Chemical analysis

Learning outcomes

After studying this section, you should be able to describe how to test for:

  • metal ions using flame tests
  • metal ions and ammonium ions using sodium hydroxide solution
  • carbonate ions, sulfate ions, and halide ions
  • common gases.


Flame tests

Flame tests are used to identify different metal ions in solution. The table shows the flame test colours of some of these ions.
Metal ionFlame test colour
lithium, Li+red
sodium, Na+yellow
potassium, K+lilac
calcium, Ca2+orange-red
copper(II), Cu2+blue-green
barium, Ba2+light green
Carrying out a flame test

Tests using sodium hydroxide solution

Metal ions

Metal ions may form precipitates when they react with hydroxide ions. The table shows the colours of these precipitates in sodium hydroxide solution.

Metal ionColour of precipitate
copper(II), Cu2+Light blue
iron(II), Fe2+Green (turns brown after a while)
iron(III), Fe3+Red-brown
chromium(II), Cr3+Green, pale green solution in excess NaOH
aluminium, Al3+White, colourless solution in excess NaOH
calcium, Ca2+White
zinc, Zn2+White, colourless solution in excess NaOH
Three different metal hydroxide precipitates
Left to right: Cu2+, Fe3+, Fe2+

Ammonium ions

Ammonia is given off when a solution containing ammonium ions mixed with aqueous sodium hydroxide, then warmed. The ammonia then turns damp red litmus paper blue.

Tests for negatively charged ions (anions)

Carbonate ions

Bubbles of gas are given off when dilute acids react with substances containing carbonate ions. Limewater is used to confirm that the gas is carbon dioxide.

Sulfate ions

A white precipitate forms when aqueous barium chloride or aqueous barium nitrate is added to a solution containing sulfate ions. The solution must be appropriately acidified.

Halide ions

Precipitates form when aqueous silver nitrate is added to solutions containing halide ions. The solution must be acidified with dilute nitric acid. The table shows the colours of the silver halide precipitates.

Halide ionColour of silver halide precipitate
chloride, Clwhite
bromide, Brcream
iodide, Iyellow
Testing for iodide ions

Common gases

The table shows how to test for five common gases. Some courses require you to know the test for sulfur dioxide, SO2:
  • acidify the sample with dilute aqueous potassium manganate(VII) solution with dilute sulfuric acid
  • bubble the gas through the acidified potassium manganate(VII) solution
  • sulfur dioxide causes a colour change from purple to colourless.
GasWhat you doWhat you see
ammonia, NH3Hold damp red litmus paper in the gasPaper turns blue
carbon dioxide, CO2Bubble through limewaterLimewater turns milky
chlorine, Cl2Hold damp litmus paper in the gasPaper is bleached white
hydrogen, H2Put a lighted splint in the gasGas ignites with a 'pop'
oxygen, O2Put a glowing splint in the gasSplint relights
Potassium manganate(VII), KMnO4, dissolves in water to form a purple solution