Question

A gas turns moist blue litmus red but bleaches the colour of litmus is:

• A. sodium oxide
  
• B. sulfur dioxide
  
• C. magnesium oxide
  
• D. copper dioxide
  

Answer 1

The correct option is B sulfur dioxide

Sulfur dioxide gas turns moist blue litmus red but bleaches the colour of litmus. Sulfur dioxide gas dissolves in water to form sulphurous acid.
$S{O}_2+H_{2}O\to H_{2}S{O}_3$
When blue litmus changes its color to red, it indicates acidic solution.
Sulfur dioxide gas acts as a bleaching agent because in presence of water, it is oxidised to sulphuric acid with liberation of nascent hydrogen which reduces colouring matter to colourless.
$S{O}_2+2H_{2}O\to H_{2}S{O}_4+2\left[H\right]$
Colouring matter $+2\left[H\right]\rightleftharpoons$ colourless matter