Separating Substances
SOLUTIONS
A mixture contains two or more different substances which are not chemically
joined together. Because they are not chemically bonded together, the components
of a solution can be separated fairly easily. You meet a wide variety of mixtures in
everyday life, for example:
• vinegar is a mixture of two liquids (water and ethanoic acid)
• hair gel is a mixture of a solid and a liquid
• shaving foam is a mixture of a liquid and a gas
One particular type of mixture is called a SOLUTION. A solution is obtained when
a solid is dissolved in a liquid, which is most commonly water. Solutions are clear
and look the same all the way through because the dissolved solid is completely
mixed with the water.
The liquid part of a solution is called the SOLVENT. The solid part of a solution is
called the SOLUTE. (Solutions can also be obtained by dissolving some gases and
some liquids in water). Not all solids dissolve in a particular liquid. The ones that do
dissolve are said to be SOLUBLE; the ones that do not dissolve are said to be
INSOLUBLE. The more soluble a substance is, the more of it dissolves in water.
Water, ethanol, propanone, petrol, white spirit and turpentine are all examples of
solvents.
To re-cap:
SOLUTE: the solid that is dissolved
SOLVENT: the liquid that the solute is dissolved into
SOLUTION: the mixture formed by dissolving the solute into the solvent.
IGCSE TOPIC 10.2: SEPARATING SUBSTANCES
1
Dissolving
When a substance dissolves, the solute and the solvent simply mix. Dissolving is
therefore known as a physical process as no new substances are formed.
The diagram shows a model of sugar particles dissolving in water particles. As the
sugar particles separate, the smaller water particles fit into the spaces between
the larger sugar particles. The water and sugar particles are attracted to each
other, so they move closer together when they are mixed. This is why the total
volume is often slightly less than the volumes of the two separate components.
Dissolving can be speeded up by heating the solvent up (sugar dissolves much
faster in a hot cup of tea than in cold water). The hotter the solvent, the faster
the molecules are moving, the more energy they have and the more collisions there
between solvent and solute. The solute molecules will also have more energy.
Stirring the mixture also speeds up dissolving as does grinding the solid into
smaller pieces.
Choosing a suitable separation method:
Method of separation
Used to separate
Filtration
Evaporation
Crystallization
Distillation
Fractional distillation
Chromatography
a solid from a liquid
a solid from a solution
a solid from a solution
a solvent from a solution
liquids from each other
different substances from a solution
IGCSE TOPIC 10.2: SEPARATING SUBSTANCES
2
Purity
Purity is important in drugs and foodstuffs since they cannot contain harmful
substances. The ability to separate substances to obtain pure samples is therefore
very important. The purity of a substance can be inferred from its melting point
(m.p.) and boiling point (b.p.) after separation. All substances have a unique m.p. and
b.p. which can be looked up in data books. Pure substances melt sharply at these
temperatures. Compared to the data book, the lower the m.p. and/or higher the
b.p. a substance is, the less pure it is. Impure substances also melt and boil over a
range of temperatures (i.e. a few degrees) rather than changing sharply.
METHODS OF SEPARATION
Filtration
The mixture goes in a funnel with
filter paper, into a flask. The residue
is insoluble and stays in filter paper.
The filtrate, which is the soluble part,
goes through the paper.
IGCSE TOPIC 10.2: SEPARATING SUBSTANCES
3
Crystallisation
Some water in the solution is evaporated so the solution becomes more
concentrated. One drop is placed on a microscope slide to check if crystals are
forming. The solution is left to cool and crystallise. Crystals are filtered to
remove solvent.
Evaporation
To obtain a solute from a solution, another technique is evaporation. Here the
solution is heated to remove solvent (often water) and leave the solute behind.
Care must be taken to heat very gently near the end as the solute may
decompose if heated too strongly.
Simple distillation
Distillation (left) evaporates a solvent from a
solution leaving the solute behind.
Fractional distillation
Fractional distillation (right) removes a
liquid from a mixture of liquids, because
the liquids have different boiling points.
Used to separate substances in crude oil
and get ethanol from the products of
fermentation.
IGCSE TOPIC 10.2: SEPARATING SUBSTANCES
4
1) The mixture is heated to evaporate the substance with the lowest boiling
point
2) Some of the other liquid(s) will evaporate too. A mixture of gases condense
on the beads in the fractional column. So the beads are heated to the boiling
point of the lowest substance in this case, so that the substance being removed
cannot condense on the beads. The other substances continue to condense and
will drip back into the flask. The beaker can be changed after every fraction.
Paper chromatography
To separate substances a drop of the mixture is placed at the centre of a piece
of filter paper and allowed to dry. Three or four more drops are added to it.
Water is dripped on, drip by drip, so the ink spreads creating different
coloured circles. Paper + rings = chromatogram. Rings are created because
different substances travel at different rates. (To identify substances) Spots
of substances placed onto a pencilled line (as ink would separate) which is called
the origin, and labelled. Paper goes in solvent, and solvent travels up paper, then
paper is taken out. There are spots which have travelled different distances.
Interpreting simple chromatograms
1) Number of rings/dots = number of substances
2) If two dots travel the same distance up the paper
they are the same substance.
3) You can calculate the Rf value to identify a
substance, given by the formula:
Rf value = distance moved by substance
distance moved by solvent
IGCSE TOPIC 10.2: SEPARATING SUBSTANCES
5
To make colourless substances visible you use a locating agent:
1) Dry paper in oven
2) Spray it with locating agent
3) Heat it in the oven for 10 minutes.
The stationary phase is the material on which the separation takes place (e.g.
the paper).
The mobile phase consists of the mixture you want to separate, dissolved in a
solvent.
Measuring purity
Pure substances have a definite, sharp meting/boiling point; a substance +
impurity has lower melting point and higher boiling point, at a range of
temperatures; more impurity means bigger change. This is why salt is used on
roads to prevent the formation of ice or to melt ice.
IGCSE TOPIC 10.2: SEPARATING SUBSTANCES
6
Topic 2 Separating substances
Summary questions
1
2
IGCSE TOPIC 10.2: SEPARATING SUBSTANCES
7
3
4
(ii) On the diagram above fractions X and Y have not been named. State the name of
Fraction X ………………………………………………….
Fraction Y ………………………………………………….
IGCSE TOPIC 10.2: SEPARATING SUBSTANCES
8
5
6
IGCSE TOPIC 10.2: SEPARATING SUBSTANCES
9
7
IGCSE TOPIC 10.2: SEPARATING SUBSTANCES
10