Question

A flame always points upwards. Why do you think this is so?
(HINT: Gases produced in a flame are hot, and hence lighter.)

Answer 1

1. A candle flame points upwards because the flame is extremely hot, and thus less dense than air (by a routine approximation using the Ideal Gas Law), and thus rises. A hot air balloon floats for the exact same reason.
2. Hot gas is generally less dense than cool gas. Flame is a very hot gas. As the flame burns, it takes the oxygen from the atmospheric air and heats the surrounding atmospheric gases. Therefore, the hot air around the flame is pushed up because it is less dense.
3. As the air around the flame is pushed up, the air around the flame is drawn towards the flame to take the space of the lighter gases, which are pushed up.
4. Meanwhile, the surrounding cold air is pulled down by gravity and the flame is pressurized all around by the atmospheric gases. This elongates the flame upward.