Question

1. Give only one suitable chemical test to identify the following gases.
   1. Ammonia
   2. Sulphur dioxide
   3. Hydrogen chloride
   4. Chlorine
   5. Carbon dioxide
   6. Oxygen
   7. Hydrogen
2. Select a basic gas mentioned in Q. (a). How is the basic nature suspected?
3. Select acidic gases from the gases mentioned in Q. (a). How is the acidic nature suspected?
4. The two gases A and B are bleaching agents. A is greenish-yellow and bleaches due to its oxidizing property while B is a colourless gas that bleaches due to reduction. Identify A and B?
5. Which gas turn blue cobalt chloride paper light pink?
   Give one similarity in the test between
   1. Cl₂ and HCl
   2. SO₂ and CO₂.

Answer 1

1. 1. Ammonia: If a rod dipped in concentrated hydrochloric acid is brought near ammonia gas, dense white fumes of ammonium chloride (NH₄Cl) are formed.
      \[\ce{NH3 + HCl -> \underset{Dense white fumes}{NH4Cl}}\]
   2. Sulphur dioxide: It decolorises pink-coloured potassium permanganate solution.
      \[\ce{2KMnO4 + 2H2O + 5SO2 -> K2SO4 + 2MnSO4 + 2H2SO4}\]
   3. Hydrogen chloride: When HCl gas is passed through AgNO₃ solution, white precipitates of AgCl are formed, which get dissolved in excess of NH₄OH.
      \[\ce{AgNO3(aq) + HCl ->AgCl\downarrow + HNO3}\]
   4. Chlorine: It turns moist starch iodide paper (KI + starch solution) blue-black.
   5. Carbon dioxide: When this gas is passed through lime water, it turns milky due to the formation of white precipitates of CaCO₃ and on passing excess of carbon dioxide gas, this milkiness disappears.
      \[\ce{Ca(OH)2 + CO2 -> \underset{Milky}{CaCO3} + H2O}\]
   6. Oxygen: This gas is absorbed in a colourless alkaline solution of pyrogallol and turns it dark brown.
   7. Hydrogen: It is neutral to litmus. It burns with a pop sound and pale blue flame when a burning splint is brought near it. On burning with oxygen, the condensed product turns CaCl₂ pink.
2. Ammonia is a basic gas, and its basic nature is suspected through the litmus paper test because it changes the colour of red litmus paper to blue.
   \[\ce{NH3 + H2O -> NH4OH}\]
3. Chlorine, carbon dioxide, hydrogen chloride, hydrogen sulphide, and sulphur dioxide are acidic gases since they convert blue litmus to red.
4. A is chlorine and B is sulphur dioxide.
5. Light pink water vapour forms on cobalt chloride paper when hydrogen combines with oxygen and condenses.
   1. A white ppt is created when the gas is passed through a silver nitrate solution.
   2. The gas turns milky when it passes through the lime water.