science world 1 – chapter 3
mixing & separating
WHAT IS A MIXTURE?
The materials around you can be grouped into two categories:
Pure substances contain only one type of substance e.g. sugar,
gold, pure water. They always have the same properties no
matter where they come from.
Most materials around you are mixtures – several different
substances mixed together e.g. lipstick, concrete, air. Not all
mixtures of the same substances have the same properties. The
amounts of each part, called their proportions, can vary widely.
SOLUTION TERMINOLOGY
Dissolve – when a solid seems to disappear into a liquid.
Solute – the substance being dissolved e.g. coffee, sugar.
Solvent – the substance doing the dissolving e.g. water.
Solution – is formed when a solute dissolves in a solvent.
Soluble – a substance that can dissolve in another substance e.g.
sugar in water.
Insoluble – a substance that does not dissolve in another
substance e.g. chalk in water.
Suspension – when some or all of the particles settle out,
eventually forming a sediment e.g. muddy water.
Sediment – when insoluble substances settle to the bottom of
the container they are in e.g. orange juice pulp.
Some solutes may be soluble in some solvents but not in
another. For example, salt dissolves in water but not in alcohol.
Solutions are not always made up of a solid dissolved in a
liquid. Liquids and gases can also dissolve in solvents.
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SOLUBILITY
Liquid solutions come in many different strengths. The colour
of a solution can often give some idea of the strength of the
solution. We describe the strengths of solutions using the
following terms:
Dilute – small amount of solute in a certain amount of solvent
(i.e. weak).
Concentrated – large amount of solute in a certain amount of
solvent (i.e. strong).
Saturated – a solution when no more solute will dissolve in the
solvent (i.e. extremely strong).
Unsaturated – any solution that can dissolve more solute.
The amount of solute needed to saturate a solution depends on
the temperature. Most solids are more soluble in warm water
than cold water. We say that their solubility increases as the
temperature increases.
SEPARATING SUSPENSIONS
The method we choose for separating suspensions depends on
the properties of the substances involved.
Decanting – used when you have a solid sediment mixed in
with a liquid. You slowly pour the liquid off the
top, leaving the sediment on the bottom e.g.
pouring water off peas in a saucepan.
Centrifuging – used when you have a liquid made up of
different density substances. The particles are
spun in a centrifuge and the heavier particles
settle on the bottom e.g. separating red blood
cells from plasma.
Filtering – is the process in which insoluble particles in a liquid
are screened out by a filter. The material that passes
through the filter is called the filtrate. The material
that is held back by the filter is called the residue.
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SEPARATING SOLUTIONS
Filtration cannot be used to separate the substances in a solution
because the particles of the solute are small and pass through
any filter paper.
Evaporation is a process in which a liquid can be removed from
a solution by using heat, leaving the solute behind. Heating
seawater will evaporate the water leaving the salt behind.
Distillation is a method used to separate the liquid part of a
mixture by evaporating the solvent, collecting the solvent
vapour and then condensing it to form a liquid again. The
material collected during distillation is called the distillate. The
residue is the mixture that remains in the original container.
SEPARATING SOLIDS
Sometimes we need to separate a mixture of solids from each
other. The five methods below all depend on differences in the
properties of the solids.
1.
2.
3.
4.
5.
If one solid has larger particles than the other, you can use
a sieve to separate them.
If one solid is soluble in water and the other is not, you
can add water and filter the mixture.
If one solid is attracted to a magnet and the other is not,
you can use magnetic separation.
If one insoluble solid floats on water and the other sinks,
you can add water to the mixture and skim off the floating
solid.
If one solid is heavier than the other, you can use gravity
separation. A stream of water is used to move the lighter
particles (e.g. sand) away from the heavier particles (e.g.
gold).
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SEPARATING COLOURS
Paper chromatography can be used to separate a mixture of
coloured substances. For example, this method will separate the
coloured substances in black ink. This involves dissolving the
ink in a solvent (water) and allowing the solvent to carry the ink
along some filter paper. The colour which dissolves the best in
the solvent will be carried the furthest along the filter paper (i.e.
the yellow ink).
SPELLING WORDS
No.
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EASY
substance
material
mixture
solution
filter
petrol
settle
light
pure
property
sieve
decant
soluble
crystal
colour
funnel
magnet
condense
sugar
water
HARD
separation
identify
dissolve
gravity
solute
solvent
suspension
magnetic
solubility
filtration
evaporation
proportion
distillation
residue
chromatography
centrifuge
dilute
concentrated
saturated
sediment
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