1. Chemical Analysis AQA 2016 Chemistry topic 8 W Richards
10/11/2018
10/11/2018
Chemical Analysis
AQA 2016 Chemistry topic 8
W Richards
The Weald School

2. 8.1 Purity, Formulations and Chromatography
10/11/2018
8.1 Purity, Formulations and Chromatography

3. Are the following pure or impure?
Pure Substances
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A “pure” substance is defined as a substance that doesn’t contain any impurities (i.e. anything other than the substance it’s supposed to be).
Are the following pure or impure?

4. Boiling Points of Mixtures
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Pure elements and compounds boil at specific temperatures (e.g pure water boils at 100OC). However, what happens when you have a mixture?
% of ethanol
% of water
Boiling Point (OC)
100 78 80 20 82 60 40 90 94 98
Some questions:
What temperature does pure ethanol boil at?
What effect does adding ethanol have to the boiling point?
What would be the boiling point of a 50/50 mix?

5. Formulations
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Formulations are mixtures that have been made from specifically measured quantities in order to give the mixture a particular purpose. Some examples:

6. Words – chemicals, paper, solvent, sizes, reference
Chromatography
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Chromatography is a technique used to find out what unknown mixtures are made of. Substances are separated by the movement of a “mobile phase” through a “stationary phase”.
In paper chromatography, the _____ is the stationary phase and the ______ is the mobile phase.
R G B
Different _____ spread out and stop at different points due to their different ____ and properties. They can then be identified by comparing to standard ________ materials.
Words – chemicals, paper, solvent, sizes, reference

7. Chromatography
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Chromatography can be used to separate a mixture of different inks. Some example questions…
R G B X Z
1) Ink X contains two different colours. What are they?
2) Which ink is ink Z made out of?

8. Rf value
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The Rf value is a way of measuring how far a substance has moved:
This line marks the distance travelled by the solvent
R G B
Rf value =
Distance travelled by substance
Distance travelled by solvent

9. Example questions Calculate the Rf values of the following: 10cm 8cm
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Calculate the Rf values of the following:
R G B
10cm
2cm
8cm
5cm

10. 8.2 Identification of Common Gases
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11. Testing for hydrogen
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“POP”

12. Testing for Oxygen
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Oxygen will relight a glowing splint

13. Testing for Carbon Dioxide
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Limewater
Limewater turns milky/cloudy
Calcium carbonate + hydrochloric acid calcium chloride + carbon dioxide + water

14. Testing for Chlorine Chlorine “bleaches” damp indicator paper
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Chlorine “bleaches” damp indicator paper

15. 8.3 Identification of ions by spectroscopic means (Chemistry only)
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8.3 Identification of ions by spectroscopic means (Chemistry only)

16. Flame tests
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Compound
Colour of flame

17. Q. What would happen if you used a mixture of chemicals?
Flame tests
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Compounds containing lithium, sodium, potassium, calcium and copper metal ions (“cations”) can be recognised by burning the compound and observing the colours produced:
Lithium
Red
Sodium
Yellow
Potassium
Lilac
Calcium
Brick red
Copper
Green
Q. What would happen if you used a mixture of chemicals?

18. Precipitation Reactions
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A precipitation reaction occurs when an insoluble solid is made by mixing two ionic solutions together.
Method:
1) Mix the reactants together
2) Filter off the precipitate
3) Wash the residue
4) Dry the residue in an oven at 50OC

19. Metal Hydroxides
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Sodium hydroxide can be used to identify some metal ions (cations). Consider calcium chloride:
Ca2+(aq) OH Ca(OH)2 (s) 2
Metal ion
Precipitate formed
Colour
Calcium Ca2+
Calcium hydroxide: Ca2+(aq) + OH-(aq) Ca(OH)2 (s)
White
Aluminium Al3+
Aluminium hydroxide: Al3+(aq) + OH-(aq) Al(OH)3(aq)
Magnesium Mg2+
Magnesium hydroxide: Mg2+(aq) + OH-(aq) Mg(OH)2 (s)
Copper(II) Cu2+
Copper hydroxide: Cu2+(aq) + OH-(aq) Cu(OH)2 (s)
Blue
Iron(II) Fe2+
Iron(II) hydroxide: Fe2+(aq) + OH-(aq) Fe(OH)2 (s)
Green
Iron(III) Fe3+
Iron(III) hydroxide: Fe3+(aq) + OH-(aq) Fe(OH)3(aq)
Brown
Q. Can you complete this table?

20. Testing for Carbonates
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Carbonates are compounds containing carbon and ________. When an acid is added to a carbonate the carbonate starts to _______. A gas called ______ _______ is produced.
Carbon dioxide is tested for using limewater – it turns limewater _____.
Words – cloudy, fizz, carbon dioxide, oxygen

21. Testing for halide ions
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For each test state: 1) The colour of the precipitate
2) What compound it is
Test 1: Chloride ions
Add a few drops of dilute nitric acid to the chloride ion solution followed by a few drops of silver nitrate.
Precipitate formed = silver chloride (white)
Ag+(aq) + Cl-(aq) AgCl(s)
Test 2: Bromide and iodide ions:
Add a few drops of dilute nitric acid followed by a few drops of silver nitrate solution again. A pale yellow precipitate should be formed for bromide ions and a darker yellow precipitate for iodide ions.

22. Testing for Sulphate ions
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Method:
Add a few drops of dilute hydrochloric acid to the sulphate ion solution followed by a few drops of barium chloride.
Precipitate formed = barium sulphate (white again)

23. Instrumental Methods
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Hi. We’re industrial scientists and we want to test for ions but use instrumental methods instead of things like flame tests. What are the advantages of doing these tests instrumentally?
Advantages of instrumental methods:
More accurate
More sensitive
Quicker

24. Flame Emission Spectroscopy
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Flame emission spectroscopy is an example of an instrumental method:
Step 1: Burn the chemical and pass the light through a spectroscope:
Step 2: Analyse the line spectrum output to identify the metal ion:
Unknown ion
Potassium
Calcium