This practical demonstrates how convection currents from hot liquids rise and from cold liquids fall. It is usually conducted as a demonstration however it is simple enough to conduct as a class practical if some preparation time is given.
Equipment typically required (per demonstration):
- Large transparent trough or tank
- Water at room temperature
- Ice cube containing water and blue food colouring
- Red food colouring
- Small transparent glass bottle and bung
The main preparation for this practical involves making an ice cube which contains water and a small amount of blue food colouring.
The trough or tank needs to be filled with water and left to settle so that it is at room temperature by the time the practical is to be conducted.
A small drop of red food dye is added to the bottle and then filled with hot water from a kettle. This water does not have to be boiling, just hotter than the temperature of the water in the trough. The bottle should be filled to the brim and a bung inserted.
The bottle is carefully placed on the bottom of the trough and the bung removed gently. You should be able to see the dyed water rise from the bottle and upwards towards the surface of the trough water.
The dyed ice cube can be added to the water at the same time as the bottle. The cube should float on the surface and as it melts, the blue dyed water should fall to the bottom of the trough.
Just as hot air rises, the hot water in the bottle does the same. The hot water is less dense than the surrounding water and so it rises. If the temperature of the water is not particularly hot, you may see it fall again as it cools near the top of the trough.
The melted water from the ice cube is more dense that the surrounding water and so it falls to the bottom of the trough.
The visible convection currents created by the hot and cold liquids are similar to those created by potassium manganate VII when heated in a beaker. This is the traditional way of conducting the convection practical. For another approach, using a convection tube see the link below.