Module 2: Chemical Tests for Biomolecules
These free OCR A Level Biology Chemical Tests for Biomolecules revision notes have been written for specification points 2.1.2(q) and 2.1.2(r).
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 |