Pick out the incorrect statement.

• A

  Mg also burns in gases such as $\mathrm{C}{\mathrm{O}}_2$ and $\mathrm{S}{\mathrm{O}}_2$
• Correct answer
  B

  Excess of $\mathrm{C}{\mathrm{O}}_2$ when passed in lime-water turns it milky
• C

  $\mathrm{M}\mathrm{g}\mathrm{C}{\mathrm{l}}_2.6{\mathrm{H}}_2\mathrm{O}$ on heating hydrolyses to form MgO (magnesia), which is refractory
• D

  Alkaline-earth metals are denser and harder than alkali metals

Solution

Here statement in option B is incorrect as when $\mathrm{C}{\mathrm{O}}_2$ is passed in lime-water, it becomes milky due to the formation of $\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3$ but excess of $\mathrm{C}{\mathrm{O}}_2$ leads to the formation of clear solution, due to the formation of soluble $\mathrm{C}\mathrm{a}(\mathrm{H}\mathrm{C}{\mathrm{O}}_3{)}_2$ .

A. $\mathrm{M}\mathrm{g}+\mathrm{C}{\mathrm{O}}_2\to 2\mathrm{M}\mathrm{g}\mathrm{O}+\mathrm{C}$
and $2\mathrm{M}\mathrm{g}+\mathrm{S}{\mathrm{O}}_2\to 2\mathrm{M}\mathrm{g}\mathrm{O}+\mathrm{S}$

D. Inter-atomic forces (or metallic bonding) in alkaline-earth metals are stronger than in alkali metals.

Hence, the correct answer is option: 'B'.