Question

How many of the following will turn moist red litmus blue and finally white?
${Li}_{2}O.{KO}_3,Rb{O}_2,{Cs}_2{O}_2,BeO,MgO,Ba{O}_2,SrO$

Answer 1

The standard oxides Li2O, MgO and SrO are water-soluble and hydrolyse to form hydroxides and a basic solution. BeO is insoluble in water.

RbO2 is superoxide which in aqueous solution undergoes disproportionation to O2 and OH- so will give a blue litmus test.

The peroxides Cs2O2 and BaO2 react with water to give hydrogen peroxide and the hydroxides, so again a basic solution . The hydrogen peroxide produced will bleach the litmus paper.

KO3 - Potassium Ozonide, highly unstable, decomposes in water to oxygen and KOH

The oxides which will NOT decolourise litmus are Li2O, MgO, BeO and SrO. The peroxides (Cs2O2 and BaO2), the superoxide (RbO2) and the ozonide $\left(K{O}_3\right)$ will decolourise litmus.