analysing substances

aqa chemsitry gcse

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Testing for Cations
Cations are positively charged ions so they have lost electrons. (In electrolysis the cathode is
negatively charged in order to attract the cations, positively charged ions, towards it.) We can
identify certain unknown substances by doing flame tests. Flame tests test for metal ions. If
the substance is a metal ion then it will produce a distinctive colour which you can use to
identify the substance. This is often used in forensic science.
Heat a loop of wire on a blue flame on a Bunsen burner to remove any substances already
present and prevent contamination Then place this wire in the substance. Hold the wire
containing the substance against a blue Bunsen flame. If the substance is a metal ion it will
produce a distinctive colour. Learn the following colours produced:
Problems: Risk of contamination/error and the colour depends of you perception.
Metal ion Colour of flame
Li+ (Lithium) Crimson
Na+ (Sodium) Yellow
K+ (Potassium) Lilac
Ca2+ (Calcium) Brick red
Ba2+ (Barium) Green
To sterilise the loop of wire you could also dip it in a solution of hydrochloric acid and then
place it in a Bunsen flame. If the flame does not change colour then the wire is clean and not

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Some metals form a coloured precipitate when reacted with sodium hydroxide (NaOH)
If you react a solution of your metal with sodium hydroxide you will produce a metal hydroxide
+ sodium. Metal hydroxides are insoluble and form a precipitate. We can use this to identify
the metal in the solution as certain metals give different coloured precipitates.
Add a few ml of the solution your testing into a test tube. Add a few drops of sodium hydroxide
into the solution.…read more

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Anions are negatively charged ions so they have gained electrons. ( In electrolysis the anode
is positively charged to attract the anions, the negatively charged particles.)
Test for carbonates
Carbonates react with dilute acid to form carbon dioxide. To check that carbon dioxide gas is
definitely given off you bubble the gas produced through limewater ­ If it is carbon dioxide
then the lime water will turn cloudy/milky.…read more

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Test for Halides (chloride/bromide/iodide ions)
To test for chloride/bromide/iodide ions add dilute nitric acid (HNO3) followed by silver nitrate
solution. (AgNO3.) This will leave a precipitate of silver (bromide/iodide/chloride) which will
be different colours which you can use to identify which halide the substance is.
Chloride gives a white precipitate of silver chloride.…read more

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Test for Organic compounds ­ hydrocarbons
All organic substances contain carbon. When organic compounds are burned in air the
hydrogen is always oxidised (gains oxygen) to become water. Carbon is also oxidised to
make carbon dioxide.
If there is plenty of air available then complete combustion takes place and so all of the
hydrocarbon is burned to produce carbon dioxide and water.(On a blue Bunsen flame as air
is sucked in when the hole is open).…read more

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Empirical formula of an organic compound by burning
Note: Ignore CGP guide
You could get a question like
0.4g of an organic hydrocarbon is burnt completely in oxygen. 1.1g of carbon dioxide and
0.9g of water is formed.…read more

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Technique How it works
White light is made up of a spectrum. Samples of
substances absorb certain colours, but not others.
The pattern is distinctive for each element. The
amount of light absorbed reflects the amount of of
substance in the sample It is used to identify
Bonds of different types absorb different wavelengths
Infra red of infra red in a distinctive pattern than can be picked
spectroscopy up by sensitive detectors and interpreted by a
computer.…read more

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­ Expensive to run and maintain
Automic absorbtion spectromity is used in the steel industry to check the coompositon of
steel to ensure it is right for it's specific use. This way it only takes minutes as opposed to
days with lab methods. It is also more reliable as it doesn't have to be done with the human
eye.…read more


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