Science 7–10 · Year 7

Design a separation: iron filings, sand, sawdust and salt

Solutions and mixtures

PracticalLow risk

The idea

Each separation technique exploits one property difference, so a multi-part mixture needs a sequence of techniques chosen from the properties of its parts.

Safety card

Low riskAn adult supervises

Hazards

iron filings can scratch the eye and rust in a sink
hot basin near dryness

Controls

eye protection
magnet kept inside a bag so filings are easy to release
iron and sand into the bin, not the sink

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson. CSIS Appendix F lists iron filings under chemicals to be used with caution because they react strongly with sulfur, chlorine or bromine; none of these is in this mixture.

What you need

mixture of about 2 g iron filings, 3 g sand, 1 g sawdust and 3 g salt
bar magnet in a plastic bag, 250 mL beaker, water, spoon, sieve or tea strainer
filter funnel and paper, conical flask, evaporating basin, tripod, gauze, Bunsen burner
balance reading to 0.1 g

How to do it

Examine the mixture and list one property for each part that the others do not share (magnetic, floats, dissolves, sinks and does not dissolve).
Write a step sequence before touching the equipment and have it checked.
Carry out the plan: magnet through the bag to lift iron; add water and skim or sieve the floating sawdust; filter off the sand; evaporate the filtrate for salt.
Dry and weigh each recovered part and compare with the starting masses.
Evaluate: which step lost the most material and how the plan could be changed.

What you should see

Four separate samples: dark iron filings on the magnet, damp sawdust from the surface, sand on the paper, white salt in the basin. The four recovered masses add to less than the starting total; the sawdust is usually the least clean fraction because it holds water and dissolved salt, so it is weighed only after drying. The learner knows it worked when no iron is drawn from the remaining sand by a second magnet pass and the filtrate is clear.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

One technique should separate everything at once.
Filtering will remove dissolved salt.
The order of the steps does not matter.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC4-SOL-01 SC4-WS-03 SC4-WS-07
Australian Curriculum v9AC9S7U06AC9S7I02

Sources

The pages the author read to write this activity.

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