Sorting materials with a magnet
Chemistry · Key Stage 2 · The School
Pass a magnet over a tray of objects and some jump to it. That is a property test with a yes or no answer, exactly like the conductivity tests. What it tells you is narrow but reliable: this material is attracted, that one is not.
Iron, steel, nickel and cobalt are attracted. Copper, aluminium, gold and silver are not, and they are unmistakably metals. So "metal" and "magnetic" are different groups that overlap, and a magnet cannot be used to answer the question "is this a metal?".
Iron filings mixed into sand cannot be sieved apart, because the grains are the same size. A magnet lifts the iron out and leaves the sand behind. The property doing the work is one the two materials do not share, which is what every separation needs.
Drinks cans are made of both. Run a magnet along a recycling bin and the steel ones cling while the aluminium ones drop. Recycling plants use exactly this, on a conveyor belt with a large magnet, to sort thousands of cans without anyone reading a label.
Hold a paperclip on one side of a sheet of paper and a magnet on the other, and the clip still moves. The pull passes through paper, card, plastic and even a thin sheet of glass. It is not blocked by materials that are simply non-magnetic, which is how a fridge magnet holds a whole stack of drawings against the door without touching a single one of them.