Chemistry Unit 3 Revision Notes


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Testing for Cations

Testing for Cations

- Flame tests identify metal ions.

- Compounds of some metals burn with a characteristic colour:

  • Lithium (Li+) => Burns with a crimson-red flame.
  • Sodium (Na+) => Burns with a yellow-orange flame.
  • Potassium (K+) => Burns with a lilac flame.
  • Calcium (Ca2+) => Burns with a brick-red flame.
  • Barium (Ba2+) => Burns with a green flame.

- Some metals form a coloured precipitate with NaOH.

- Many metal hydroxides are insoluble and precipitate out of a solution when formed.

- If after adding a few drops of sodium hydroxide solution you get a coloured insoluble hydroxide, you can tell which metal was in the compund.

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Testing for Cations

Metal                              Colour of precipitate                                         Ionic Reaction

Calcium, Ca2+                White                                                                Ca2+(aq) + 2OH-(aq) => Ca(OH)2(s)

Copper(II), Cu2+            Blue                                                                 Cu2+(aq) + 2OH-(aq) => Cu(OH)2(s)

Iron(II), Fe2+                  Sludgy green                                                   Fe2+(aq) + 2OH-(aq) => Fe(OH)2(s)

Iron(III), Fe3+                 Reddish brown                                                Fe3+(aq) + 3OH-(aq) => Fe(OH)3(s)

Aluminium, Al3+               White                                                               Al3+(aq) + 3OH-(aq) => Al(OH)3(s)

                                      (colourless in excess NaOH)                          Al(OH)3(s) + 2OH-(aq) => Mg(OH)2(s)

Magnesium, Mg2+           White                                                               Mg2+(aq) + 2OH-(aq) => Mg(OH)2(s)

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Testing for Cations

Ammonia (NH3)

- Ammonia (NH3) smells very distinctive.

- This is a good way to tell if there is ammonia present.

- Alternatively, damp (red) litmus paper can be used - it will turn blue in the presence of ammonia (NH3).


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Testing for Anions

Testing for Anions

- Test for carbon dioxide (CO2) - turns lime water cloudy.

- acid + carbonate => salt + water + carbon dioxide

- Sometimes a colour change during a reaction gives an indication as to the identity of the substance.

- By putting one spatula of carbonate into a test tube and heating strongly, and then allowing it to cool...

- Copper carbonate (CuCO3) => turns from green to black and stays black.

- Zinc carbonate (ZnCO3) => turns from white to yellow, but then cools to white.

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Testing for Anions

Testing for sulphate (SO42-) ions

- You can test for certain ions by seeing if a precipitate is formed after these reactions.

- To test for sulphate ion (SO42-), add dilute hydrochloric acid (HCl).

- Then add barium chloride solution (BaCl2).

- A white precipitate of barium sulphate means the original compound was a sulphate.

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Testing for Anions

Testing for chloride (Cl-), bromide (Br-) or iodide (I-) ions

- Add dilute nitric acid (HNO3).

- Then add silver nitrate solution (AgNO3).

  • Chloride => white precipitate of silver chloride - Ag+(aq) + Cl-(aq) => AgCl(s)
  • Bromide => cream precipitate of silver bromide - Ag+(aq) + Br-(aq) => AgBr(s)
  • Iodide => yellow precipitate of silver iodide - Ag+(aq) + I-(aq) => AgI(s)
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Testing for Anions

Testing for nitrates (NO3-)

- To test for nitrates (NO3-) mix some of the compound with aluminium powder.

- Then add a few drops of sodium hydroxide solution and heat.

- If you started with a nitrate, it will be reduced to ammonia, so therefore...test for ammonia.

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This is very good.

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