Chemistry C3 - Identifying Ions

Revising how to identify anions and cations - the tests and the results.

HideShow resource information

Identifying Ions

When you have an ionic compound, you have to test and identify two different ions - the cation (the atom that loses electrons, so positively charged) and the anion (the atom that gains electrons, so negatively charged).

There are two types of analysis:

  • qualitative - what is present
  • quantitative - how much is present

There are several reasons why you would need to identify ions:

  • to determine alcohol levels in blood
  • to check for toxic substances in food
  • for water companies to check the purity of water
  • for companies to check the purity of their products (eg copper pipes)
1 of 5

Cation Tests

Calcium (Ca 2+)

  • add sodium hydroxide solution drop by drop
  • white cloudy suspension, insoluble in excess

Copper (Cu 2+)

  • add sodium hydroxide solution drop by drop
  • blue gelatinous precipitate, insoluble in excess

Iron II (Fe 2+)

  • add sodium hydroxide solution drop by drop
  • moss-green gelatinous precipitate, insoluble in excess

to be continued...

2 of 5

Cation Tests (cont.)

Iron III (Fe 3+)

  • add sodium hydroxide solution drop by drop
  • rust-red gelatinous precipitate, insoluble in excess

Aluminium (Al 3+)

  • add sodium hydroxide solution drop by drop
  • white gelatinous precipitate, dissolves to colourless solution in excess

Ammonium (NH4 4+)

  • add sodium hydroxide, warm gently and test gas given off with damp UI paper
  • no precipitate, UI paper turns blue, smell of ammonia
3 of 5

Anion Tests

Hydroxide (OH-)

  • add some ammonium chloride, warm, and test gas given off with damp UI paper
  • UI paper turns blue - ammonia given off

Carbonate (CO3 2-)

  • add hydrochloric acid and bubble gas given off through limewater
  • limewater turns cloudy - carbon dioxide given off

Sulphite (SO3 2-)

  • add hydrochloric acid, warm gently and test gas given off with potassium dichomate solution on a piece of filter paper
  • potassium dichromate solution turns from orange to green

to be continued...

4 of 5

Anions Tests cont.

Chloride (Cl )

  • add nitric acid, then silver nitrate drop by drop
  • white precipitate

Bromide (Br -)

  • add nitric acid, then silver nitrate drop by drop
  • cream precipitate

Iodide (I -)

  • add nitric acid, then silver nitrate drop by drop
  • yellow precipitate - turns grey in the sunlight

Sulphate (SO4 2)

  • nitric acid then barium chloride
  • white gelatinous precipitate
5 of 5

Comments

RATM33

wow, these are amazing! great detail

thanks =D

Tiula

Thanks - you have my very scary Chemistry teacher to thank for these.

ruby1234

thanks this helped :) x

Similar Chemistry resources:

See all Chemistry resources »See all Testing and analysing substances resources »