explain – a hollow glass sphere which floats with its entire volume submerged in water at 4C sinks when water is heated abov 4C

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. This is the reason why the hollow sphere stays a float and will only sink after the water is heated above the 4 degrees celcius. The sinking is because, when the water is heated above the 4 degrees celcius, it starts to expand and its density falls hence not able to keep the sphere afloat.