Reaction of Magnesium with Steam

In this classic demonstration, magnesium (Mg) reacts with steam. The products are magnesium oxide (a white solid) and hydrogen (a flammable gas).
The (hot) steam ensures this reaction is very fast. Very clean magnesium ribbon has a very slight reaction with cold water. After several minutes, some bubbles of hydrogen form on its surface, and the coil of magnesium ribbon usually floats to the surface. However, the reaction soon stops because the magnesium hydroxide formed is almost insoluble in water and forms a barrier on the magnesium preventing further reaction.
As a general rule, if a metal reacts with cold water, you get the metal hydroxide. If it reacts with steam, the metal oxide is formed. This is because the metal hydroxides thermally decompose (split up on heating) to give the oxide and water.
A similar reaction, involving zirconium in place of magnesium, was partly responsible for the Chernobyl disaster in 1986. Zirconium is used in nuclear reactors to provide the cladding, or outer covering, for the cylindrical fuel rods that power a nuclear reaction. Packed inside the zirconium cladding are pellets of uranium oxide or other fissionable materials. A hydrogen explosion can occur if this zirconium comes into contact with steam, which is used in power stations to drive electrical turbines.