How can we test for the presence of positive and negative ions? Chemical Analysis How can we test for the presence of positive and negative ions? Starter: Why might we need to know what chemicals are present in different substances?

Flame Tests Metal ions (+ve) produce flames with particular colours. Watch the demo on how flame tests are done.

Method 1) Put a Small amount of compound in a cleaned nichrome wire loop. 2) Hold the loop in a roaring flame. 3) Observe the colour. (to clean use conc. Hydrochloric acid and heat)

Metal Ion Colour Li+ Na+ K+ Ca 2+ Ba2+

Results Metal Ion Colour Li+ Crimson Na+ Yellow K+ Lilac Ca 2+ Red Ba2+ Green

2) Reactions with sodium hydroxide (NaOH) Useful in identifying certain positive (metal) ions. Test ion with NaOH AL 3+, Ca 2+ and Mg2+ all form a white precipitate. Add more NaOH the precipitate formed with Al will dissolve. Distinguish between Ca and Mg with a flame test. Ca Red/crimson, Mg NO Colour

Aluminium nitrate Copper II sulfate http://www.youtube.com/watch?v=VONKsXtUS7M

3) Coloured precipitates with NaOH Ion Precipitate colour Copper (II) ions Blue Iron (II) ions Green Iron (III) ions Brown How could I tell if a substance contained Iron II or Iron III ions?

3) Testing for negative ions A) Carbonates React a carbonate with acid to release CO2, - test with lime water.

Nitrates (NO3-) Add NaOH (sodium hydroxide) and Aluminium powder. Ammonia gas will be released with a pungent smell.

Halides (Halogen compounds) Chlorides, Bromides and Iodides. Add dilute Nitric acid Add silver nitrate (we say it is silver nitrate acidified with nitric acid) Look for precipitate – if present, Halide present. Check colour

More Halide tests 1) add an unsaturated organic compound to bromine water – it will go from red to colourless.

Results What do you think the precipitates are? Halide Colour precipitate Cloride White Bromide Cream Iodide Pale yellow What do you think the precipitates are? Silver chloride, Silver bromide etc

Sulfates Test for sulfates by adding dil. Hydrochloric acid to remove carbonate ions that would react with barium ions. Then add barium chloride solution. White precipitate forms = barium sulfate.

Recognising Ions

Recognising Ions – pair these up SO4 2- Acid ion CO3 2- Barium Ions OH - Carbonate ions Ag+ Sulfate ions H+ Silver ions Alkali ion Ba 2+

Recognising Ions – pair these up SO4 2- Sulfate ions CO3 2- Carbonate ions OH - Alkali ion Ag+ Silver ions H+ Acid ion Ba 2+ Barium Ions

Positive ion tests Unknown chemical . Add NaOH Coloured Precipitate Redish brown ……… Light blue ………….. Light green, turns slowly brown. …………… White precipitate Al3+, Ca2+ or Mg2+ Precipitate redissolves in NaOh ……….. Flame test …………………………

Positive ion tests Unknown chemical . Add NaOH Coloured Precipitate Redish brown Fe3+ Light blue Cu2+ Light green, turns slowly brown. Fe2+ White precipitate Al3+, Ca2+ or Mg2+ Precipitate redissolves in NaOh Al3+ Flame test for Ca or Mg