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Later-course unknowns can require carbonyl chemistry, phenols, amines, carboxylic-acid derivatives and transition-metal evidence. Choose tests according to the supplied candidates and use independent information such as molecular formula, infrared or NMR data when provided. A molecular-ion mass alone does not uniquely reveal a formula or structure.
Video by Royal Society of Chemistry. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Combine qualitative and spectroscopic evidence for later-course unknowns
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Which conclusion is justified by 2,4-DNPH alone?
An aldehyde or ketone carbonyl is supported in the suitable candidate set. Use additional selective and structural evidence.
After watching 2: What does dissolving a copper hydroxide precipitate in excess ammonia normally support?
Formation of a soluble ammine complex. Classify the ligand process separately from redox.
What you'll be able to do
Later-course unknowns can require carbonyl chemistry, phenols, amines, carboxylic-acid derivatives and transition-metal evidence. Choose tests according to the supplied candidates and use independent information such as molecular formula, infrared or NMR data when provided. A molecular-ion mass alone does not uniquely reveal a formula or structure.
A carbonyl derivative with 2,4-DNPH supports an aldehyde or ketone; a selective aldehyde oxidation test can narrow that distinction. Phenol may decolourise bromine water with a white precipitate and give an iron(III) chloride colour. These overlapping responses show why one test cannot be assigned a unique meaning without context. Work on fresh small portions using prepared reagents and approved conditions.
Record small-addition and excess-reagent stages separately. Copper(II) can form a blue hydroxide precipitate then a deep-blue solution with excess ammonia. Aluminium hydroxide is white and dissolves in excess hydroxide under the usual test conditions, while iron(III) gives a brown precipitate. A green iron(II) precipitate can brown on standing through oxidation, so elapsed time is part of the observation.
Distinguish proton-transfer precipitation, ligand substitution and redox change. A colour change is not automatically a change in oxidation state. A sample mixture can show more than one process or hide a precipitate; use reference comparisons and report uncertainty rather than forcing a single-ion conclusion where the candidate set allows mixtures.
Use a table linking each observation to supported and excluded candidates. If a supplied infrared spectrum has a strong carbonyl absorption but no broad O–H feature, combine that with chemical tests and the given formula. A negative aldehyde test supports a ketone only if the reagent and conditions were demonstrably suitable. A control prevents a failed test from masquerading as structural evidence.
If a new reliable observation contradicts the proposed identity, revise the hypothesis or investigate contamination; do not silently discard the result. Compare purity and identity separately. More than one chromatographic component can explain a mixed test pattern, and one matching retention or Rf value still needs corroboration. Keep source spectra distinct from observations made on the laboratory sample.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: A blue precipitate dissolves in excess ammonia to a deep-blue solution. Does the final colour prove reduction to copper metal?
No. It supports ligand substitution, not reduction to copper metal.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.