Triple science-Chemistry- test for anions and cations

Triple Science AQA - tests for cations and anions

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Flame Tests

Dip nichrome wire into HCl acid and then into unknown substances.Then place on the peak of the blue flame on the bunsen burner:


Pottasium Lilac

Calcium Brick Red

Sodium Yellow

Barium Apple Green

Lithium Red

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Many metal hydroxides form a precipitate with NaOH

Add a few drops of NaOH into mixture and then you can tell what the metal is by the coloured precipitate:


Calcium white precipitate ammonium compound+NaOH gives off

Aluminium white precipitate which redissolves a stinky aroma and turns

Magnesium white precipitate red litmus blue.

Copper blue precipitate

Iron II green precipitate

Iron III brown/red precipitate

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Testing for carbonates

You can test if there is CO2 in the mixture by bubling it through limewater- if the limewater goes cloudy then CO2 is present

Acid+carbonate = CO2 + H20 + salt

Or you can decompse the mixture to see for colour change

copper carbonate= copper oxide + CO2

green to black colour

zinc carbonate=CO2 + zinc oxide

white to yellow to white colour

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Testing for sulfates and halides and nitrates


Add HCl and thenbarium chloride. If a precipitate of barium sulfate comes out then the former compound was a sulfate


To test for chloride, iodide or bromide, addHNO3 and thensilver nitrate solution, these are the results

Chloride White precipitate

Bromide Cream precipitate

Iodide Yellow precipitate


Mix mystery compound with aluminium compound

Add a few drops of NaOH , if it was a nitrate before hand it will reduce to ammonia

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Tests for organic compounds

Organic compounds burn when heated

When there is plenty of air available burning a Hydroncarbon produces CO2 and H2O. If amount of air is reduced then then carbon monoxide and carbon could also be produced.


Compounds with double carbon bonds decolourise bromine water

If organic compound is unsaturated it will decolourise bromine water

If it is saturates the brominw ater will stay brown.

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Instrumental methods

Machines are useful for medical purposes, police forensic work, environmental analysis, checking whether an athlete has taken a banned substamce, analysis of products in industry

Advantages of using machines

  • can be operated by technicians. Lab methods need trained chemists to do everything.
  • More accurate than lab method and can detect the tiniest amounts
  • Much faster than lab methods and tests can be automated


  • Its very epenisve to buy , run and maintain the machines

Atomic absorption spectroscopy identifies metals

The patterns of lights absorbed by the metals in the sample area analysed , each metal present in the produces a different pattern

Its much faster and more reliable than the human eye

Used in the steel industry to check the composition of metals - only takes a couple of minute compared to lab method.

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Other techniques

1) Infrared Spectrometry -identifies which frequencies of infrared radiation are absorbed - the pattern of absorbion is different for each compound .

2) UV Spectroscopy- similar to above but with UV rays

3) Nuclear magnetic spectroscopy -used for organic compounds - helps find the structure of the molecule .

4) Gas-liquid chromotography- similar principles to paper chromotography, used to identify liquids and gases

5)Mass spectroscopy- for elements and compounds - tells you the mass of each particle.

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