Why does magnesium ribbon start floating on water?
Magnesium is a very reactive metal. It reacts with hot water and forms magnesium hydroxide and hydrogen gas. Magnesium hydroxide being insoluble in water deposits on the surface of the magnesium ribbon and cuts off the contact of magnesium with water and the reaction stops. Hydrogen gas bubbles formed on the surface of the magnesium ribbon makes it lighter than water. Hence magnesium ribbon floats on water.
Mg(s) +2H2O(I) → Mg (OH)2(aq) + H2