A sealed glass bulb containing mercury (incompletely filled) just floats in water at 4°C. If the water and bulb are (i) cooled to 2°C and (ii) warmed to 8°C, the bulb
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The bulb floats when its average density equals water's density. Water has maximum density at 4°C (ρmax). Mercury and glass contract when cooled and expand when warmed.
At 2°C: Water density decreases (ρwater < ρmax). Mercury contracts more than glass, increasing bulb's average density. Since ρbulb > ρwater, it sinks.
At 8°C: Water density also decreases (ρwater < ρmax). Mercury expands more than glass, decreasing bulb's average density. Since ρbulb < ρwater, it floats.
Final answer:
(i) sinks (ii) floats