Question

Give the characteristic tests for the following gases :

1. CO₂
2. SO₂
3. O₂
4. H₂

Answer 1

1. CO₂ gas : It is a colourless and odourless gas. When passed through lime water, it will initially become milky.
   `underset("Lime  water")("Ca"("OH")_2("aq")) + "CO"_2("g") -> "CaCO"_3("s") + "H"_2"O"("aq")`
   The milkiness will disappear on passing the gas in excess.
   `"CaCO"_3("s") + "CO"_2("g") + "H"_2"O" ("aq") -> underset("Colourless")("Ca"("HCO"_3)_2("aq"))`
2. SO₂ gas : It is a colourless but a pungent smelling gas. The gas when passed through an acidified solution of potassium permanganate (pink) will decolourise it.
   `underset("(Orange)")("K"_2"Cr"_2"O"_7) + 3"SO"_2 + "H"_2"SO"_4 -> "K"_2"SO"_4 + underset("(Light  green)")("Cr"_2("SO"_4)_3) + "H"_2"O"`
3. Oxygen gas : The gas is a supporter of combustion. It can be tested by bringing a burning or smouldering match-stick near the mouth of the test tube from which the gas escapes. The match-stick will burn more brightly.
4. Hydrogen gas : The gas itself is combustible. It will burn with a pop sound when a burning candle or match- stick is brought near it.
   `2"H"_2("g") + "O"_2("g") -> 2"H"_2"O"("g")`