Tests for ions and gases Cambridge IGCSE Chemistry (9–1) revision
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In plain words
Chemists are detectives: they work out what an unknown substance is from simple tests. A splint tells you which gas you have. A flame's colour gives away the metal. A few drops of the right solution make the hidden ions show themselves as a coloured solid.
Three things to know
- Gases: hydrogen gives a squeaky pop with a lighted splint. Oxygen relights a glowing splint. Carbon dioxide turns limewater milky. Chlorine bleaches damp litmus paper. Ammonia turns damp red litmus paper blue.
- Flame tests: lithium red, sodium yellow, potassium lilac, calcium orange-red, copper(II) blue-green.
- In solution: sodium hydroxide gives a light blue precipitate with copper(II), green with iron(II) and red-brown with iron(III). Silver nitrate (with dilute nitric acid) gives white with chloride, cream with bromide, yellow with iodide. A barium solution (with dilute acid) gives white with sulfate. Acid makes carbonates fizz.
Tips and tricks
- Glowing splint for oxygen, lighted splint for hydrogen. Mixing them up is the commonest mistake.
- Always give the result as well as the test: "add silver nitrate: a white precipitate forms".
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Tests for ions and gases: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
Which gas relights a glowing splint?
Things burn much better in oxygen.
What colour does sodium give to a flame?
Lithium is red and potassium is lilac.
Which gas turns limewater milky?
It forms insoluble calcium carbonate.
Silver nitrate solution gives a yellow precipitate with which ion?
Chloride is white, bromide cream, iodide yellow.
Sodium hydroxide solution gives a light blue precipitate with which ion?
Iron(II) gives green and iron(III) red-brown.
What is the test for ammonia gas?
Ammonia is the only common alkaline gas.
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Tests for ions and gases
Cambridge IGCSE Chemistry (9–1) 0971 · 7 marks · papermunch.org
Name ______________________________ Date ______________
Describe the test for oxygen and the test for hydrogen, with the results.[2]
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Oxygen: a glowing splint relights. Hydrogen: a lighted splint gives a squeaky pop.
Describe the test for chloride ions in solution, and give the result.[2]
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Add dilute nitric acid and then silver nitrate solution. A white precipitate forms.
Sodium hydroxide solution is added to a solution and a red-brown precipitate forms. Name the metal ion present.[1]
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Iron(III), Fe³⁺.
Describe the test for sulfate ions in solution, and give the result.[2]
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Add dilute acid and then barium chloride (or barium nitrate) solution. A white precipitate forms.
Answers: Tests for ions and gases
- 1. Oxygen: a glowing splint relights. Hydrogen: a lighted splint gives a squeaky pop.
- 2. Add dilute nitric acid and then silver nitrate solution. A white precipitate forms.
- 3. Iron(III), Fe³⁺.
- 4. Add dilute acid and then barium chloride (or barium nitrate) solution. A white precipitate forms.