Q.Write the balanced equation for the reaction of calcium metal with cold water and describe what is observed.

Group 2 metals react with oxygen, water, hydrogen and the halogens in patterns that broadly parallel those of the alkali metals, but with two important differences: reactivity is generally somewhat lower for a given period (owing to Group 2's higher ionization enthalpy), and beryllium in particular shows several genuine exceptions to the group trend because of its unusually small size.

With oxygen. Beryllium and magnesium form a thin, strongly adherent protective oxide film that passivates the metal at room temperature (magnesium ribbon must be cleaned before it will ignite), but once ignited magnesium burns brightly in air to give a mixture of the oxide and nitride: $2\text{Mg}+\text{O}_2\rightarrow 2\text{MgO}$ and $3\text{Mg}+\text{N}_2\rightarrow \text{Mg}_3\text{N}_2$. Calcium, strontium and barium tarnish and burn readily in air, and the heavier three can additionally form a peroxide when the oxide is heated strongly in excess oxygen, echoing (in a limited way) the alkali-metal oxide/peroxide pattern.

With water. Reactivity rises sharply down the group. Beryllium is the standout exception: it does not react with water even at red heat, because its exceptionally thin but tenacious oxide film seals the metal completely against further attack. Magnesium reacts only slowly with cold water but readily with hot water/steam ($\text{Mg} + 2\text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2 + \text{H}_2$). Calcium reacts moderately with cold water, forming the sparingly soluble hydroxide as a characteristic milky suspension ($\text{Ca} + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2$); strontium and barium react still more vigorously. …