Carrying out a flame test
Definition
Flame test
A test that identifies some metal ions from the colour they give to a blue Bunsen burner flame.
- Clean a nichrome wire loop by dipping it in hydrochloric acid and holding it in a blue Bunsen flame until it gives no colour.
- Cleaning removes traces of other ions, which would add their colours to the flame.
- Dip the clean loop into the solid sample or a solution of it.
- Hold the loop at the edge of a blue flame and note the colour produced.
- A blue flame is used because the yellow safety flame would hide the colour of the ion.
- Clean the wire again, or use a fresh loop, before testing the next sample.
Practical
- Aim: identify the metal ion in a solid from the colour it gives a flame.
- Apparatus: a Bunsen burner, a heatproof mat, a nichrome wire loop with a handle, dilute hydrochloric acid in a small beaker or watch glass, the solids to test, known samples of lithium, sodium, potassium, calcium and copper(II) compounds, and wooden splints.
Method
- Light the Bunsen burner and open the air hole to give a roaring blue flame.
- Dip the wire loop in dilute hydrochloric acid and hold it in the flame, repeating until the flame shows no colour.
- Dip the clean loop in the acid again, then into a small amount of the solid so that some sticks to it.
- Hold the loop at the edge of the flame and record the colour.
- Clean the loop again, or use a new one, before each sample.
- Compare each colour with the flames from the known samples.
Accuracy and safety
- A clean loop and a blue flame stop other colours hiding the colour of the metal ion.
- Testing known samples helps to tell apart similar colours, such as the reds from lithium and calcium ions.
- If no nichrome wire is available, soak a wooden splint in water, dip it in the solid and hold it in the flame, taking it out before the wood catches fire.
- Wear eye protection when using hydrochloric acid, and tie back long hair near the flame.
Five metal ions give characteristic flame colours
Definition
Cation
An ion with a positive charge, such as a metal ion.
- Lithium ions, Li+\text{Li}^{+}Li+, give a red flame.
- Sodium ions, Na+\text{Na}^{+}Na+, give a yellow-orange flame.
- Potassium ions, K+\text{K}^{+}K+, give a lilac flame.
- Calcium ions, Ca2+\text{Ca}^{2+}Ca2+, give an orange-red flame.
- Copper(II) ions, Cu2+\text{Cu}^{2+}Cu2+, give a green flame.
- The flame test identifies only the metal ion, so a second test is needed to identify the negative ion in the compound.
Common Mistake
- Do not rely on a flame test for a mixture, because the strong yellow-orange of sodium ions can hide the colours of other ions.
- Lithium and calcium give similar red colours, so compare an unclear flame with flames from known samples.
Sodium hydroxide gives coloured metal hydroxide precipitates
Definition
Precipitate
An insoluble solid that forms when two solutions are mixed and react.
- Add a few drops of sodium hydroxide solution to a solution of the sample.
- The metal ions react with hydroxide ions to form an insoluble metal hydroxide, which appears as a precipitate.
- Copper(II) ions give a blue precipitate of copper(II) hydroxide: CuSO4(aq)+2NaOH(aq)→Cu(OH)2(s)+Na2SO4(aq)\text{CuSO}_4\text{(aq)} + 2\text{NaOH(aq)} \rightarrow \text{Cu(OH)}_2\text{(s)} + \text{Na}_2\text{SO}_4\text{(aq)}CuSO4(aq)+2NaOH(aq)→Cu(OH)2(s)+Na2SO4(aq)
- Iron(II) ions give a green precipitate of iron(II) hydroxide.
- The green precipitate slowly turns orange-brown at its surface as it reacts with oxygen in the air.
- Iron(III) ions give an orange-brown precipitate of iron(III) hydroxide.
Example
A pale green solution gives a green precipitate with sodium hydroxide solution, so it contains iron(II) ions.
Tests for carbonate, halide and sulfate ions
Definition
Anion
An ion with a negative charge, such as a halide, sulfate or carbonate ion.
Definition
Halide ion
The negative ion formed when a halogen atom gains one electron, such as chloride, bromide or iodide.
- Carbonate ions, CO32−\text{CO}_3^{2-}CO32−: add dilute acid and bubble any gas given off through limewater.
- Fizzing, with the limewater turning milky, shows that carbon dioxide formed, so carbonate ions are present.
- Halide ions: add dilute nitric acid, then silver nitrate solution.
- Chloride ions, Cl−\text{Cl}^{-}Cl−, give a white precipitate of silver chloride.
- Bromide ions, Br−\text{Br}^{-}Br−, give a cream precipitate of silver bromide.
- Iodide ions, I−\text{I}^{-}I−, give a yellow precipitate of silver iodide.
- Sulfate ions, SO42−\text{SO}_4^{2-}SO42−: add dilute hydrochloric acid, then barium chloride solution.
- A white precipitate of barium sulfate shows that sulfate ions are present: Na2SO4(aq)+BaCl2(aq)→BaSO4(s)+2NaCl(aq)\text{Na}_2\text{SO}_4\text{(aq)} + \text{BaCl}_2\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)} + 2\text{NaCl(aq)}Na2SO4(aq)+BaCl2(aq)→BaSO4(s)+2NaCl(aq)
Common Mistake
- Do not acidify with hydrochloric acid before adding silver nitrate, because its chloride ions would give a white precipitate whatever the sample contains.
- The acid goes in first to remove carbonate ions, which would otherwise form a precipitate of their own and give a false result.
Combining the results for both ions identifies a compound
- Test separate portions of the sample for each ion, so that one reagent does not interfere with another test.
- Use a flame test or sodium hydroxide solution to identify the positive ion.
- Use the carbonate, halide and sulfate tests to identify the negative ion.
- Put the two ions together, using their charges to name the compound and write its formula.
- Give the ion names, such as bromide rather than bromine, because the sample contains ions and not the element.
Example
Identifying a compound from two ion tests
- A white solid gives a red flame, and a solution of it gives a cream precipitate with dilute nitric acid and silver nitrate solution; identify the solid.
- The red flame shows lithium ions, Li+\text{Li}^{+}Li+.
- The cream precipitate with acidified silver nitrate shows bromide ions, Br−\text{Br}^{-}Br−.
- The charges cancel in a 1:11:11:1 ratio, so the solid is lithium bromide, LiBr\text{LiBr}LiBr.
Self review
- How is a nichrome wire cleaned before a flame test?
- What flame colour do potassium ions give?
- What colour is the precipitate that iron(III) ions give with sodium hydroxide solution?
- Why is dilute nitric acid, rather than hydrochloric acid, added before silver nitrate?
- A solution gives a blue precipitate with sodium hydroxide and a white precipitate with acidified barium chloride; what is the compound?
