What it shows
A mixture keeps the properties of each part, so each part can be removed by a method that suits it: a magnet for iron, dissolving for salt, filtering for sand and evaporation to get the salt back. The project shows four separation methods in one go.
The science
In a mixture the substances are only mingled, not chemically joined, so each keeps its own behaviour. Iron is magnetic but sand and salt are not. Salt dissolves in water but sand does not. The sand grains are too large to pass through filter paper, while dissolved salt passes through with the water. Finally, evaporation removes the water and leaves the salt. Each step uses one property difference.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 2 spoons each of clean sand, table salt and iron filings (or steel wool rubbed on sandpaper)
- A fridge magnet wrapped in a plastic bag
- A cup of water
- A funnel and a coffee filter or clean cotton cloth
- Three bowls and a shallow plate
- A kitchen balance, optional
Safety
For this project: Iron filings are sharp and must not go near eyes; an adult must make them from steel wool if needed. Wash hands afterwards.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Mix the sand, salt and filings in a bowl and weigh the mixture if you can.
Hold the bagged magnet just above the mixture and move it around; the filings jump up and stick to the bag.
Pull the bag off the magnet over a second bowl so the filings drop off cleanly; repeat until no more filings are picked up.
Pour the water on the remaining sand and salt and stir for two minutes so all the salt dissolves.
Set the funnel with the filter over a third bowl and pour the muddy water through slowly; the sand stays in the filter.
Spread the sand on a paper to dry and pour the clear salty water into the shallow plate.
Leave the plate in the sun or a warm place for a day until only white salt remains.
Weigh each recovered part and compare the total with the starting mass; write down where any loss happened.
What you should see
Three separate piles, iron filings, dry sand and white salt, with the total mass close to the original. Most loss is salt left in the filter, which the student can explain.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the magnet pick up only the filings?
Only iron is magnetic; sand and salt are not attracted.
2Why can filtering not remove salt?
Dissolved salt particles are tiny and pass through the filter with the water.
3How is the salt recovered from the water?
By evaporating the water, which leaves the salt behind.
4Is this a mixture or a compound?
A mixture, because the parts keep their own properties and can be separated physically.
Take it further
Add sawdust to the mixture and work out an extra step (floating on water) to remove it.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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