Question

Calcium and Magnesium are heavier than water but still float over it. Explain.

Answer 1

calcium reacts with cold water to form  calcium hydroxide and hydrogen gas.
Heat produced is so less in which is not enough to burn hydrogen gas.
The calcium metal floats on water because the bubbles of Hydrogen gas formed during reaction stick to its surface.
Magnesium metal reacts with hot water to form magnesium hydroxide and hydrogen gas.
The magnesium metal floats on water because the bubbles of Hydrogen gas formed during reaction stick to its surface.

Answer 2

Calcium metal floats on surface of water because of the bubble of hydrogen produce which sticks on surface of calcium when it react with cold water.

Similarly the bubble of hydrogen produce sticks on magnesium when it react with hot water