The Cloudy Crystal in the Lab
A student dissolves 5.0 g of an unknown white solid in 50 mL of warm water. The solution is clear. On cooling, long needle-like crystals form. The student filters them, dries them, and finds the mass is 4.2 g. A flame test on a fresh sample gives a persistent lilac colour. Adding dilute hydrochloric acid to a separate portion produces brisk fizzing and a gas that turns limewater milky. The student concludes the solid is 'pure sodium carbonate' and writes that the missing 0.8 g 'evaporated as gas.' A teacher notes the flame colour and asks the student to reconsider.
1. Identify which of the student's two claims is contradicted by the flame test, and explain what the flame colour indicates.
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The flame test contradicts the identification as sodium carbonate. Sodium gives a strong persistent yellow-orange flame, not lilac. A persistent lilac flame is characteristic of potassium. So the cation is more likely potassium than sodium, and the compound is probably a potassium salt rather than a sodium salt. The fizzing with acid and the milky limewater confirm a carbonate, so a potassium carbonate is a better hypothesis than sodium carbonate. The student's conclusion fails on a single, easily checked observation.
2. Explain why the 0.8 g mass difference is not evidence of gas escaping during drying, and give a better explanation.
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The 0.8 g was never part of the recovered crystals: it stayed dissolved in the 50 mL of water that was filtered away. Solubility is not zero at the filtration temperature, so some solute remains in the mother liquor. A rough check: if the solution still held about 0.8 g in 50 mL, that is roughly 16 g per 100 mL of remaining dissolved solid, which is plausible for a soluble carbonate at cool room temperature. Gas escaping during drying would require an acid or heat decomposition, but drying alone does not drive off carbonate as CO₂. A better explanation is incomplete crystallisation plus losses on the filter paper.
3. The student wants to confirm the identity. Describe two independent tests and the result each should give for a potassium carbonate.
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Test 1: flame test on a clean nichrome wire dipped in the redissolved solid should give a persistent lilac flame, confirming potassium; a cobalt-blue glass viewed through the flame can help filter out sodium yellow if contamination is suspected. Test 2: add dilute acid and pass the gas through limewater; a potassium carbonate should fizz and turn limewater milky, confirming carbonate. A third useful check is to weigh a known mass, react it fully with excess acid, and compare the CO₂ volume or mass loss with the value predicted for K₂CO₃, which also distinguishes it from potassium hydrogen carbonate. Agreement of two independent tests on the same sample supports the identification.