Chemical and Physical Tests in AQA AS Level Unit 2 Chemistry

Notes on Chemical and Physical Tests in AQA Unit 2 Chemistry

Preview of Chemical and Physical Tests in AQA AS Level Unit 2 Chemistry

Extracted text:

Physical Tests and Chemical Tests to
Test for Chemicals! YAAAAAY! by Marco Narajos ;]
Group 7 chemicals
Testing for the presence of halide ions
o Use acidified silver nitrate
White precipitate (silver chloride) for chloride
Cream precipitate (silver bromide) for bromide
Yellow precipitate (silver fluoride) for iodide
No precipitate for fluoride so you cannot use this to test for fluoride
presence
Acidified with sulphuric acid or nitric acid to remove ions that may
interfere such as carbonate ions, but hydrochloric acid is NOT used
because this contains chloride ions which would give a false positive.
o For a secondary test, use aqueous ammonia
Dilute ammonia dissolves white precipitate
Concentrated ammonia dissolves cream precipitate
Yellow precipitate is not soluble in concentrated ammonia
You can use this to check for a mixture ­ the formation of a white
precipitate and a yellow precipitate may look like a cream precipitate,
but using aqueous ammonia, then concentrated ammonia, you can
check to see if it is a mixture.
Testing for the reducing power of halide ions (X-)
o The reducing power of halide ions increases as you go down the group
o This can also be used as a test for the presence of halide ions.
o Use concentrated sulphuric acid (oxidation state of S = +6) ­ add it to the
aqueous halide

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Page 2

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Fluoride ions react in an acid-base reaction (with the fluoride acting as
the base) to form hydrogen fluoride (steamy fumes) and a hydrogen
sulphate.
NaF + H2SO4 ­> NaHSO4 (stays in solution) + HF (steamy fumes)
This is NOT a redox reaction. (S oxidation state = +6)
Chloride ions react in the same way, forming hydrogen chloride
(steamy fumes of HCl). Again, no redox reaction takes place because
chloride ions are too weak.

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Group 2 chemicals
Test for sulphate ions
o Add acidified barium chloride (acidified to remove carbonate ions)
Acidified with hydrochloric acid, NOT sulphuric acid (as this contains
sulphate ions)
White precipitate if sulphates are present (forms insoluble barium
sulphate)
Organic Chemistry
Test to differentiate between primary/secondary alcohols and a tertiary alcohol
o Heat with acidified potassium dichromate(VI)
Turns from orange to green when heated with a primary or a
secondary alcohol
Remains green with a tertiary alcohol as tertiary alcohols do not
oxidise with acidified

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Test for an acid (like a carboxylic acid)
o Add sodium hydrogen carbonate (NaHCO3)
Fizzes in the presence of an acid as CO2 is released (water is also
formed
o Or pH indicators: universal ­ yellow/orange/red; blue litmus ­ turns red
Physical Tests
You are unlikely to need to know this in detail for the exam but it's all good
Chemistry knowledge and may help acquire a few marks if you're really stuck with
the chemical tests (or if Chemistry examiners are closet Biologists and

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