Chemistry Unit 3 Chemical Tests

AQA Chemistry Unit 3- Identifying ions from chemical tests. Last minute, but I hope it helps.

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  • Created by: Harriet
  • Created on: 24-05-11 19:59

Tests for Cations


Metal Ions can be identified using flame tests:

Burn an organic compound and the colour of the flame can identify the metal ion present:

Lithium produces a crimson-red flame.

Sodium produces an orange-yellow flame.

Potassium produces a lilac flame.

Calcium produces a brick-red flame.

Barium produces a green flame.

The problem with flame tests is that if there is more than one ion present, their colours can mask each other, so it is unclear which ions the substance contains.

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Tests for Cations

Some metal ions produce a coloured precipitate with NaOH:

Copper produces a blue precipitate.

Iron (II) produces a sludgy green precipitate.

Iron (III) produces a reddish-brown precipitate.

These are not soluble in excess NaOH.

Calcium, Aluminium and Magnesium produce a white precipitate.

Aluminium's precipitate re-dissolves in excess NaOH, which is how you can differentiate between these three.

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Tests for Cations/Anions

Some compounds produce ammonia:

Ammonium compounds: Adding sodium hydroxide will produce ammonia, which turns red litmus paper blue.


Nitrates also produce ammonia, but aluminium powder must be added before the sodium hydroxide. The ammonia produced turns red litmus paper blue.

Carbonates produce CO2:

When an acid (hydrochloric, nitric, sulfuric...) is added to a carbonate, CO2 is produced. This gas turns limewater milky.

Some carbonates change colour when heated:

Copper carbonate changes from green to black, and stays black when cool.

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Tests for Anions

Zinc carbonate changes from white to yellow, but as it cools it changes back to white.

Sulphates produce a precipitate with Barium Chloride solution:

Add dilute HCL to your substance, then barium chloride solution. If a white precipitate was formed, your substance was a sulfate.

Halides (chlorine, bromine and iodine) produce a precipitate with silver nitrate:

Add acidified silver nitrate (nitric acid and silver nitrate) to a substance.

white precipitate means it was a chloride Cl- (think Clouds are white)

A cream precipitate means it was a bromide Br- (think cream is cold Brrrrrrr!)

A yellow precipitate means it was an iodide I- (think eyes (I) are yellow. I know it makes no sense...)

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Tests for Organic Compounds

1. They burn when heated

2. Compounds with C=C bonds decolourise bromine water:

Unsaturated organic compunds (i.e ones with double or triple bonds) will turn bromine water from orange to colourless when they are added to it.

It does not turn colourless with saturated (single bond compounds).

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