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Figure 4 shows a source of heat placed at equal distances from two identical flasks X and Y containing air. The surface of X is painted black while Y is clear.

X and Y are linked by a U-tube filled with water whose level S and T are initially the same; it is later observed that S falls while T rises. Explain this observation.

Answer Text:
- X being black absorbs heat faster than Y. the gas in X therefore is heated – it expands pushing the water level S downwards,
which causes the level in T to rise.