Question

why calcium metal from after reaction with water starts floating on it surface

Answer 1

n the reaction between calcium an metal , the calcium hydroxide and hydrogen is the product. As soon as hydrogen is produced it makes bubbles in the water of itself. The reaction is like 

Ca + 2H20 ⇒ Ca(OH)2 +H2

Hydrogen bubbles has the tendency to stick to the metal like calcium as the result calcium become lighter also, so this takes the calcium with bubbles to the surface and starts floating. In this reaction very small amount of heat is also released.