Flame Test and Emission Lines in an Unknown Chloride
During a unit on atomic structure, a lab group heats an unknown chloride salt in a Bunsen flame and records a strong yellow-orange color. Their handheld spectrometer logs two strong lines at 589.0 and 589.6 nm and weak lines at 568.2, 616.1, and 670.8 nm. The group concludes the unknown is pure sodium chloride because 'sodium's D lines are the strongest.' The teacher knows the sample was supplied as a single salt, the nichrome loop was cleaned with dilute HCl between trials, and another group's sodium chloride standard shows only the 589.0/589.6 pair. The class has reference spectra for Li, Na, K, Ca, Sr, Ba, and Cu, but no certified detection-limit data for the handheld spectrometer. The teacher must decide whether to accept the conclusion, what follow-up evidence to require, and how to use the discrepancy to teach evidence standards.
1. Is the group's conclusion that the unknown is pure NaCl justified by the evidence? Justify your answer using the line data and the limits of the method.
Read the worked answer
No. The strong 589.0/589.6 nm pair identifies sodium, but it does not establish purity. The weak line at 670.8 nm coincides with a lithium emission line, and the 568.2 and 616.1 nm lines are not explained by sodium. They could come from a trace impurity in the sample, contamination on the nichrome loop, or a second metal. A purity claim requires that no other lines appear above the method's detection limit. The class has no detection-limit data, and flame color alone is subjective. The appropriate conclusion is 'sodium is present; purity is not established.'
2. Design a follow-up procedure that would help distinguish sample contamination from loop contamination. Include at least two controls.
Read the worked answer
Run a flame blank with the cleaned loop and no sample, then a reagent blank of the HCl used for cleaning, under the same flame and spectrometer settings. Repeat the unknown with a new loop or a platinum wire, and run a known NaCl standard spiked with a trace of Li at a level near the weak-line intensity. If the weak lines appear in the blank or disappear with a new loop, contamination is supported; if they persist in the sample with a clean loop and are absent from the NaCl standard, a sample impurity is supported. Use identical flame position and integration time.
3. How should the teacher respond to a student who says 'the strongest lines are the only ones that count'?
Read the worked answer
Explain that line intensity depends on concentration, transition probability, and detector response, so a trace component can give weak lines while the major component gives strong lines. A purity decision is not 'strongest line wins'; it is a comparison of all peaks against a calibrated detection threshold. Ask students to state the threshold and to design a blank. This turns the anomaly into a lesson on evidence standards rather than a hunt for the 'right color.'