The Playground Mixture That Lost 17 Grams
A class received a 60 g sample of a playground mixture: fine sand, table salt, iron filings, and small plastic beads. Available materials included magnets, filter paper, funnels, beakers, a 2 mm screen, water, and a hot plate. A group planned these steps: (1) use a magnet to remove iron; (2) add 100 mL water and stir; (3) skim off plastic beads that float and set them aside without weighing; (4) pour the rest through filter paper; (5) evaporate the liquid that passes through. Results: the magnet removed 9 g; the filter caught 28 g of wet solid that dried to 23 g; evaporation left 11 g of white crystals. The group said, 'We started with 60 g, but 9 + 23 + 11 is only 43 g. Matter must have been destroyed.' They did not weigh the skimmed beads.
1. Identify what each step separates and the property that makes the separation possible.
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Step 1 removes iron because iron is magnetic and the other materials are not. Step 2 adds water, which dissolves the salt but not the sand or plastic. Step 3 skims floating plastic beads, using density: the beads float while sand and iron sink. Step 4 filters out sand because sand is insoluble and its particles are too large to pass through the filter paper, while salty water passes through. Step 5 evaporates the water, leaving salt crystals because water changes to vapor at a lower temperature and the salt does not evaporate with it. The plan relies on magnetism, solubility, density, particle size, and boiling point.
2. Explain the missing 17 g and evaluate the claim that matter was destroyed.
Read the worked answer
The 43 g total does not include the plastic beads, because the group skimmed them off and never weighed them. If the beads have a mass of 17 g, then 9 g + 17 g + 23 g + 11 g = 60 g. Matter was not destroyed; some parts were simply not measured. Other small losses are possible, such as bits of sand stuck to the filter, spills, or water remaining in the dried solid. To test the explanation, weigh the beads and check whether all dried parts add to 60 g. Conservation of matter predicts that the masses of all recovered parts must equal the original 60 g, not counting the added water that was later evaporated.
3. The group did not use the 2 mm screen. Would using it first have separated salt from sand? Explain, and describe one improvement to the procedure.
Read the worked answer
A 2 mm screen separates materials by particle size. Salt crystals and most sand grains are smaller than 2 mm, so both would pass through the screen. The screen would not separate salt from sand. It might catch larger plastic beads, but the beads were already removed by floating. A better improvement is to weigh every separated part, including the beads, and dry the sand thoroughly before weighing. The magnet should be used before adding water so wet iron filings do not stick to the filter. The revised order keeps each property test clear and makes the mass check complete.