Why calcium floats over water during the reaction with water?

Calcium floats on water during its reaction with water because bubbles of hydrogen gas, produced in the reaction, stick to the surface of the calcium metal and reduce its effective density, making it lighter than water. The reaction is:

Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)

The hydrogen gas bubbles form on and adhere to the calcium, increasing buoyancy so that the calcium floats. This is not because calcium itself is less dense than water, but because the gas bubbles overall make the system less dense than water. The reaction is relatively mild (not very exothermic), so the hydrogen does not ignite, and the gas can accumulate on the metal surface.