Recovering Four Components from a 15.0 g Mixture
Each candidate in a Grade 11 practical assessment receives a sealed, dry 15.0 g sample containing iron filings, sand, ammonium chloride and sodium chloride. The label gives no masses. The bench provides a bar magnet in a plastic bag, filter funnels and paper, beakers, evaporating basins, a tripod and gauze, a Bunsen burner, a balance reading to 0.01 g, distilled water, a fume cupboard and a supply of watch glasses. Candidates must recover a dry sample of each component and record its mass; marks are awarded for purity as well as for recovery. Candidate A adds 60 cm³ of water to the whole sample, stirs, filters, then heats the filtrate strongly in an open basin until a white solid remains. Candidate B first removes one component with the magnet, then heats the remaining solid gently in the fume cupboard under an inverted watch glass, then treats what is left with water. Candidate A recovers 6.50 g of white solid, 3.10 g of damp iron filings and 4.90 g of sand, and reports the ammonium chloride as 'lost as gas'.
1. Explain which candidate's order of steps is more likely to give four pure, dry samples, justifying the position of the magnetic step and of the heating step.
Read the worked answer
Candidate B's order is the sound one. Each step must be placed where it separates one component without contaminating another.
- Magnet first, while everything is dry. Iron is the only magnetic component. If water is added first, the filings rust, and wet filings smear through the sand and are hard to pull out cleanly; drying a wet iron/sand mixture also risks losing fine particles.
- Gentle heating second, before any water. Ammonium chloride is the only component that leaves a solid again on cooling: on heating it dissociates into ammonia and hydrogen chloride, which recombine on the cool watch glass as a white solid. If water is added first (Candidate A), ammonium chloride dissolves in the same water as the sodium chloride, so it can no longer be separated by heating without the sodium chloride also being present in the solid left behind.
- Water, filtration and crystallisation last. Sodium chloride dissolves; sand does not. Filter, wash the sand with a little distilled water and dry it. Evaporate the filtrate only to the point of crystallisation, cool, filter the crystals and press them dry. Evaporating to dryness risks spitting and gives a less pure product.
2. Use Candidate A's masses to judge whether the 6.50 g of white solid can be accepted as pure sodium chloride, and state the test that would settle the question.
Read the worked answer
It cannot be accepted as pure without evidence. Mass balance: 6.50 + 3.10 + 4.90 = 14.50 g, so 0.50 g of the original 15.0 g is unaccounted for. Two things may be true at once: some solid was lost (spitting from the open basin, powder clinging to the filter paper, damp filings), and the white solid may still contain ammonium chloride, because heating a solution to dryness does not guarantee that all the ammonium salt has been driven off.
Because the original composition is not given, the expected mass of sodium chloride is not known, so the mass alone cannot prove purity. A 0.50 g discrepancy is, however, large enough to require a test:
- Warm a small sample with dilute sodium hydroxide solution. Ammonia gas that turns damp red litmus blue shows ammonium ions.
- A flame test giving a persistent yellow colour shows sodium.
- Adding silver nitrate solution gives a white precipitate with chloride from either salt, so that test does not distinguish them; the ammonium test is the decisive one.
3. Explain why gentle heating in a fume cupboard under a watch glass is preferred to strong heating in an open basin for removing ammonium chloride, and give one safety precaution.
Read the worked answer
Ammonium chloride does not simply melt and stay in the basin. On heating it dissociates into ammonia and hydrogen chloride, both irritant gases. In an open, strongly heated basin these gases escape into the room, the solid is lost, and the yield falls. With a watch glass held above, the gases meet a cool surface and recombine there as a white solid that can be scraped off.
Strong local heating also causes spitting and bumping, especially once sodium chloride or sand is present, which throws material out of the basin and contaminates the sublimate. Gentle, even heating keeps the sublimate white and limits mechanical loss.
Safety: wear eye protection and carry out the heating in a fume cupboard or a well-ventilated area, keeping the flame moving and never pointing the hot watch glass or the mouth of the basin towards anyone.