Ask “will anything happen to the candle when I add the vinegar to the baking soda? Do you have a hypothesis?” They guessed that the candle would go out, because (a) candles need oxygen to burn and (b) carbon dioxide is heavier than air.“OK, so let’s test the hypothesis that CO2 is heavier than air. If it is, the candle should go out, right?”Pour the vinegar:
The candle went out (note the wisps of smoke):
Yes, indeed, carbon dioxide (CO2) is heavier than air.