1. 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?

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

3. 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)

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

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

6. 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

7. Aluminium nitrate Copper II sulfate

8. 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?

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

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

11. 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

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

13. 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

14. 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.

15. Recognising Ions

16. 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+

17. 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

18. 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 …………………………

19. 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