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Science

Forensic science

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Forensic scientists analyse substances from a crime scene using many methods - including checking for traces of metals that might indicate a link between significant people or objects in the case.

Individual transition metals can be identified by the colour of their precipitate with sodium hydroxide solution. Flame tests can be used to identify a particular metal.

Transition metal precipitates

Transition metals form coloured compounds with other elements. Many of these are soluble in water, forming coloured solutions. If sodium hydroxide solution is then added, a transition metal hydroxide is formed. These are insoluble. They do not dissolve but instead form solid precipitates. Here are the equations for copper sulphate solution reacting with sodium hydroxide solution:

copper sulphate + sodium hydroxide    →    copper hydroxide + sodium sulphate

CuSO4     +     2NaOH        →        Cu(OH)2     +     Na2SO4

(blue solution + colourless solution → blue precipitate + colourless solution)

the solutions are added together in a test tube

Copper solutions form a blue precipitate with sodium hydroxide

Different transition metals form different coloured precipitates.

Some common transition metals and the colours of their precipitates.

transition metalcolour of precipitate
iron(II)green - turns orange-brown when left standing
iron(III) orange-brown
copperpale blue
zincwhite

Example

How could you tell if an unknown substance contained iron(II) nitrate or iron(III) nitrate? You would add a few drops of sodium hydroxide solution. If you got a dark green precipitate it would show that the unknown substance was iron(II) nitrate; if you got an orange-brown precipitate it would show that the unknown substance was iron(III) nitrate.

Note that it is the metal that determines the colour. So you would get the same result whether you used zinc chloride or zinc nitrate - a white precipitate in this example.

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