SIEVES AND FILTERS
- To understand that sieving is used as
a means of separating materials by particle size.
- To understand
that particles smaller than the hole size of the sieve will pass
through, and those that are larger will not.
- To appreciate
that this technique is used within the colour industry to separate
pigments according to particle size.
English National Curriculum: Science 3: 3a
All the images of filtration on this gateway
depict the separation of solids from liquids. When making a pigment,
a liquid called a 'slurry' is produced. The solid (powder) pigment
in the slurry must be separated from the liquid. (The pigment is
the dried and packaged).
The gateway consists of:
- A modern filter press
- An early 20th century filter press
- Animated graphics of a working filter press
- A production worker holding a 'filter cake'.
Modern filter press: This picture shows
a filter press used in the manufacture of red pigment on an industrial
site near Manchester.
Animated graphics of a working filter press:
The slurry enters the press from the left. It passes under pressure
(i.e. it is 'squeezed') through the filter membrane or cloth. The
solid pigment remains on one side of the filter, and the liquid
passes through. When the press is full, it is opened, and the filter
cake ('squashed, damp powder') removed.
Filter cake: This man, on a production
site in Brazil, is holding a filter cake after its removal from
a press. Most of the liquid has been removed, but it is still damp.
The last traces of liquid are removed by drying (see the activity,
Drying methods and removing water (link)).
Whilst looking at the gateway, ask the children
some of the following questions:
- What do all the pictures
have in common?
- What similarities/differences can you see between
the pictures of the modern and the old presses? (e.g. number of
people shown - due to changes in automation, lack of safety in
historical image - loose wiring etc.)
- How does the filter press work?
- What is a filter cake?
Link between the gateway and the classroom
activity. Tell the children that in industry, pigment particles
are also separated according to their size. Ask how this might be
done. After gathering the ideas, the children can be introduced
to the 'soup separation' activity.
Approximate time required: 30
2 blue pigment samples of different particle size
in transparent containers (see List
Packet of dried soup or pot snack - try a different soup for each group
(cheaper brands have a narrower range of ingredients than known brands
and are harder to identify)
Plastic container (similar size to colander)
4 food bags
4 adhesive labels (or paper strips and sellotape)
2-3 hand lenses
Initial whole class discussion, followed by children
working in groups of 2-3.
This can be a teacher demonstration, or children can try this activity
as part of a circus of related activities where they move on, in mixed
ability groups, from one to the next, in rotation.
Carrying out the activity
The activity can be used for revision purposes
or as an assessment exercise for Year 5/6 children. It is intended to
provide a novel context for a sieving activity.
Introduce the activity by asking the children
to think of as many uses as possible that people have for sieves and filters
- e.g. baking/cooking, gardening, in tumble driers, in coffee machines,
The class are first shown the packet soup or
pot snack and asked the following questions:
- What ingredients do
you think might be in here?
- How do you think we
might be able to separate some/all of them?
- How could we identify the ingredients?
answers to these questions may include the use of filter papers or water
to dissolve some ingredients. The teacher should decide whether these
avenues are pursued. Often, it is a good idea to allow the children who
suggested these methods to try out their theories. This offers a more
open-ended approach to the task and also can help to remove misconceptions
where they exist.
Either a structured or an open-ended approach
can then be adopted:
1. Children are asked to draw and explain the
means by which they will separate the mixture.
2. Children are given the colander, sieve and
tea-strainer and asked to plan how to separate the ingredients using these
pieces of equipment.
Children can be asked to prepare a means of
recording their results or they can be given a blank table, such as the
one shown on the Soup Separation
sheet. Each group then carries
out the chosen method of separation and records its findings. The children
keep the ingredients separated at each stage in a food bag and label this
bag with the filter they have used. The groups' results can also be recorded
more visually, by stapling transparent food bags containing each set of
separated ingredients to a piece of A4 paper, e.g.:
Hand lenses can be used in conjunction with the ingredients list from
the soup packet to identify ingredients. For example, a Batchelor's cupa-soup
produced the following results:
dried red peppers
large bits of parsley & coriander
small bits of pepper, leeks, tomatoes
grains of salt
small bits of parsley & coriander
tiny bits of pepper & tomatoes
small bits garlic
powder, made up of white, green and red bits
The class gathers to discuss their findings.
The following questions can be asked during the discussion:
- How did you separate the ingredients?
- How successful was your method?
- Were all ingredients clearly separated?
- Were some ingredients in more than
one filter? Why?
- What are the differences between the
Through this discussion, the children should
realise that particles that pass through filters are smaller than the
holes in the filter. Therefore, filters can be designed to let specific
sizes of particles pass through. The children are now shown the two pigment
samples. The teacher explains that the only difference between
these pigments is the particle size (which results in the colour difference)
and that they will each pass through filters with specific sizes of holes
In industry, the mesh sizes used are often from 1
to 5 or 6 microns. In the colour industry, 4 or 5 grades of sieves are
used for different, fine grades of pigment:
Extensions / links
The children can be challenged to separate other mixtures into different
size particles, such as muesli.
Children can carry out the Filter
Holes activity, in which they investigate the size of holes
in a range of household filters.