Separating a Three-Component Mixture
A student is given a beaker containing a mixture of sand, table salt, and small iron filings, plus a little water. The mixture is stirred and looks cloudy. The student is asked to obtain a dry sample of each of the three solids. Available equipment: a magnet, filter paper and funnel, a tripod and gauze, a Bunsen burner, an evaporating basin, a glass rod, and a balance. The student's plan is: (1) pour the whole mixture through filter paper, (2) heat the filtrate to dryness, (3) scrape the residue off the filter paper and call it sand. A teacher says the plan will not give pure samples of all three solids.
1. Explain why the student's plan fails to give pure samples, identifying which steps are wrong and why.
Read the worked answer
Step 1 is incomplete: iron filings are not separated by filtration. They will be trapped with the sand on the filter paper, so the 'sand' residue is contaminated with iron. Iron must be removed first, using a magnet, before any wet separation.
Step 2 is acceptable in principle: the filtrate contains dissolved salt, and evaporating to dryness gives solid salt. However, heating to complete dryness can cause spitting and loss of salt, and overheating can decompose some salts. Gentle evaporation with a glass rod to stir is safer.
Step 3 is wrong for two reasons: (a) the residue contains iron as well as sand, and (b) scraping wet filter paper gives a mixture of sand, paper fibres, and residual moisture - not a dry pure solid. The residue should be washed with a little distilled water, dried, and the iron removed magnetically beforehand.
So the plan gives contaminated 'sand' and possibly impure salt, and no separate iron sample at all.
2. Write a corrected sequence of steps that would yield dry samples of iron, sand, and salt, and explain the principle behind each separation.
Read the worked answer
Remove iron with a magnet. Iron is magnetic; sand and salt are not. Pass a magnet (wrapped in plastic for easy release) through the mixture and collect the filings. Principle: difference in magnetic properties.
Add water and stir. Salt dissolves; sand does not. Principle: difference in solubility.
Filter. Sand is trapped on the filter paper; salt solution passes through as filtrate. Principle: difference in particle size - dissolved particles pass through, undissolved solids do not.
Wash and dry the sand residue. Rinse with a little distilled water to remove salt, then dry in a warm oven or air-dry. Weigh to confirm constant mass.
Evaporate the filtrate gently in an evaporating basin over a water bath or low flame, stirring with a glass rod, until crystals form. Do not heat to charring. Principle: salt has a much higher boiling point than water, so water leaves as vapour and salt remains.
Dry and weigh each sample. Confirm that the sum of masses is close to the original mass, allowing for handling losses.
3. The student recovers 4.8 g of salt, 6.1 g of sand, and 1.9 g of iron from a starting mixture of 13.0 g. Evaluate the recovery and suggest sources of loss.
Read the worked answer
Total recovered = 4.8 + 6.1 + 1.9 = 12.8 g from 13.0 g, a recovery of 98.5%. This is good but not perfect.
Possible sources of loss:
- Sand or iron left behind on the filter paper or in the beaker during transfer.
- Salt solution retained in the filter paper and not fully washed through.
- Spitting during evaporation, or salt left on the glass rod.
- Small amounts of material still dissolved in the final washings.
Possible sources of gain (making recovery too high):
- Salt not fully dried, so the salt mass includes water.
- Impurities in the 'sand' fraction.
Judgement: a 1.5% shortfall is consistent with handling losses and is acceptable for a school practical. To improve, the student should wash the residue thoroughly, dry each sample to constant mass, and repeat the separation to check consistency. A recovery greater than 100% would indicate incomplete drying and would be a warning sign.