sblock Elements - Result Question 43

####53. A white solid is either Na2O or Na2O2. A piece of red litmus paper turns white when it is dipped into a freshly made aqueous solution of the white solid.

(i) Identify the substance and explain with balanced equation.

(ii) Explain what would happen to the red litmus if the white solid were the other compound.

(1999,4M)

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Solution:

  1. The substance is Na2O2. When Na2O2 is dissolved in water, it forms NaOH and H2O2. In this case, NaOH is a strong base while H2O2 is a weak acid.

(i) Na2O2+2H2O2NaOH+H2O2

H2O2 decolourises red litmus paper due to its bleaching action which is due to its oxidising character.

H2O2H2O+[O] Bleaches colour   of red litmus 

(ii) If the compound is Na2O, it will hydrolyse to form NaOH.

Na2O+H2O2NaOH

NaOH solution formed above will change colour of red litmus paper into blue.



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