Tests for Metal Ions

• metal ions can be identified using simple laboratory tests
• some metal ions can be identified using flame tests
• different metal ions in compounds produce different and distinctive colours in flame tests
• Lithium, Li+ = red flame
• Sodium, Na+ = yellow-orange flame
• Potassium, K+ = lilac/violet flame
• Calcium, Ca2+ = orange-red flame
• Copper, Cu2+ = blue-green flame
• you have to clean the flame test loop in acid each time, rinse with water and check it is clean in a Bunsen burner flame
• to test a substance, dip the clean loop in a solution of the ions and hold at the edge of a blue flame
• some metal ions form coloured hydroxide precipitates when they react with acidic solutions
• hydroxide precipitates test - the sample solution is placed in a test tube and a few drops of dilute sodium hydroxide solution are added
• some metal ions form white precipitates with sodium hydroxide solution - calcium ions (Ca2+), aluminium ions (Al3+)
• copper, Cu2+ = blue precipitate
• iron(II), Fe2+ = green precipitate
• iron(III), Fe3+ = brown precipitate
• copper and iron are transition metals, many of these elements form coloured compounds. although sodium hydroxide is soluble in water, most hydroxides are insoluble so form precipitates in these tests
• aluminium and calcium are not transition metals, so they form white or colourless compounds. both these precipitates are white
• the aluminium hydroxide precipitate reacts with the sodium hydroxide, forming a colourless solution
• in a flame test, it is hard to identify a mixture of ions as the different colours interfere with each other