SC25 Qualitative Analysis
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What is a flame test used for?
To identify metal ions by the colour of the flame they produce
Describe how to carry out a flame test
• Clean a wire loop by dipping it into HCL and then holding it in a blue flame until it burns without altering the colour of the flame • Dip the clean wire loop into the unknown salt and then hold the loop in the blue flame of a Bunsen burner. Record the colour • Repeat for other unknown samples
What colour flame does Lithium produce as the outcome of a flame test?
Red
What colour flame does Potassium (K+) produce as the outcome of a flame test?
Lilac
What colour flame does Sodium (Na+) produce as the outcome of a flame test?
Yellow
What colour flame does Calcium (Ca2+) produce as the outcome of a flame test?
Orange-red
What colour flame does Copper (Cu2+) produce as the outcome of a flame test?
Blue-green
Give some examples of instrumental methods
• Gas chromatography • Mass spectrometry
What are the advantages of using instrumental methods?
Accurate, sensitive and rapid
How can metal cations that are dissolved in a solution be identified?
Through the colour of the precipitate formed after the addition of sodium hydroxide
Describe how to carry out a hydroxide precipitate test
• Place a little of the unknown solution in a test tube and add a few drops of dilute sodium hydroxide solution. Record any observations • If a white precipitate forms, add an excess of sodium hydroxide and record any observations
What is the effect of adding sodium hydroxide to aluminium?
Forms a white precipitate, dissolves in excess NaOH to form a colourless solution
What is the effect of adding sodium hydroxide to calcium?
White precipitate, insoluble, so remains in excess sodium hydroxide
What is the effect of adding sodium hydroxide to copper?
Light blue precipitate, insoluble in excess
What is the effect of adding sodium hydroxide to iron II?
Green precipitate, insoluble in excess
What is the effect of adding sodium hydroxide to iron III?
Red-brown precipitate, insoluble in excess
How can we test for ammonium ions using sodium hydroxide?
• Gently heat the solution after adding the NaOH • If ammonium ion is present, it produces ammonium gas and can be tested with red litmus paper
What is the chemical test for ammonia?
Turns red litmus paper blue
Describe the test for ammonia using HCl
• React the gas with HCl • A white smoke is produced if ammonium gas is present of ammonium chloride
How can we test for the presence of a carbonate ion?
Bubble the gas through limewater or any dilute acid. If the limwater goes cloudy, it indicates the presence of CO2
How can we test for the presence of a sulfate ion?
• Add dilute HCl followed by a barium chloride solution • A white precipitate of barium sulfate will form when sulfate ions are in the solution
How can we test for the presence of halide ions?
• Add dilute nitric acid followed by the addition of silver nitrate solution • This will form a silver halide precipitate • Depending on the halide present, a different colour precipitate is formed, allowing us to identify the halide ion
What colour does chloride turn when added to a silver precipitate?
White
What colour does bromide turn when added to a silver precipitate?
Cream