8. Chemical analysis

Question 1

How to calculate Rf value

Answer 1

Rf= spot distance/solvent distance

Question 2

The amount of time spent by molecules in each phase depend on 2 things:

Answer 2

How soluble they are in the solvent

How attracted they are to the paper

Question 3

What are the 2 phases in chromotography

Answer 3

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

Question 4

How do you carry out a flame test

Answer 4

Dip wire into concentrated HCl then into solid sample

Place in blue Bunsen flame

Question 5

Why is concentrated HCl used in flame tests

Answer 5

It sterilises the wire to remove any contaminants

Question 6

Result of Lithium flame test (Li+)

Answer 6

Scarlet flame

Question 7

Result of Sodium flame test (Na+)

Answer 7

Yellow flame

Question 8

Result of Sodium flame test (Na+)

Answer 8

Yellow flame

Question 9

Result of Potassium flame test (K+)

Answer 9

Lilac flame

Question 10

Result of Calcium flame test (Ca2+)

Answer 10

Orange-red flame

Question 11

Result of copper flame test (Cu2+)

Answer 11

Green flame

Question 12

How is a NaOH ion test carried out

Answer 12

Add NaOH to a solution of the compound

Question 13

NaOH test result of Copper (Cu2+)

Answer 13

Blue copper hydroxide precipitate

Question 14

NaOH test result of Iron II (Fe2+)

Answer 14

Green iron II hydroxide precipitate

gradually turns brown as Iron II hydroxide is oxidised to iron III hydroxide

Question 15

NaOH test result of Iron III (Fe3+)

Answer 15

Brown iron III hydroxide precipitate

Question 16

NaOH test result of Calcium (Ca2+)

Answer 16

White calcium hydroxide precipitate

Question 17

NaOH test result of Magnesium (Mg2+)

Answer 17

White magnesium hydroxide precipitate

Question 18

NaOH test result of Aluminium (Al3+)

Answer 18

White aluminium hydroxide precipitate

Question 19

How can the 3 white precipitates be told apart

Answer 19

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

Question 20

How is the presence of Sulphate ions (SO4 2+) proved

Answer 20

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

Question 21

How are Halide ions tested (silver nitrate)

Answer 21

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

Question 22

Halide ion test results

Answer 22

Chloride - white silver chloride precipitate
Bromide - cream silver bromide precipitate
Iodide - yellow silver iodide precipitate

(milk, cream, butter)

Question 23

What can be done in chromatography to make the dye travel further

Answer 23

Change the solvent
Different paper
Let the experiment run for longer