8. Chemical analysis Flashcards
How to calculate Rf value
Rf= spot distance/solvent distance
The amount of time spent by molecules in each phase depend on 2 things:
How soluble they are in the solvent
How attracted they are to the paper
What are the 2 phases in chromotography
A mobile phase - where the molecules can move. This is always a liquid or gas
A stationary phase - where the molecules can’t move. This can be a solid or a really thick liquid
How do you carry out a flame test
Dip wire into concentrated HCl then into solid sample
Place in blue Bunsen flame
Why is concentrated HCl used in flame tests
It sterilises the wire to remove any contaminants
Result of Lithium flame test (Li+)
Scarlet flame
Result of Sodium flame test (Na+)
Yellow flame
Result of Sodium flame test (Na+)
Yellow flame
Result of Potassium flame test (K+)
Lilac flame
Result of Calcium flame test (Ca2+)
Orange-red flame
Result of copper flame test (Cu2+)
Green flame
How is a NaOH ion test carried out
Add NaOH to a solution of the compound
NaOH test result of Copper (Cu2+)
Blue copper hydroxide precipitate
NaOH test result of Iron II (Fe2+)
Green iron II hydroxide precipitate
gradually turns brown as Iron II hydroxide is oxidised to iron III hydroxide
NaOH test result of Iron III (Fe3+)
Brown iron III hydroxide precipitate
NaOH test result of Calcium (Ca2+)
White calcium hydroxide precipitate
NaOH test result of Magnesium (Mg2+)
White magnesium hydroxide precipitate
NaOH test result of Aluminium (Al3+)
White aluminium hydroxide precipitate
How can the 3 white precipitates be told apart
Add excess NaOH
Aluminium hydroxide precipitate is soluble in excess NaOH
Calcium and magnesium hydroxide precipitates are insoluble in excess NaOH
Use a flame test for Mg and Ca. Mg - no flame colour Ca - orange-red flame
How is the presence of Sulphate ions (SO4 2+) proved
Dilute HCl is added to sample followed by a few drops of barium chloride
A white precipitate of barium sulphate is formed to show presence of sulphate ions
HCl is added to the sample first to remove any ion impurities that would produce a precipitate
How are Halide ions tested (silver nitrate)
A few drops of nitric acid is added to the sample followed by a few drops of silver nitrate
The precipitate colour tells us which halide ion is present
Halide ion test results
Chloride - white silver chloride precipitate
Bromide - cream silver bromide precipitate
Iodide - yellow silver iodide precipitate
(milk, cream, butter)
What can be done in chromatography to make the dye travel further
Change the solvent
Different paper
Let the experiment run for longer
