Hii

The reaction ofcalciumwithwateris less violent. So, the heat released is not sufficient for the hydrogen to catch fire.Calciumstartsfloatingbecause the bubbles of hydrogen gas which are formed during the reaction stick to the surface of the metal and hencecalcium floatsonwater.

Good luck

Hii

The reaction of calcium with water is less voilent. The reaction is as follows:
Ca (s) + 2H2O (l) → Ca(OH)2 (aq) + H2 (g)
 Since the reaction is less voilet, the heat released is not sufficient for the hydrogen to catch fire.Calcuim starts to float as the bubbles of hydrogen gas which are formed during the reaction stick to the surface of the metal and hence calcium floats on the surface.

Hope this helps
Feel free to revert if you have any doubt.
Thanks!!

hello aspirant,

Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g)

 The reaction of calcium  with water  is less violent. so the heat released is not sufficient  for the hydrogen to catch the fire. 

calcium starts floating because the bubbles of hydrogen gas which are formed during reaction stick to the surface of the metal and hence calcium floats on water 

hope you understood,

thank you