Ammonia and chlorine
Definition
Bleaching
The removal of colour from a substance, such as damp litmus paper turning white.
- Ammonia, NH3\text{NH}_3NH3: hold damp red litmus paper in the gas, and it turns blue.
- Ammonia is an alkaline gas, so it is the only gas in this set that turns red litmus blue.
- The paper must be damp, because ammonia has to dissolve in water to form the alkaline solution that changes the colour of the litmus.
- Chlorine, Cl2\text{Cl}_2Cl2: hold damp litmus paper in the gas, and it is bleached white.
- Damp blue litmus paper may turn red for a moment before it is bleached, because chlorine forms an acidic solution in water.
- Chlorine is a pale yellow-green gas, but its colour alone is not accepted as a test.
- Ammonia and chlorine are both toxic, so these tests are done in a fume cupboard.
Carbon dioxide and sulfur dioxide
Definition
Limewater
An aqueous solution of calcium hydroxide, used to test for carbon dioxide.
- Carbon dioxide, CO2\text{CO}_2CO2: bubble the gas through limewater, and it turns milky.
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Carbon dioxide reacts with the dissolved calcium hydroxide to form calcium carbonate, a fine white insoluble solid that makes the liquid cloudy:
Ca(OH)2(aq)+CO2(g)→CaCO3(s)+H2O(l) \mathrm{Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)} Ca(OH)2(aq)+CO2(g)→CaCO3(s)+H2O(l) -
The gas is passed through limewater using a delivery tube, or drawn up into a teat pipette and bubbled through limewater in a test tube.
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- Sulfur dioxide, SO2\text{SO}_2SO2: pass the gas through acidified aqueous potassium manganate(VII), which turns from purple to colourless.
- Sulfur dioxide acts as a reducing agent, so it reacts with the purple manganate(VII) ions and removes the colour.
- This is the same colour change as in the test for sulfite ions, because sulfite ions and sulfur dioxide are both reducing agents.
Hydrogen and oxygen
- Hydrogen, H2\text{H}_2H2: hold a lighted splint at the mouth of a test tube of the gas, and the gas burns with a squeaky pop.
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The pop is a small explosion as hydrogen reacts rapidly with oxygen from the air to form water:
2H2(g)+O2(g)→2H2O(l) \mathrm{2H_2(g) + O_2(g) \rightarrow 2H_2O(l)} 2H2(g)+O2(g)→2H2O(l)
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- Oxygen, O2\text{O}_2O2: put a glowing splint, one that has been lit and blown out so that it only smoulders, into the gas, and it relights.
- Oxygen supports burning, and the gas contains far more oxygen than air does, so the glowing wood bursts back into flame.
- A lighted splint tests for hydrogen and a glowing splint tests for oxygen.
Practical
Testing the gas from a reaction
- Aim: to identify the gas given off in a reaction by testing it as it is produced.
- Apparatus: boiling tubes, a bung fitted with a delivery tube, test tubes, a test-tube rack, wooden splints, a Bunsen burner, red and blue litmus paper, a wash bottle of distilled water, limewater, acidified aqueous potassium manganate(VII), dilute hydrochloric acid, magnesium ribbon, marble chips, hydrogen peroxide solution and manganese(IV) oxide.
- Method, tests that use a solution:
- Add dilute hydrochloric acid to marble chips in a boiling tube and fit the bung and delivery tube straight away.
- Dip the end of the delivery tube into about 2 cm32\ \text{cm}^32 cm3 of limewater in a test tube and let the gas bubble through for about 30 s30\ \text{s}30 s.
- Use the same method with acidified aqueous potassium manganate(VII) in place of limewater when testing for sulfur dioxide.
- Method, tests that use a splint:
- Add dilute hydrochloric acid to magnesium ribbon in a boiling tube and hold an empty test tube upside down over its mouth to collect the hydrogen, which is less dense than air.
- After about 30 s30\ \text{s}30 s, bring a lighted splint to the mouth of the upturned test tube.
- Add a little manganese(IV) oxide to hydrogen peroxide solution in another boiling tube and lower a glowing splint into the top of the tube.
- Method, tests that use litmus paper:
- Wet the litmus paper with distilled water before use.
- Hold the damp paper at the mouth of the tube without letting it touch the glass or the liquid, because the reaction mixture itself could change its colour.
- Results: the gas from marble chips turns limewater milky, the gas from magnesium pops with a lighted splint, and the gas from hydrogen peroxide relights a glowing splint.
- Conclusion: the three gases are carbon dioxide, hydrogen and oxygen.
- Watch out: test the gas while the reaction is still producing it, because if you wait too long most of the gas escapes and the result may be negative.
- Safety: wear eye protection, test chlorine, ammonia and sulfur dioxide only in a fume cupboard, because they are toxic, and keep lighted splints away from flammable liquids.
Exam technique
Describing gas tests
- Name the test material and the result, such as "damp red litmus paper turns blue".
- For carbon dioxide, write that limewater turns milky or cloudy.
- For chlorine, write that damp litmus paper is bleached, because "turns red" alone is not the test result.
- Use "pops" for hydrogen and "relights" for oxygen.
Common Mistake
- Do not use dry litmus paper, because a gas changes the colour of litmus only after it dissolves in water.
- Do not write that hydrogen relights a glowing splint or that oxygen pops with a lighted splint.
- Do not test for carbon dioxide with a lighted splint, because many gases put out a flame, so this does not identify carbon dioxide.
Self review
- What happens to damp red litmus paper in ammonia?
- Why must litmus paper be damp when it is used to test a gas?
- Which substance forms and makes limewater turn milky with carbon dioxide?
- What colour change identifies sulfur dioxide?
- How do the splint tests for hydrogen and oxygen differ?
