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2.45 - 2.50 Chemical tests (for ions)

 2.45 - 2.46 Up in flames

Fireworks get their colours from the metal cations in them

Fireworks produce a whole range of different colours. All fireworks release energy in the form of heat energy and are therefore exothermic.  Explosions and rocket power is provided by gunpowder and similar explosive mixtures.

Colours are achieved by mixing compounds which contain various metal ions into the explosive. When hot, these metal ions produce light of specific colours. The colour produced by a particular metal ion can be used in the laboratory as way of identifying the metal ion present  in a compound. 

Assumed background knowledge

 2.45. Activity 1

 Students should:

  • 2.45 describe how to carry out a flame test

Study the "positive ions part 1" video 

  • Use the video to write a sequence of  numbered instructions which would allow a friend to successfully carry out a flame test on an unknown substance.
  • outline the safety precautions which you should take 
  • explain why it is important to clean the nichrome wire thoroughly for each substance

How to perform a flame test


 2.46 Activity 2. Colour coding - to learn

You should learn these flame colours.

 Students should:

  • 2.46 know the colours formed in flame tests for the cations: lithium, sodium, potassium, calcium, copper
Use the two images to match the metal to the colour. Make a copy of the blank table. Use your own words to describe each of the flame colours and write your descriptions in the appropriate column.

Flame emission spectroscopy

Flame colours can help to identify the metal ion in a compound Flame tests can be used to identify some metal ion...
Metal cation  flame colour 
lithium  red ("crimson") 
potassium lilac
calciumorange ("brick") red
copper blue-green

 2.47 Activity. Testing for some other Cations:

 Students should:

  • 2.47 describe tests for these cations: Ammonium, Copper II, Iron II and Iron III.

Use the video find out the results obtained when sodium hydroxide solution is added to solutions containing either ammonium (NH4+) ions, Copper (II) ions (Cu2+), Iron II ions (Fe2+) or iron III ions (Fe3+) . Add your results to a copy of the table 

Name of ion Formula  Result observed
Name of ion Formula Observation
Ammonium  NH4+ Warming with sodium hydroxide produces an alkaline gas
Copper(II)Cu2+blue precipitate produced
Iron(II)Fe2+green precipitate produced
Iron(III)Fe3+brown precipitate produced

The positive result for the presence of ammonium ions is when warmed with sodium hydroxide solution,  ammonia gas is produced. This is detected using damp red litmus paper which turns blue in the presence of  the alkaline gas ammonia.

 2.48 Activity. Testing for anions: Carbonate ions

 Students should:

  • 2.48 describe tests for these anions: chloride, bromide, iodide, sulfate and carbonate

The test for carbonate ions is to add an acid to the unknown substance. Bubbles of carbon dioxide will be formed if a carbonate is present.

 The equation below is for the reaction of sodium carbonate with sulfuric acid.

The reaction shown in the video uses calcium carbonate as the carbonate being tested. 

The acid used is hydrochloric acid.

Use the equation above and the fact that the reactants used in the video are calcium carbonate and hydrochloric acid to help you predict the full symbolic equation for the reaction in the video.

How do they prove that the gas is carbon dioxide?

carbonates react with acids to produce carbon dioxide gas

The gas produced is bubbled through limewater which turned cloudy.  This suggests the presence of limewater.

 2.48 Activity. Testing for anions: Sulfate ions

The insolubility of barium sulfate is why barium ions are used to test unknown solutions for the presence of sulfate ions. Acidified barium sulfate solution is added to the unknown solution

A white precipitate indicates the presence of sulfate.

Barium ions react with sulfate ions to produce a white precipitate
The ionic equation leaves out those ions which remain in solution

 2.48 Activity. Testing for anions: chloride, bromide and iodide ions

Acidified silver nitrate solution is added to the unknown solution.  A precipitate forms if chloride, bromide or iodide are present.

This test for chloride ions bromide and iodide ions uses the fact that silver chloride , silver bromide and silver iodide are all insoluble in water.

Silver chloride is white, silver bromide is cream and silver iodide is yellow. 

Ammonia solution will dissolve silver chloride but not silver iodide.  

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