Identifying ions required practical

Aim: To use flame tests and chemical tests to identify the positive and negative ions in unknown single ionic compounds.

Equipment

• Nichrome wire loop and dilute hydrochloric acid for cleaning it
• Bunsen burner and heatproof mat
• Test tubes, test tube rack and dropping pipettes
• Dilute sodium hydroxide solution (NaOH)
• Dilute hydrochloric acid (HCl) and dilute nitric acid (HNO₃)
• Silver nitrate solution (AgNO₃)
• Barium chloride solution (BaCl₂)
• Limewater and a delivery tube (or dropping pipette)
• Unknown solid samples and their solutions

Method

1. Flame test: dip a clean nichrome loop in dilute hydrochloric acid, then into the solid sample, and hold it in the edge of a blue Bunsen flame. Record the colour: Li⁺ crimson, Na⁺ yellow, K⁺ lilac, Ca²⁺ orange-red, Cu²⁺ green.
2. Clean the loop in acid between samples until it gives no colour in the flame.
3. Hydroxide test: add a few drops of sodium hydroxide solution to about 2 cm³ of the sample solution and note any precipitate: Cu²⁺ blue, Fe²⁺ green, Fe³⁺ brown, and Al³⁺, Ca²⁺ and Mg²⁺ white.
4. If a white precipitate forms, add excess sodium hydroxide: only the aluminium hydroxide precipitate dissolves.
5. Carbonate test: add dilute hydrochloric acid to the sample; if it fizzes, pass the gas through limewater. Limewater turning milky (cloudy) shows carbon dioxide, so a carbonate (CO₃^2-) is present.
6. Halide test: add dilute nitric acid, then a few drops of silver nitrate solution: a white precipitate shows chloride, cream shows bromide and yellow shows iodide.
7. Sulfate test: add dilute hydrochloric acid, then a few drops of barium chloride solution: a white precipitate shows sulfate (SO₄^2-).
8. Record all observations in a table and combine the positive and negative ion to name each unknown compound.

Safety

• Wear eye protection throughout: sodium hydroxide is corrosive or irritant and the acids are irritants.
• Barium chloride is toxic and silver nitrate stains skin and is an irritant; avoid skin contact and wash hands afterwards.
• Take care with the hot wire loop and Bunsen flame; let the loop cool before cleaning it.
• Do not heat samples in the flame for too long and keep the room ventilated.

Results and calculations

Record the test, what you observed (colour of flame, colour of precipitate, fizzing and limewater result) and your conclusion about the ion in a results table. Ionic equations can be written for precipitates, e.g. Cu²⁺(aq) + 2OH^-(aq) → Cu(OH)₂(s) and Ag⁺(aq) + Cl^-(aq) → AgCl(s).

Common mistakes and improvements

• A dirty loop gives the wrong flame colour (sodium impurities give a yellow flame that masks others); clean the loop in acid until it gives no colour.
• Using hydrochloric acid in the halide test adds chloride ions and gives a false positive; use nitric acid instead.
• Adding too much sodium hydroxide at once can hide whether a white precipitate dissolves; add a few drops first, then excess.
• Colours can be hard to judge in a mixture of ions; compare with known samples, use a white background, and consider flame emission spectroscopy for more reliable results.

Exam tips

• Learn the flame colours, hydroxide precipitate colours and halide precipitate colours; these are frequently worth one mark each.
• Explain why acid is added first in the halide and sulfate tests: to remove carbonate ions, which would also form a precipitate and give a false result.
• Explain how to tell apart Al³⁺, Ca²⁺ and Mg²⁺: only aluminium hydroxide dissolves in excess NaOH, and calcium gives an orange-red flame while magnesium gives no flame colour.
• Use all the observations given in a question to name the compound fully, e.g. iron(III) sulfate.
• Compare with instrumental methods: they are more accurate, sensitive and rapid, and can identify ions in mixtures.