Give reason: A hollow glass sphere which floats with its entire volume submerged in water at 4 degrees centigrate sinks when the water is heated above 4 degrees centigrate

At 4 ^oC ,hollow sphere floats in water due to the maximum density of water.Because we know water shows unusual expansion when heated from 0 ^oC to 4 ^oC. Instead of expanding with rise in temperature, water contracts till the temperature 4 ^oC.and then starts expanding. Thus, water has its maximum density and minimum volume at 4 ^oC.Above this temperature density becomes less,so sphere sinks.