the air more gradually, since the foam has to dissipate before the bubbles are released.