Question

GIVE REASON: A hollow glass sphere which floats with its entire volume submerged in water at 4degree celsius sinks when water is heated avove 4deg. C

Answer 1

When the water is heated above 4 degree celcius then the particles start to vibrate and then the density of water decreases . When the density of water will decrease more than that of the object then the object will sink.

Answer 2

 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.