Question

A hollow glass sphere which floats with its entire volume submerged in water at 4degree Celsius , sinks when water is heated above 4 degrees Celsius . Why?

Answer 1

Cold water is slightly heavier than Hot water, which in turn makes the Cold water denser than Hot water, both taken in the same volume.
This then alters the relative density of the of the sphere.
The temperature causes the hydrogen bonds to break and hence the Kinetic energy of the molecules increases and they start moving away due to less London forces and thus increasing volume and density (as Mass remains constant)

Answer 2

Answer:

This is because at 4 degrees celcius, water is at its peak density. This high density is due to the fact that water at this temperature gets more contracted than usual and will have a low volume with a higher density.