Methods of separating and purifying substances

Question 1

What is a mixture?

Answer 1

Contains 2 or more elements or compounds
that are not chemically combined together.
The chemical properties of each substance
in the mixture are unchanged.

Question 2

What is a pure substance?

Answer 2

A single element or compound, not

mixed with any other substance.

Question 3

How can you use melting point data to distinguish

between pure substances and mixtures?

Answer 3

Pure substances have a sharp exact
melting point whereas mixtures melt over
a range of temperatures since they
consist of several elements / compounds.

Question 4

When is simple distillation used?

Answer 4

Used to separate a pure liquid from a
mixture of liquids. Only works when the
liquids have different boiling points.

Question 5

How can ethanol be separated from water?

Answer 5

Ethanol has a lower boiling point than water so can be
separated from water by simple distillation:

• Distillation apparatus set up.
• Mixture boiled.
• Ethanol evaporates first.
• Ethanol vapour cools in the condenser, condensing
  back to a liquid before being collected.

Question 6

When is fractional distillation used to separate

mixtures?

Answer 6

Fractional distillation is used to separate
all the elements/ compounds in a
mixture. These chemicals must have
different boiling points to be separated.

Question 7

What is the difference between fractional and simple

distillation?

Answer 7

Simple distillation is only able to separate the
liquid with the lowest boiling point from the
mixture.

Fractional distillation separates all the chemicals
in a mixture, using their different boiling points.

Question 8

How does fractional distillation work?

Answer 8

• Oil is heated until it evaporates into the
  fractionating column.
• Vapours rise up the fractionating column and
  condense at the different fractions, depending
  on the relative boiling point of each substance.

Question 9

What sort of mixtures can filtration be used to

separate?

Answer 9

Filtration is used to separate an insoluble

substance from a solution.

Question 10

Describe how to separate an insoluble substance

from a solution

Answer 10

• Place filter paper in a funnel.
• Pour the solution containing an insoluble substance
  through the funnel into a conical flask.
• The insoluble substance will collect on the filter
  paper and the solution will collect in the conical
  flask.

Question 11

When is the process of crystallisation used to

separate a mixture?

Answer 11

To separate a soluble solid from a
solution if the solid decomposes when
heated

Question 12

How could you separate a soluble solid from a

solution, if the solid decomposes when heated?

Answer 12

Crystallisation:
- Pour the solution into an evaporating dish and heat gently.
- When the crystals start to form, remove the dish from the heat
and leave to cool.
- Once cold, filter the crystals out of the solution and leave
them in a warm place to dry.

Question 13

What process can be used to identify soluble

substances in a mixture?

Answer 13

Chromatography

Question 14

How does paper chromatography work to separate a

mixture?

Answer 14

• The mobile phase (solvent) moves through the
  stationary phase (paper) so anything dissolved in
  the mobile phase will move with up the paper.
• Compounds interact differently with each phase so
  will move different distances through the stationary
  phase meaning they will be separated.

Question 15

How can chromatography show the composition of a

mixture?

Answer 15

Different coloured substances in the
mixture will separate as they have
different solubilities in the solvent and
will travel at different rates.

Question 16

Why should pencil be used to draw the line along the

bottom of the chromatography paper?

Answer 16

It will not affect the experiment as it is

insoluble in the solvent.

Question 17

Why should the water (solvent) in the beaker for
paper chromatography be no deeper than 1 cm
deep?

Answer 17

If it is deeper it will wash away the
substances placed on the line on the
chromatography paper.

Question 18

Why should you use a lid when carrying out paper

chromatography?

Answer 18

To prevent the solvent evaporating.

Question 19

How many spots will a pure substance produce on a
chromatogram? How would this be different for an
impure substance?

Answer 19

Pure substances produce one spot.

An impure substance contains more than
one compound so will produce more spots
(one spot for each chemical).

Question 20

What is an Rf value?

Answer 20

The Rf value is the ratio between the
distance travelled by the dissolved
substance (the solute) and the distance
travelled by the solvent.

Question 21

How do you calculate Rf values?

Answer 21

Rf =

Distance travelled by substance/Distance travelled by solvent

Question 22

When measuring the distance moved by a
substance on the chromatography paper, where
should you measure between?

Answer 22

Measure from the pencil baseline to the

middle of the spot of the substance.

Question 23

How can you use chromatography to see if a certain

substance is present in a mixture?

Answer 23

Run a pure sample of this substance alongside
the unknown mixture.

If the Rf value of the pure substance matches the
value of one of the spots from the mixture, it is
likely to be present.

Question 24

True or false?
‘Substances with a higher solubility in the solvent will
travel further up the chromatography paper’

Answer 24

TRUE

They will stay dissolved in the solvent
(mobile phase) for longer.

Question 25

How could you separate salt from a mixture of salt

and sand?

Answer 25

• Salt is soluble in water, sand isn’t.
• Add water to the mixture.
• Filter the solution to remove the insoluble
  sand.
• Evaporate the water to collect the salt crystals.

Question 26

How can liquids be separated if they have different

densities?

Answer 26

If liquids have different densities they will
naturally separate into two layers.
To separate these layers, use a flask with a
tap on the bottom. Open the tap to collect the
bottom liquid (has the highest density).

Question 27

What does potable mean?

Answer 27

Safe to drink

Question 28

How can waste and groundwater be made potable?

Answer 28

1. Sedimentation: Large insoluble particles sink to the
   bottom after the water is left still for a while.
2. Filtration: Removes small insoluble particles by
   passing the water through layers of sand and filters.
3. Chlorination: Kills bacteria and microorganisms
   which are too small to be removed by filtration.

Question 29

How can seawater be made potable?

Answer 29

Distillation

1. Filter to remove insoluble particles.
2. Boil.
3. Cool and condense the water vapour.

Question 30

What issues surround the process of making

seawater potable?

Answer 30

• Extremely expensive as it requires a lot
  of energy to boil large volumes of water.
• Wastewater is toxic due to the high
  concentration of salt so must be
  disposed of carefully

Question 31

What is deionised water?

Answer 31

Water that has had metallic ions (such as

copper or calcium ions) removed.

Question 32

Why is deionised water used in experimental

analysis?

Answer 32

Deionised water is used to prevent ions in
the water interacting with the substances
under analysis. If water wasn’t deionised,
false positive results may be produced.