Three colourless-to-pale gases were collected over water as shown, and each was tested. The gases were chlorine, ammonia and sulfur dioxide, in a random ord...
Question 1 Report
Three colourless-to-pale gases were collected over water as shown, and each was tested. The gases were chlorine, ammonia and sulfur dioxide, in a random order. Some results are shown in the table.
| gas | test | result |
|---|---|---|
| gas from tube 1 | damp blue litmus paper held in the gas | litmus turns red then is bleached white |
| gas from tube 2 | damp red litmus paper held in the gas | ...... |
| gas from tube 3 | acidified potassium manganate(VII) added | purple colour changes to colourless |
(a) Identify the gas from tube 1. [1]
(b) Complete the table by giving the result expected for the gas from tube 2, which is ammonia. [1]
(c) Identify the gas from tube 3. [1]
(d) State the colour of chlorine gas. [1]
(e) Describe how you would test an unknown gas to show that it is carbon dioxide, giving the test and the positive result. [2]
(f) Suggest why these gas tests should be carried out in a fume cupboard. [2]
Answer Details
This tests the identification of common gases by their action on litmus and on acidified potassium manganate(VII), plus safe handling. The key idea is that chlorine bleaches, ammonia is alkaline, and sulfur dioxide is a reducing agent.
(a) A gas that turns damp blue litmus red and then bleaches it white is chlorine [1]. Bleaching (not just a colour change) is the distinctive feature of chlorine.
(b) Ammonia is alkaline, so the damp red litmus paper turns blue [1]. The completed table is:
| gas | test | result |
|---|---|---|
| gas from tube 1 | damp blue litmus paper held in the gas | litmus turns red then is bleached white |
| gas from tube 2 | damp red litmus paper held in the gas | litmus turns blue |
| gas from tube 3 | acidified potassium manganate(VII) added | purple colour changes to colourless |
(c) A gas that turns acidified potassium manganate(VII) from purple to colourless is a reducing agent; here that gas is sulfur dioxide [1].
(d) Chlorine gas is (pale / yellowish) green [1].
(e) To show a gas is carbon dioxide, bubble it through limewater [1]; the limewater turns milky / cloudy [1].
(f) These tests should be done in a fume cupboard because chlorine and sulfur dioxide are toxic / poisonous / harmful to breathe [1]; the fume cupboard draws the gases away so they are not inhaled [1].
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