Question

What will happen when ash of magnesium is dissolved in water? Is solution acidic or basic? What effect does litmus show in case of oxides of metal?

Answer 1

Ash of magnesium is MgO which is basic
when it will be dissolved in water it will become Mg(OH)2 which is basic
it will turn red litmus to blue

Answer 2

Ash of magnesium is magnesium oxide.

This is obtained from burning magnesium ribbon. When this magnesium oxide dissolves in water, it will liberate hydroxyl [OH-] ions in water and forms magnesium hydroxide. This solution is basic in nature.  

$2 \mathrm{Mg}+\mathrm{O} 2 \rightarrow 2 \mathrm{MgO}\\ \mathrm{MgO}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Mg}(\mathrm{OH})_{2}$

When litmus paper is introduced into magnesium oxide, it will turn blue litmus red indicating acidic nature of metallic oxides; whereas, magnesium hydroxide is basic and will turn red litmus blue.