Question

why does magnesium ribbon start floating on water

Answer 1

First u need to know magnesium is not more reactive as calcium hence calcium can react with react with cold water but magnesium cannot is can react with hot water .
So when it reacts with hot water it forms magnesium hydroxide and hydrogen .
The hydrogen gas evolved doesn't catch fire because the heat evolved is very low . But it forms bubbles and stick to the surface of magnesium and thus as a result magnesium starts floating on water due to the hydrogen gas

hence
magnesium ribbon start floating on water
Hope this helped u :-)

Answer 2

Answer:

as it is lighter than water it reacts slowly with boiling water to form HgO and H2