Arrange the metals: calcium, iron, magnesium and sodium in order of their activity with water placing the most reactive first.

Being highly reactive, sodium is placed at the top of the reactivity series. It reacts violently with cold water forming sodium hydroxide and hydrogen gas and may explode with hot water. Calcium, is placed next to sodium in the order; it does not react violently with water. Magnesium reacts very slowly with cold water, whereas red hot magnesium reacts rapidly with steam to form magnesium oxide and hydrogen gas. Iron does not react with cold water. However, red hot iron reacts with steam and forms tri iron tetroxide and hydrogen. This is a reversible reaction. Therefore, the decrease in the order of reactivity of metals with water is as follows:
Sodium > Calcium > Magnesium > Iron