Methods of separating and purifying substances Flashcards Preview

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Flashcards in Methods of separating and purifying substances Deck (32)
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1
Q

What is a mixture?

A

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

2
Q

What is a pure substance?

A

A single element or compound, not

mixed with any other substance.

3
Q

How can you use melting point data to distinguish

between pure substances and mixtures?

A

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

4
Q

When is simple distillation used?

A

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

5
Q

How can ethanol be separated from water?

A

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.
6
Q

When is fractional distillation used to separate

mixtures?

A

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

7
Q

What is the difference between fractional and simple

distillation?

A

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.

8
Q

How does fractional distillation work?

A
  • 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.
9
Q

What sort of mixtures can filtration be used to

separate?

A

Filtration is used to separate an insoluble

substance from a solution.

10
Q

Describe how to separate an insoluble substance

from a solution

A
  • 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.
11
Q

When is the process of crystallisation used to

separate a mixture?

A

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

12
Q

How could you separate a soluble solid from a

solution, if the solid decomposes when heated?

A

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.

13
Q

What process can be used to identify soluble

substances in a mixture?

A

Chromatography

14
Q

How does paper chromatography work to separate a

mixture?

A
  • 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.
15
Q

How can chromatography show the composition of a

mixture?

A

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

16
Q

Why should pencil be used to draw the line along the

bottom of the chromatography paper?

A

It will not affect the experiment as it is

insoluble in the solvent.

17
Q

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

A

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

18
Q

Why should you use a lid when carrying out paper

chromatography?

A

To prevent the solvent evaporating.

19
Q

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

A

Pure substances produce one spot.

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

20
Q

What is an Rf value?

A

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

21
Q

How do you calculate Rf values?

A

Rf =

Distance travelled by substance/Distance travelled by solvent

22
Q

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

A

Measure from the pencil baseline to the

middle of the spot of the substance.

23
Q

How can you use chromatography to see if a certain

substance is present in a mixture?

A

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.

24
Q

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

A

TRUE

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

25
Q

How could you separate salt from a mixture of salt

and sand?

A
  • 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.
26
Q

How can liquids be separated if they have different

densities?

A

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).

27
Q

What does potable mean?

A

Safe to drink

28
Q

How can waste and groundwater be made potable?

A
  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.
29
Q

How can seawater be made potable?

A

Distillation

  1. Filter to remove insoluble particles.
  2. Boil.
  3. Cool and condense the water vapour.
30
Q

What issues surround the process of making

seawater potable?

A
  • 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
31
Q

What is deionised water?

A

Water that has had metallic ions (such as

copper or calcium ions) removed.

32
Q

Why is deionised water used in experimental

analysis?

A

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.