Tests for Ions, Gases and Flame Tests
Tests for ions, gases and flame tests are the exact reagents and results used to identify gases and ions in the laboratory: the four standard gas tests, flame tests for metal ions, the sodium hydroxide test that distinguishes calcium, magnesium, aluminium, copper, iron(II) and iron(III), the test for ammonium ions, and the precipitation tests for carbonate, halide and sulfate ions, including the ionic equations for the precipitation reactions.
Method
- Learn the four gas tests precisely, including the exact observation: a lit splint held in the gas gives a squeaky pop for hydrogen; a glowing splint relights (bursts back into flame) in oxygen; limewater (calcium hydroxide solution) turns cloudy when carbon dioxide is bubbled through it; damp blue litmus paper is bleached and turns white in chlorine gas.
- Learn the flame test colours in order: lithium (Li+) burns red, sodium (Na+) burns yellow, potassium (K+) burns lilac, calcium (Ca2+) burns orange-red, and copper (Cu2+) burns green (sometimes described as blue-green).
- Use sodium hydroxide solution to distinguish the six common metal ions by the colour of the precipitate and its behaviour in excess: calcium gives a white precipitate insoluble in excess; magnesium gives a white precipitate insoluble in excess; aluminium gives a white precipitate that dissolves in excess sodium hydroxide to give a colourless solution; copper gives a blue precipitate; iron(II) gives a green precipitate; iron(III) gives a brown precipitate.
- Write the ionic equations for the hydroxide precipitation reactions, for example Ca2+(aq) + 2OH-(aq) gives Ca(OH)2(s), Cu2+(aq) + 2OH-(aq) gives Cu(OH)2(s), and for aluminium the extra step in excess sodium hydroxide: Al(OH)3(s) + OH-(aq) gives AlO2-(aq) + 2H2O(l).
- Test for ammonium ions (NH4+) by warming the sample gently with sodium hydroxide solution; ammonia gas is released, recognised by its pungent smell and because it turns damp red litmus paper blue.
- Test for carbonate ions by adding dilute acid; carbonates fizz and release carbon dioxide, which is confirmed by bubbling it through limewater, which turns cloudy: CO3^2-(aq) + 2H+(aq) gives H2O(l) + CO2(g).
- Test for halide ions by first adding a few drops of dilute nitric acid, which removes any carbonate ions that would otherwise give a false positive, then adding silver nitrate solution: chloride gives a white precipitate (AgCl), bromide gives a cream precipitate (AgBr), iodide gives a yellow precipitate (AgI).
- Test for sulfate ions by first adding dilute hydrochloric acid, which removes carbonate interference, then adding barium chloride solution; a white precipitate of barium sulfate (BaSO4) forms if sulfate ions are present: Ba2+(aq) + SO4^2-(aq) gives BaSO4(s).
Worked example
A white solid Y gives a brick-red (orange-red) flame test. When dilute hydrochloric acid is added to a sample of Y, bubbles of gas are produced that turn limewater cloudy. Identify Y and write a balanced symbol equation for its reaction with dilute hydrochloric acid.
- Interpret the flame test: an orange-red (brick-red) flame identifies calcium ions, Ca2+.
- Interpret the gas test: fizzing with acid, producing a gas that turns limewater cloudy, identifies carbon dioxide, which shows a carbonate ion, CO3^2-, was present.
- Combine the ion identities: Y is calcium carbonate, CaCO3.
- Write the word equation: calcium carbonate + hydrochloric acid gives calcium chloride + water + carbon dioxide.
- Write and balance the symbol equation, checking each element: CaCO3(s) + 2HCl(aq) gives CaCl2(aq) + H2O(l) + CO2(g).
- Check the balance: 1 Ca, 1 C, 3 O and 2 H and 2 Cl on each side, so the equation is balanced.
Practice questions
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Q1State the flame test colour for sodium ions.Show answer
Answer: Yellow.
Q2A solution forms a green precipitate with sodium hydroxide solution. Which ion is present?Show answer
Answer: Iron(II), Fe2+.
Q3Explain how the sodium hydroxide test distinguishes aluminium ions from calcium ions, given that both give a white precipitate.Show answer
Answer: The aluminium precipitate dissolves when excess sodium hydroxide solution is added, forming a colourless solution, whereas the calcium precipitate stays as a white solid and does not dissolve in excess.
Q4Describe the test, and the positive result, for ammonium ions.Show answer
Answer: Warm the sample with sodium hydroxide solution; if ammonium ions are present, ammonia gas is given off, which has a pungent smell and turns damp red litmus paper blue.
Q5Why must dilute nitric acid be added before silver nitrate solution when testing for a halide ion?Show answer
Answer: To remove any carbonate ions present first, since carbonate ions would also react with silver nitrate to give a precipitate, causing a false positive result for a halide.
Q6A solution gives a cream precipitate when acidified with dilute nitric acid and then treated with silver nitrate solution. Name the halide ion present.Show answer
Answer: Bromide, Br-.
Q7Write the ionic equation for the precipitation reaction between barium ions and sulfate ions.Show answer
Answer: Ba2+(aq) + SO4^2-(aq) gives BaSO4(s).
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A student is given four unlabelled test tubes, each containing a solution of a different metal chloride: calcium chloride, copper chloride, iron(II) chloride and aluminium chloride. Describe how the student could use sodium hydroxide solution alone to identify which solution is in each test tube, including what would be observed for each.
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A colourless gas is collected and tested with a lit splint, which is extinguished with a squeaky pop. Identify the gas and write a balanced symbol equation to show it burning in air.
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This topic is chapter 26 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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