Question

why a hollow glass sphere which floats with its entire volume submerged in water at 4 degree Celsius sinks when water is heated above 4 degree celcius

Answer 1

It so because of the unique property of water, almost every liquid contracts when cooled /frozen but in the case of water, it contracts till 4 degree celsius and expands if  it's temperature is reduced further ,at this temperature (4 degree celsius) point it has the highest density and so the sphere floats. Now, if we decrease or increase it's temperature it will expand, lowering it's density, hence the sphere sinks.

Answer 2

The anomalous expansion of water causes the density of water more and when heated its density decreases.