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  • 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.

Curriculum links

English National Curriculum: Science 3: 3a

Gateway story

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).

Gateway elements

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)).

Gateway discussion

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 minutes

Resources needed

2 blue pigment samples of different particle size in transparent containers (see List of Suppliers)

Per group:

Soup Separation sheet
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

Suggested organisation

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, etc.

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?

Note: Children's 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:

Product for separation Ingredients separated by …
cupa-soup colander sieve tea-strainer remaining

large noodles

dried leeks

dried tomatoes

dried red peppers

dried onions

small noodles

large bits of parsley & coriander leaf

small bits of pepper, leeks, tomatoes and onions

grains of salt

small bits of parsley & coriander leaf

tiny bits of pepper & tomatoes

small bits garlic

fine 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 filters?

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 them.

Background information

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.