This demonstration requires:
The glass tube should be filled with water and clamped firmly to a stand above a candle. A few small crystals of potassium manganate VII are dropped into the neck of the tube.
- KMnO4 crystals
- Heatproof mat
- Clamp and stand
One corner of the tube is heated with a candle until whisps of purple KMnO4 travelling around the tube can be witnessed. The glass walls of these types of tubes are very thin and may become warped if heated using a bunsen burner.
The heat transferred to the liquid makes it less dense and so it rises within the tube. The cooler parts of the liquid fall creating a moving current. Particles of KMnO4 are carried by this current around the tube giving a more visual representation of the currents involved.
If you do not have a convection tube, the same effect can be created by filling a large one litre beaker with water and dropping some KMnO4 crystals in at the bottom. Heating from below with a Bunsen burner will create noticeable convection currents which because of the size of the beaker, can be viewed by many in a demonstration environment.
Another way to see convection currents involves a slide projector and a candle. The candle is lit and placed on a table about 50cm away from a blank, light coloured wall. The projector is then aimed towards the candle from across the room. In a darkened room, the light being projected should produce a silhouette of the candle on the wall with moving convection currents producing a visible mirage effect above the candle.