Question

why iron nail floats on mercury but sinks in water

Answer 1

Because the density of mercury is more than iron and the waters density is far more less than iron... hence iron floats on mercury and sinks in water...
Hope This Works Out...
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Answer 2

IRON NAIL FLOATS ON THE WATER BECAUSE THE DENSITY OF IRON NAIL AND WATER ARE LESS BUT WHEREAS WHEN WE SEE TOWARDS MERCURY ITS DENSITY IS MORE COMPARED TO WATER