describe an experiment to show convection current in liquid

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Convection currents in liquids

When a vessel containing a liquid is heated at the bottom a current of hot liquid moves upwards and its place is taken by a cold current moving downwards. Unlike conduction, where heat is passed on from one section of the substance to another as described in the previous post ( transmission of heat energy explained), the heat is here actually carried from one place to another in the liquid by the movement of the liquid itself. This phenomenon is called convection. The same process occurs when a gas is heated.

Experiment to demonstrate convection currents in water

Convection currents in water

Convection currents in water may be shown by filling a large spherical flask with water and dropping a single large crystal of potassium permanganate ( KMnO₄) to the bottom of it through a length of glass tubing. A finger is placed over the end of the tube, which is then removed, together with the colored water it contains.

This method of introducing the crystal ensures getting it in the centre and also prevents it from coloring the water before it is required. On heating the bottom of the flask with a very small gas flame an upward current of colored water will ascend from the place where heat is applied. This colored stream reaches the top and spreads out.

After a short time it circulates down the sides of the flask, showing that a convection current has been set up.

Explanation of convection currents

When a portion of liquid near the bottom of a vessel is heated it expands. Since its mass remains unaltered, it becomes less dense ( you remember that Density = mass / volume ), and therefore rises. Thus a warm convection current moves upwards; for the same reason a cork rises in water or a hydrogen-filled balloon rises in air. In effect, convection is an application ofArchimedes' principle. For further illustration please read Balloons and floating bodies.

If, on the other hand, some liquid in a vessel is heated at the top, the liquid there expands and remains floating on the denser liquid beneath. No convection current is set up, and the only way in which heat can travel downwards under these conditions is by conduction.