In the school lab, a box of fine iron filings has spilled into a tray of fine sand, and the grains…
In the school lab, a box of fine iron filings has spilled into a tray of fine sand, and the grains of both are about the same size. Nirmal says the quickest way to separate them is to shake the mixture through a sieve. Dhruv says a sieve won't help here — they should use a magnet. Whose plan will work?
Who do you think is right? Pick one.
The answer: Dhruv is right, the difference between them is magnetism, not size
Why
Nirmal reaches for a method he knows works — sieving is great for separating things like stones from flour. But every separation method works only by using a property in which the two parts are different. A sieve sorts by size, and here the iron filings and the sand grains are about the same size: whatever passes through the holes, both kinds pass; whatever stays, both stay. The sieve can't tell them apart. So the smart move is to ask what IS different about them. Iron is attracted to a magnet; sand is not. Move a magnet wrapped in a thin plastic bag over the tray and the iron filings jump onto it, leaving the sand behind; pull the magnet out of the bag and the filings drop off. Test: before choosing any method, name the difference first — size → sieve, weight with wind → winnowing, dissolving → evaporation, magnetism → magnet. Dhruv is right: when the sizes match, a sieve can't separate them, but a magnet can, because only the iron is magnetic.
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