Edexcel iGCSE Biology-2.9 practical: food samples- Study Notes- New Syllabus
Edexcel iGCSE Biology-2.9 practical: food samples- Study Notes- New syllabus
Edexcel iGCSE Biology-2.9 practical: food samples- Study Notes -Edexcel iGCSE Biology – per latest Syllabus.
Key Concepts:
2.9 practical: investigate food samples for the presence of glucose, starch, protein and fat
Practical: Investigate Food Samples for Glucose, Starch, Protein and Fat
📝 Aim
Test different food samples to find whether they contain reducing sugar (glucose), starch, protein and fat.
🔬 Materials (per group)
- Food samples (glucose solution, bread, potato, milk, egg white, oil, cheese)
- Test tubes & rack, droppers/pipettes, distilled water
- Benedict’s solution (reducing sugars), Iodine solution (starch), Biuret reagent (protein), Sudan III or ethanol (lipids)
- Water bath / Bunsen with tripod + gauze, test tube holders, spotting tiles/white tile
- Labels, forceps, safety goggles, gloves, lab coat, waste container
⚖️ Controls & Preparation
- Positive controls: glucose (sugar), starch paste, egg white (protein), vegetable oil (fat).
- Negative control: distilled water.
- Label all test tubes clearly before running tests.
1) Test for Reducing Sugar (Benedict’s test) 
Method:
Place 2 cm³ of sample in tube.
Add 2 cm³ Benedict’s solution.
Heat in water bath for 2–5 minutes.
Observe colour change.
Positive result: Blue → green → yellow → orange → brick red.
Negative: Remains blue.
2) Test for Starch (Iodine test) 
Method:
Add 1–2 drops iodine to sample on spotting tile/test tube.
Observe colour change.
Positive result: Yellow/brown → blue-black.
Negative: Stays yellow/brown.
3) Test for Protein (Biuret test)
Method:
Add 2 cm³ sample solution.
Add 2 cm³ Biuret reagent. Mix.
Positive result: Pale blue → lilac/purple.
Negative: Remains blue.
4) Test for Lipids (Fats & Oils) 
A – Sudan III stain:
- Add 2 cm³ sample. Add few drops Sudan III. Shake.
- Positive: red-stained oil layer separates.
- Negative: no red layer.
B – Grease spot / Ethanol test:
- Rub sample on filter paper, let dry.
- Positive: permanent translucent spot.
- Negative: no grease spot.
✅ Expected Results Table
| Sample | Benedict (glucose) | Iodine (starch) | Biuret (protein) | Sudan/Grease (lipid) |
|---|---|---|---|---|
| Glucose solution | Brick red | – | – | – |
| Bread / potato | Orange/green | Blue-black | – (trace) | – |
| Milk | Green/orange | – | Lilac/purple | Small oil layer |
| Egg white | – | – | Purple | – |
| Vegetable oil | – | – | – | Red-stained layer |
| Distilled water | Blue (no change) | Yellow/brown | Blue | No grease spot |
⚠️ Safety & Practical Tips
- Wear safety goggles, gloves and lab coat.
- Handle hot test tubes with a holder.
- Benedict’s & Biuret contain metals → avoid skin contact.
- Iodine stains clothes/skin → handle carefully.
- Dispose of chemical waste properly; avoid pouring reagents into sink.
🔎 Common Sources of Error & Fixes
- Not heating Benedict’s properly → false negative. Heat full 5 mins.
- Contaminated pipettes → false positives. Use clean equipment.
- Too dilute sample → faint colour. Use more sample/concentrated solution.
- Coloured foods (e.g., beetroot) interfere. Dilute or use controls.
- Mixing up reagents. Label everything clearly.
🧠 Recording & Reporting Results
- Draw a results table.
- Record colours + whether positive/negative.
- State test type: qualitative (presence/absence).
- Write conclusion, e.g., “Sample X contains starch and protein but no fat or reducing sugar.”
⚡ Quick Recap
Benedict’s + heat → reducing sugar → brick red.
Iodine → starch → blue-black.
Biuret → protein → lilac/purple.
Sudan III / grease spot → lipid → red layer or translucent spot.