Question

Question 40
Give the characteristic tests for the following gases
(a) ${\text{CO}}_2$
(b) ${\text{SO}}_2$
(c) ${\text{O}}_2$
(d) ${\text{H}}_2$

Answer 1

(a) Lime water test: Carbon dioxide $\left({\text{CO}}_2\right)$ gas turns lime water milky when passed through it due to the formation of insoluble calcium carbonate.

${\text{Ca(OH)}}_2+{\text{CO}}_2\to {\text{CaCO}}_3+{\text{H}}_2\text{O}$

(b) Decolouration of acidified ${\text{KMnO}}_4$ or ${\text{K}}_2{\text{Cr}}_2{\text{O}}_7$: Sulphur dioxide is a strong reducing agent. When passed through acidified potassium dichromate solution (orange in colour), reduces it and turns it to green. When passed through acidified potassium permanganate solution (purple in colour), reduces it and turns it colourless. The reaction occurs as follows:

$2{\text{KMnO}}_4+2{\text{H}}_2\text{O}+5{\text{SO}}_2\to {\text{K}}_2{\text{SO}}_4+2{\text{MnSO}}_4+2{\text{H}}_2{\text{SO}}_4$

(c) Increase in intensity of a burning candle or splinter: The evolution of oxygen $\left({\text{O}}_2\right)$ gas during a reaction can be confirmed by bringing a burning candle near the mouth of the test tube containing the reaction mixture. The intensity of the flame increases because oxygen supports burning.

(d) Pop sound when hydrogen bubbles interact with a flame: Hydrogen $\left({\text{H}}_2\right)$ gas burns with a pop sound when a burning candle is brought near it. This represents a small explosion that occurs by the combustion of hydrogen gas.