Question

Why magnesium and calcium don't react with cold water?

Answer 1

Mg and Ca are not reacted completely with cold water. they are virtually unreactive with cold water
Magnesium burns in steam to produce white magnesium oxide and hydrogen gas. Very clean magnesium ribbon has a very slight reaction with cold water. However, the reaction soon stops because the magnesium hydroxide formed is almost insoluble in water and forms a barrier to the magnesium preventing further reaction.

Calcium reacts slowly with water. This is in contrast with magnesium, immediately above calcium in the periodic table, which is virtually unreactive with cold water. The calcium metal sinks in water and after an hour or so bubbles of hydrogen are evident, stuck to the surface of the metal.