C3bL6  Testing for Metals

Key Words

Flame test - burning a small sample of metal (or metal compound) in a Bunsen flame to see its colour.

Lilac - light purple

Metal ion - positively charged metal atom.

Precipitate - a solid that forms when two solutions react.

Scarlet - bright red.

Test Yourself

Homework
Physics GCSE
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Grade E

Flame tests can be used to identify metal ions in unknown salts.  You need to know the tests that are shown in this table:

Salt

Colour of flame
Lithium Scarlet
Sodium Orange
Potassium Lilac
Calcium Red
Barium Green

We can react solutions of metal salts with sodium hydroxide to give precipitates.

Salt

Precipitates
Copper II Blue
Iron II Grey-green
Iron III Red-brown
Magnesium White
Aluminium White

If we get a white precipitate, we want to know if it's a magnesium or aluminium salt.  If we add more sodium hydroxide to the precipitate, we find that the aluminium hydroxide precipitate dissolves to form a colourless solution.  The magnesium hydroxide does not.

Grade C

There are a number of ways to do a flame test.  The easiest is to clean an inoculating loop in concentrated hydrochloric acid, then dip it into the sample.  You put it into a hot Bunsen flame and observe the colour.  It is important that the loop is fully cleaned, otherwise you could get a false result.

Zinc carbonate and copper carbonate can be tested for by heating in a crucible:

copper carbonate ® copper oxide + carbon dioxide

CuCO3(s) ® CuO(s) + CO2(g)

green powder ® black powder

 

zinc carbonate ® zinc oxide + carbon dioxide

ZnCO3(s) ® ZnO(s) + CO2(g)

white powder ® yellow powder

The zinc oxide is only yellow when it's heated; on cooling, it goes back to white.

 

Precipitates are insoluble:  Here are some ionic equations for precipitations:

Cu2+(aq) + 2OH-(aq) ® Cu(OH)2(s)

Fe2+(aq) + 2OH-(aq) ® Fe(OH)2(s)

Fe3+(aq) + 3OH-(aq) ® Fe(OH)3(s)

 

Grade A

If we put a metal ion into a flame, the energy level of its electron is raised.  This means it rises to a new energy level.  The atom is excited.  Then the electron falls back to its previous level, emitting a photon (a tiny packet) of light.  The colour is distinctive.

The yellow light of sodium is particularly intense.  If we have even a small amount of sodium salt contamination, it may mask the coulour of the other metal item, leading to a false result.