- Define and explain Chemical Tests 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
How do chemists prove what a substance is? Sometimes with a flame colour or a fizzing test tube; sometimes with machines that weigh molecules or listen to atomic nuclei. Analytical chemistry is detective work with evidence. In this chapter you will run the classic tests and read mass, infrared and NMR spectra.
This lesson focuses on Chemical Tests: identify gases and ions with classic test-tube tests.
Identify gases and ions with classic test-tube tests.
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.
Distinguish between propan-1-ol and propan-2-one using a chemical test.
Warm with acidified potassium dichromate: propan-1-ol (a primary alcohol) is oxidised and the orange solution turns green; propan-2-one (a ketone) does not react and the solution stays orange.
Answer: Warm with acidified potassium dichromate: propan-1-ol (a primary alcohol) is oxidised and the orange solution turns green; propan-2-one (a ketone) does not react and the solution stays orange.
- 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.
- Forgetting that NMR needs non-equivalent environments Chemically equivalent hydrogens give one shared signal — the three hydrogens of a CH₃ group appear as a single peak (split by neighbours), not three separate peaks.
Practice
Oxygen.
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.
Quick check
Which of these best defines "fragmentation"?
The ¹H NMR spectrum of ethanol (CH₃CH₂OH) shows three signals. Explain why, and state the splitting of the CH₃ signal.
- Chemical Tests: identify gases and ions with classic test-tube tests.
- 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.
- chemical shift: The position of a peak in an NMR spectrum, in ppm, showing the environment of the nuclei.
- Watch out for: confusing the fingerprint region with functional-group peaks