During a practical lesson, a group investigates convection in water. Fig. 1 shows a large transparent tank. A small crystal of potassium manganate is placed...
Question 1 Report
During a practical lesson, a group investigates convection in water. Fig. 1 shows a large transparent tank. A small crystal of potassium manganate is placed near the base on the left. A lamp heats the water at that end of the tank. The purple colour slowly moves around the tank, even though the water is not stirred.
The group uses the colour as a tracer. They are told not to drink the solution because potassium manganate is a chemical compound. The same movement of water can occur in a lake warmed by sunlight near a shallow shore.
(a) Describe the path taken by the coloured water shown in Fig. 1. [2]
(b) Explain why water near the lamp rises. [3]
(c) Use particle ideas to explain why the water moves back along the surface away from the lamp. [3]
(d) What would happen to the convection current if the lamp were switched off and the tank was left for a long time? [2]
Answer Details
(a) The coloured water rises near the lamp [1]. It then moves across the top, sinks at the cooler end, and returns along the bottom [1].
(b) Water near the lamp gains thermal energy [1]. Its particles move faster and spread further apart [1], so the water becomes less dense and rises [1].
(c) At the surface, water moves away from the lamp [1]. It cools, so its particles move more slowly and become closer together [1]. It becomes denser and sinks, producing a circulating convection current [1].
(d) With the lamp off, temperature differences in the tank decrease [1]. The convection current slows and eventually stops [1].
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