Free Chemical Tests for Biomolecules OCR A Level Biology revision notes – covering specification points 2.1.2(q) and 2.1.2(r).
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Chemical Tests for Biomolecules
Biochemical tests can either be qualitative or quantitative.
Qualitative tests only give a positive or negative response, allowing you to identify a substance.
Quantitative tests allow you to determine the concentration of a substance in a solution.
Qualitative Tests
The table below provides an overview of the different qualitative tests for identifying biological molecules:
| Chemical | Reagent(s) Used | Positive Result | Negative Result |
|---|---|---|---|
| Biuret Test (Proteins) | Biuret reagent (or NaOH → CuSO₄) | Lilac | Stays blue |
| Benedict’s Test (Reducing Sugars) | Benedict’s reagent (+ 80 °C heat) | Green → Yellow → Orange → Brick-red | Stays blue |
| Benedict’s Test (Non-Reducing Sugars) | HCl (hydrolysis) + NaHCO₃ → Benedict’s reagent (+ 80 °C heat) | Green → Yellow → Orange → Brick-red | Stays blue |
| Iodine Test (Starch) | Iodine solution | Blue-black colour | Yellow-brown (no starch) |
| Emulsion Test (Lipids) | Ethanol → Water | Cloudy white emulsion | Solution remains clear |