Methods of Separating and Purifying Substances

Question 1

Describe the differences between a pure substance and a mixture

Answer 1

A pure substance is completely made up of a single element or compound

A mixture contains more than one compound or different elements that aren’t all part of a single compound

Question 2

How would you calculate the Rf value

Answer 2

Distance travelled by solute / distance travelled by solvent

Question 3

Explain why water used in chemical analysis must be deionised

Answer 3

These ions, although present in small amounts and harmless in tap water, can interfere with reactions. This can cause a false result

Question 4

How can fresh water be produced from sea water

Answer 4

Using simple distillation

Question 5

What are the steps needed to make fresh water suitable for drinking

Answer 5

Filtration, sedimentation, chlorination

Question 6

What happens during filtration

Answer 6

A wire mesh separates out large twigs etc., and then gravel and sand beds filter out any other solid bits

Question 7

What happens during sedimentation

Answer 7

Iron sulfate or aluminium sulfate is added to the water, which makes fine particles clump together and settle at the bottom

Question 8

What happens during chlorination

Answer 8

Chlorine gas is bubbled through to kill harmful bacteria and other microbes

Question 9

What does a pure substance look like on a chromatogram

Answer 9

A pure substance won’t be separated by chromatography, it’ll move as one blob