NCERT Solutions Curiosity Chapter 7 –947.2 Convection — Activity 7.3: Let us find out
Book page 93 Updated on2026-09-05
Q1.
Observe the movement of the coloured streak in the water.
Answer
As soon as the candle is lit, a streak of purple colour rises straight up from the centre of the beaker's base, spreads out near the top, and then comes down along the sides of the beaker [Fig. 7.5(b)]. The streak keeps going round and round until the whole beaker of water is coloured.
The convection current in Activity 7.3 — the coloured streak goes up in the middle, above the flame, and comes down along the cooler sides.
Why it happens: the potassium permanganate simply colours the water so that its movement becomes visible. The water at the bottom, right above the flame, gets heated, expands, becomes lighter and rises. The comparatively cooler and heavier water at the sides sinks to take its place, gets heated in turn, and rises again. This continuous circulation is convection, and the coloured streak is nothing but the path of the moving water particles.
Caution: handle potassium permanganate and the burning candle only under the supervision of your teacher or an adult, and keep the beaker on a tripod stand with a wire gauze.