Question

Why does magnesium ribbon start floating in hot air?

Answer 1

Hint: Magnesium is not reactive as calcium with cold water. Therefore calcium can react with cold water but magnesium cannot react with cold water.
So, magnesium reacts with water in hot air and forms magnesium hydroxide and hydrogen gas as the products.
\[Mg+2{{H}_{2}}O\to Mg{{(OH)}_{2}}+{{H}_{2}}\]

Complete step by step answer:
Magnesium hydroxide formed during the reaction of magnesium with water in hot air is insoluble in the water and deposits on the surface of the magnesium ribbon.
Magnesium hydroxide separates the magnesium surface with water in hot air and further reaction of magnesium stops reacting with water.
The Hydrogen gas formed at the initial stage of the reaction bubbles on the surface of the magnesium ribbon and makes magnesium ribbon lighter than water.
Therefore magnesium ribbon does not sink and floats on water.

Additional information:
Magnesium is used in different products because of its lightweight for example in car seats, power tools, laptops, and in cameras.
Magnesium is added to molten iron and steel to remove the sulfur present in them.
As magnesium burns easily in air with a bright light, it is used in fireworks and rockets.
The color of the magnesium ribbon is silver.

Note: The hydrogen gas evolved when magnesium reacts with water doesn't catch fire because the heat developed is very low. But hydrogen gas comes out as bubbles and sticks to the surface of magnesium ribbon. As a result the remaining magnesium ribbon floats on water.