- Define and explain Flame Tests and Precipitates in your own words
- Use key terms such as fragmentation accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Flame Tests and Precipitates: use flame colours and precipitation to identify metal ions.
Use flame colours and precipitation to identify metal ions.
Key ideas
Classic tests give fast answers
A glowing splint relights in oxygen, a burning splint pops in hydrogen, and limewater turns milky with carbon dioxide — three gas tests every chemist knows. For ions, flame tests colour the flame (lilac for potassium, brick red for calcium), while adding sodium hydroxide gives coloured precipitates with transition metal ions. These tests are quick, cheap and specific.
IR and NMR see inside molecules
Infrared spectroscopy detects bond vibrations: an O–H stretch appears as a broad peak around 3300 cm⁻¹ and a C=O stretch as a sharp peak near 1700 cm⁻¹. Proton NMR goes further, showing how many hydrogen environments a molecule has, how many hydrogens are in each (integration), and how they split each other (the n+1 rule). Together they can identify an unknown organic compound completely.
Key term — fragmentation: The breaking of a molecular ion into smaller ions, giving a fingerprint pattern of peaks.
A gas relights a glowing splint. Identify the gas.
Oxygen.
Answer: Oxygen.
- Confusing the fingerprint region with functional-group peaks Useful functional-group absorptions appear above 1500 cm⁻¹; the fingerprint region below 1500 cm⁻¹ identifies a molecule by matching its whole pattern, not single peaks.
- Reading the tallest peak as the molecular ion The tallest peak is the base peak, not necessarily the molecular ion. The molecular ion is the peak with the highest m/z (ignoring tiny isotope peaks just above it).
Practice
Chloride — silver chloride is white and dissolves in dilute ammonia (bromide needs concentrated ammonia).
C₃H₆O₂: (3 × 12) + (6 × 1) + (2 × 16) = 36 + 6 + 32 = 74 (e.g. propanoic acid or methyl ethanoate).
A carboxylic acid — the broad O–H stretch and the C=O stretch together are characteristic.
Three distinct environments: CH₃, CH₂ and OH. The CH₃ signal is split into a triplet by the two neighbouring CH₂ hydrogens (n+1 rule).
Quick check
Which of these best defines "fragmentation"?
Distinguish between propan-1-ol and propan-2-one using a chemical test.
- Flame Tests and Precipitates: use flame colours and precipitation to identify metal ions.
- Classic tests give fast answers: A glowing splint relights in oxygen, a burning splint pops in hydrogen, and limewater turns milky with carbon dioxide — three gas tests every chemist knows.
- molecular ion: The ion formed when a molecule loses one electron in a mass spectrometer; its m/z gives the relative molecular mass.
- Watch out for: confusing the fingerprint region with functional-group peaks