Junior School Mixtures and Separation Investigation

Particles, mixtures and separation: choose a method that actually separates

A separation method works when the components respond differently enough to produce the fractions you want. This junior-school paper investigation uses visible solid pieces. You will choose a sieve, follow every component, compare alternative routes and explain when the proposed method fails. The mixture, counts and perfect recoveries are invented for learning. Do not obtain, taste, heat or handle any of the materials to complete the task.

Read the materials and the target

A model tray contains large white counters, large dark counters and small grey granules. All are dry and nonmagnetic. The counters are rigid and cannot pass through an opening narrower than 12 mm. The granules can pass through a 5 mm opening. Assume that no pieces stick together, no pieces break and none are lost. These are explicit model properties, not measurements of a real classroom mixture.

Target: obtain three separate groups containing all 6 white counters, all 8 dark counters and all 20 granules. The available paper choices are a sieve with 5 mm openings, a sieve with 15 mm openings, a magnet, and visual hand-sorting of the large counters. Explain your choices using the supplied properties, rather than naming every apparatus item.

Here “particle” means a visible piece in the mixture. A 2 mm granule is not a single atom or molecule. A simple particle model of matter uses symbols for much smaller constituents; its drawn circles are not the grains in this tray. Keeping the two meanings clear prevents an incorrect claim that a classroom sieve separates individual dissolved molecules.

Worked sequence and original component-flow diagram

The first stage uses size. Both large counter types remain together, so sieving has not completed the entire job. The second stage uses appearance to divide that retained group. In a real lesson, any physical activity would need a teacher-selected safe substitute, adequate supervision and control of small-piece hazards. This page is the paper alternative and gives no instruction to handle loose granules.

The record says where each component goes, not merely that “the mixture is clean”. The count check helps detect a missing branch. It is not a mass calculation: twenty small granules need not have the same mass as twenty counters. The model assumes complete recovery; an actual record would report lost or uncertain pieces honestly.

Why the other choices fail

The 15 mm openings are wider than the supplied 12 mm counters, so all three components can pass. That sieve does not provide a selective split. A magnet also fails because every component is stipulated to be nonmagnetic. A method is useful because of a relevant difference, not because it is familiar from another question.

Visual sorting first could be a valid alternative if it selects all large counters accurately and keeps the granules separate. However, sorting only the white counters would leave dark counters mixed with granules. Explain both the chosen route and the unfinished branch. The simplest route is not necessarily the only possible route.

Suppose a learner reports “14 pure counters” after sieving. The word “pure” hides the fact that this fraction contains two component types. The useful response is: “A granule-free mixture of white and dark counters remains, and it still needs sorting.” Likewise, a liquid that looks clear need not be pure: ordinary filtration can leave dissolved material in the filtrate. Our dry-sieving model does not test liquid purity or make drinking water.

Independent separation decisions

1. A new model contains 5 red counters and 7 blue counters, all 10 mm across, plus 18 granules that can pass a 4 mm sieve. Give a complete route and the number in each final group.
2. Would a 12 mm sieve separate those components under the stated rigid-piece model? Explain.
3. Two sets of grey granules have the same size range and the same visible appearance. Can the stated sieve and visual sorting separate the sets? State what additional information would help.
4. A completed count is 5 red, 7 blue and 16 granules. Compare it with the original 30 pieces and state what can and cannot be concluded.

Explained answers and self-check: 10 marks

1. Use the 4 mm sieve: 18 granules pass; 5 red and 7 blue counters remain. Sort the retained counters by appearance. Final groups contain 5, 7 and 18 pieces. Award 1 mark for the size-based split, 1 for both fractions, 1 for the final sorting and 1 for all final counts: 4 marks.
2. No. The 10 mm counters and small granules can all pass 12 mm openings under this model, so the choice does not give the wanted size split. Award 1 mark for the decision and 1 for the property-based explanation: 2 marks.
3. No supported selective difference is given: 1 mark. Additional information could establish a different property, such as one set being magnetic while the other is not. Do not assume it without evidence: 1 mark.
4. The final count is 28, two fewer than the original 30: 1 mark. Two granules are unaccounted for, but the count alone does not establish whether they were lost, left in equipment or miscounted: 1 mark.

For review, point to the property that justifies every arrow. Name both exits of a separation step. Check whether the final groups meet the original target. If you cannot distinguish two components with the available information, say so rather than inventing a property. These are transferable investigation habits; the rubric is original practice guidance, not an official marking scheme.

Background: OpenStax: classification of matter. The dry-mixture model, component records and questions here are original; no textbook activity or image is reproduced.

More junior-school revision

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