Separating a Mixture from a School Laboratory Spill
During a practical, a learner accidentally knocks over a beaker containing a mixture of iron filings, table salt, sand and small pieces of cork. The mixture is dry. The teacher wants the class to recover as much clean salt and iron as possible, and to dispose of the rest safely. Available equipment: a strong magnet, filter paper, funnels, beakers, a tripod, gauze, a Bunsen burner, a glass rod, a measuring cylinder, water, and a heat-proof mat. The class is told that the salt dissolves in water, the sand does not, the iron is magnetic, and the cork floats. The teacher warns that heating must be done carefully and that the iron must be dried completely before storing.
1. Design a step-by-step procedure to separate the four substances, naming the technique used at each stage and stating what is collected.
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A workable sequence uses differences in magnetic attraction, density, solubility and particle size:
- Magnetic separation. Wrap a magnet in plastic or paper and pass it over the dry mixture. Iron filings stick and are removed. Unwrap to collect the iron. (Technique: magnetic separation.)
- Flotation / decanting. Add water and stir. The cork floats; skim it off with a spoon or filter it out. Cork pieces can be picked out or poured off. (Technique: flotation/decanting.)
- Filtration. Pour the remaining mixture (sand + salt water) through filter paper in a funnel. The sand stays on the paper; the salt solution passes through as the filtrate. (Technique: filtration.)
- Evaporation/crystallisation. Gently heat the salt solution in an evaporating basin (or beaker on a gauze) until water evaporates, leaving salt crystals. Do not heat to dryness too strongly; stop when crystals appear and let the rest dry. (Technique: evaporation/crystallisation.)
Each solid is then dried and stored separately. The order matters: removing iron first prevents it from rusting in water, and removing cork before filtration stops it clogging the filter.
2. A learner suggests simply heating the whole mixture until 'everything separates'. Explain two reasons why this would fail and one safety concern.
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Heating the whole mixture would not separate the components well:
- Salt does not evaporate at Bunsen temperatures in a useful way - it would remain mixed with the sand, and some salt might spit or decompose. Evaporation only works to recover salt from a solution, so the salt must first be dissolved and filtered.
- Iron would not be removed and could react. Hot iron in air can oxidise (rust), and iron filings may burn or glow, changing the substance so it is no longer clean iron. The cork would also char and burn, contaminating everything.
- Heating a dry mixture of sand and salt does not separate them - there is no change of state to exploit; the solids stay mixed.
Safety concern: heating an unknown mixture can cause spitting, smoke or fire (cork is flammable), and hot apparatus can cause burns. The teacher's warning about careful heating and using a heat-proof mat reflects this. The correct approach is to use the substances' different properties (magnetism, floating, solubility) rather than heat alone.