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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…

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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…

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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…

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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…

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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…

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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. What is the compounds empirical formula?

Ar: Hydrogen:1 Carbon:12 Oxygen: 16


First…

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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.
Atomic
The pattern is distinctive for each element. The
absorption
amount of light absorbed reflects the amount of of
spectroscopy
substance in the sample It is used to identify…

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Disadvantage
­ 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…

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