Topic 2- Separation and purification of substances (Paper 1)

Question 1

What factors affect how far a compound moves in chromatography?

Answer 1

1. How soluble they are up the solvent
2. How attracted they are to the paper

Question 2

How do you calculate an Rf value?

Answer 2

Rf value =
distance travelled by solute / distance travelled by solvent

ink/disolvy stuff 😎

Question 3

Method for carrying out a paper chromatography

Answer 3

1. Draw a line near the bottom of a sheet of filter paper with pencil as it is insoluble (this is the baseline)
2. Spot the ink by placing dot of it on the line
3. Put the sheet upright in a beaker of slovent
4. Make sure that the ink isn’t touching solvent initially as you don’t want it to be washed away.
5. Place a watch glass on top of the beaker to stop the solvent evapourating away.
6. Solvent will start to move up the paper. When the chemicals in the ink dissolve in the solvent they will move up the paper too.
7. The different dyes in the ink will move up the paper at different rates. Any insoluble inks will stay at the baseline.
8. When the solvent front has nearly reached the top of the paper, remove the paper from the beaker and draw a line with a pencil along the solvent front before leaving it to dry.

Question 4

How can you identify a pure substance from chromatography?

Answer 4

A pure substance will not be separated by chromatography and instead it will remain in one spot

Question 5

Describe the process of water purification

Answer 5

1. Filtration- a wire mesh screens out large twigs, gravel and sand beds filter out any solids
2. Sedimentation- Iron sulphate or aluminium sulphate is added to make fine particles clump together and settle at the bottom
3. Chlorination- chorine gas is bubbled through to kill harmful bacteria and other microbes

Question 6

How would sea water be made potable?

Answer 6

Distilling it- but requires a lot of energy

Question 7

Why might you not use ionised water in chemistry?

Answer 7

Mineral ions could interfere with results

Question 8

Method to crystalise soluble solids

Answer 8

1. Place evapourating dish on top of tripod and gauze mat.
2. Pour solution into evapourating dish and gently heat.
3. Once water has evapourated or crystals start to form remove dish from heat and leave to cool.
4. Dissolved compound should start to form crystals as it becomes insoluble in the cold highly concentrated solution
5. Filter the crystals out of the solution using a drying oven or desiccator.

Question 9

What are sources of water?

Answer 9

1. Surface water
2. Ground water from aquifers
3. Waste water

Question 10

In what ways can a solid be separated from a liquid?

Answer 10

Not dissolved → filtration
Dissolved → evapouration

Question 11

In what ways can a liquid be separated from a liquid?

Answer 11

Immiscible liquids (haven’t mixed) → separating funnel
Miscible liquids → fractional distillation

Question 12

In what ways can you separate two solids?

Answer 12

Dissolving → one substance is soluble but not the other
Chromatography → dissolved solids