sblock Elements - Result Question 43
####53. A white solid is either $Na _2 O$ or $Na _2 O _2$. 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.
$(1999,4 M)$
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Solution:
- The substance is $Na _2 O _2$. When $Na _2 O _2$ is dissolved in water, it forms $NaOH$ and $H _2 O _2$. In this case, $NaOH$ is a strong base while $H _2 O _2$ is a weak acid.
(i) $Na _2 O _2+2 H _2 O \longrightarrow 2 NaOH+H _2 O _2$
$H _2 O _2$ decolourises red litmus paper due to its bleaching action which is due to its oxidising character.
$$ H _2 O _2 \longrightarrow H _2 O+\underset{\substack{\text { Bleaches colour } \ \text { of red litmus }}}{[O]} $$
(ii) If the compound is $Na _2 O$, it will hydrolyse to form $NaOH$.
$$ Na _2 O+H _2 O \longrightarrow 2 NaOH $$
$NaOH$ solution formed above will change colour of red litmus paper into blue.
