Separating Mixtures
Grade 7 Science: Pure Substances and Mixtures
S. Willis
Separating Mixtures
Key Question: How can we
separate the components of a
mechanical mixtures or
solution?
Learning Goals/Success Criteria
 I can describe the processes used to separate
mixtures into their components and identify
some industry applications of these
processes.
 Key terms: Sorting, floating, settling, sieve,
sieving, filter, filtration, sewage
Separating Mixtures
 Kalia always picks the tomatoes out of her salad.
 Ali sorts the laundry before washing.
 Jen boils pasta in water for lunch. When it is cooked
she drains it through a colander.
 All of these are examples of separating
mechanical mixtures.
Sorting Mechanical Mixtures
 There are 5 main ways of separating mechanical
mixtures:





Sorting
Floating and Settling
Using a Magnet
Using Sieves and Filters
Dissolving Soluble Components
Sorting
 Definition:
 Physically separating
large pieces of
mechanical mixture so
that similar pieces are
together
Floating and Settling
 Floating: a separation
technique in which a
“lighter” component rises
to the top of a liquid
where it can be skimmed
or poured off.
Floating and Settling
 Settling: a separation
technique in which a
“heavier” component
sinks to the bottom of a
liquid and the liquid can
be poured off.
Floating and Settling
 This technique is use to separate blood which is very
helpful for doctors. A centrifuge is used to spin the
blood samples to speed up the process of settling.
Floating and Settling
Using Sieves and Filters
 Sieve: a device used to
separate a mixture, with
many visible holes that
allow smaller solid pieces
and liquids to pass
through while blocking
larger solid pieces.
 (Verb: Sieving)
 A colander is a household
sieve.
Using Sieves and Filters
 Filter: a device with many
small holes or channels
that can be used to
separate a mixture.
Filtration removes tiny
pieces of solids from a
liquid or a gas. The holes
are much smaller than a
sieve.
Using a Magnet
 Some metals and alloys,
such as iron and steel, are
attracted to magnets. If
only one part of a mixture
is attracted magnets, a
magnet can be used to
separate that part from
the rest of the mixture.
Dissolving Soluble Components
 If one part of a
mechanical mixture
dissolves easily in a
solvent, you can separate
this mixture using
dissolving.
 What would
happen if you pour
a mixture of salt
and sand in to a
glass of water?
Applying Separating Mixtures to the
Environment
 Think about the last time you visited a lake or river.
Did it look and smell clean? Often water around cities
is polluted. There are two ways that cities deal with
this:
 1. Try to stop or limit pollution from sources.
 2. Treat any water that contains pollutants, such as
waste from sinks and toilets, before it reaches the lakes
and rivers.
Sewage Treatment
 Sewage: the mixture of
water and waste that is
flushed down toilets and
sink drains.
Sewage Treatment
 Primary Treatment: half
of the solids in sewage
are removed from the
mixture through sieving,
then settling.
Sewage Treatment
 Secondary Treatment:
The remaining dirty water
passes through tanks
containing bacteria that
break down any
remaining human waste
and plant material. Then
the water is allowed to
settle again. The bacteria
settles and is removed.
Sewage Treatment
 Tertiary Treatment:
Pollutants like
phosphorus and nitrogen
(from soaps) are removed
by bacteria that is
encouraged to grow by
ultraviolet or ozone
exposure. Chlorine kills
any remaining organisms.
Sewage Treatment
Keeping our Water Clean
 Mixtures of dirty water can be very difficult to
separate. Some of the contaminants include:
 Waste produced by factories
 Fertilizers, pesticides or salt from farms, golf courses or
roads
 Accidental spills and leaks (think oil spills from ships)
Video: Sewage Treatment Process