Q1.
Why does the streak of coloured water go up in the middle and come down from the sides?
Answer
Because a convection current has been set up in the beaker — hot water rises in the middle and cooler water sinks at the sides.
Water at the bottom (above the flame) gets heated
It expands, becomes lighter and rises up the middle
Water at the sides is cooler and heavier
⇒ it comes down the sides to take the place of the rising water
This water now gets heated and rises in turn
The cycle repeats until the entire volume of water is heated
It expands, becomes lighter and rises up the middle
Water at the sides is cooler and heavier
⇒ it comes down the sides to take the place of the rising water
This water now gets heated and rises in turn
The cycle repeats until the entire volume of water is heated
Why it happens: the candle is placed right below the centre of the base, so only the water in the middle is heated directly. Heating makes water expand — the same mass now occupies more volume, so it becomes lighter than the water around it and floats upward. Nature does not leave a gap behind, so the cooler, heavier water at the sides flows down and inwards. The entire volume of water therefore gets heated by the actual movement of water particles, which is exactly what convection means.
Tip: this is also why we heat a vessel of water from below and never from the top. Heating from the top gives you no convection current at all — the point tested in question 6 of the exercise.