How many of the following will turn moist red litmus blue and finally white?

$\ce{Li2O}$, $\ce{KO3}$, $\ce{RbO2}$, $\ce{Cs2O2}$, $\ce{BeO}$, $\ce{MgO}$, $\ce{BaO2}$, $\ce{SrO}$.

Since bleach convert moist red litmus blue and then white, but how we can identify which compound will act as bleach ?


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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 a 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 reference here. The hydrogen peroxide produced will bleach the litmus paper.

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

The oxides which will NOT decolourise litmus are Li2O, MgO, BeO and SrO. The peroxides (Cs2O2 and BaO2), the superoxide (RbO2) and the ozonide (KO3) will decolourise litmus.

  • $\begingroup$ KO3 is Potassium Ozonide $\endgroup$ – saket kumar Feb 11 at 8:39
  • $\begingroup$ RbO2 is technically a superoxide. $\endgroup$ – Oscar Lanzi Feb 11 at 10:03
  • $\begingroup$ @OscarLanzi quite correct, my error. I will correct the entry $\endgroup$ – Waylander Feb 11 at 11:20
  • $\begingroup$ @Waylander Can you mention Compound Name clearly which finally turn white ? I am asking this because in my book answer of this question is given as 4 I.e. 4 Compound will turn white finally. A/c to your answer only 3 compound will turn white. $\endgroup$ – saket kumar Feb 11 at 12:55

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