Why do the reactions of Magnesium in steam and cold water differ? "Magnesium has a very slight reaction with cold water, but burns in steam.

A very clean coil of magnesium dropped into cold water eventually gets covered in small bubbles of hydrogen which float it to the surface. Magnesium hydroxide is formed as a very thin layer on the magnesium and this tends to stop the reaction.

Magnesium burns in steam with its typical white flame to produce white magnesium oxide and hydrogen." - From Chemguide.

May I know why the reactions of Magnesium in steam and cold water differ?
Magnesium is a very active metal- highest activity of all engineering metals. Your question gives you part of the answer. In cold water it forms a decent protective layer. Given greater activation energy- simple as hot water, that layers is simply damaged the oxidation of the magnesium proceeds unimpeded at a greater rate. Once its in steam the oxidation is extremely rapid.