Question

why will block of iron float in mercury but sink in the water

Answer 1

due to lower density of iron than mercury it float on mercury and high density than water so sink in it

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Answer 2

Lead is more dense, which means for the same mass it has less volume. Therefore Aluminium displaces more water. The density of mercury bigger than iron, while the density of iron is bigger than water. Therefore, iron floats in mercury, but sinks in water.