The Food Test Lab
Four reagents, four colour changes. Learn the chemical tests that reveal starch, sugar, protein and fat in your food — and never mix up which colour means what.
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The Food Test Lab 🍽️
What is really in your lunch? With four simple chemical tests you can reveal whether a food contains **starch, sugar, protein or fat** — each shown by a distinct **colour change**. The trick is remembering which reagent goes with which colour.
Testing for carbohydrates 🧪
Two tests for carbohydrates: • **Starch** — add orange-brown **iodine solution**. If starch is present, it turns **blue-black**. • **Reducing sugars** (such as glucose) — add blue **Benedict's solution** and **heat** it in a water bath. If present, it turns **brick-red** (through green, yellow and orange for smaller amounts).
Testing for protein and fat 🥚
Two more tests: • **Protein** — add **Biuret reagent**. If protein is present, the blue reagent turns **purple (lilac)**. • **Fat (lipid)** — the **ethanol emulsion test**: dissolve the sample in ethanol, then add water. If fat is present, a **cloudy white emulsion** forms.
Match each nutrient to its positive result
- Starch
- Reducing sugar
- Protein
- Fat
- Iodine solution turns blue-black
- Benedict's solution turns brick-red on heating
- Biuret reagent turns purple
- An ethanol emulsion turns cloudy white
Which test needs heat?
Which food test requires you to HEAT the sample in a water bath?
- The Benedict's test for reducing sugars
- The iodine test for starch
- The Biuret test for protein
- The ethanol emulsion test for fat
No colour change
You add iodine solution to a food sample and it stays orange-brown. What does this tell you?
- There is no starch in the sample
- The sample contains a lot of starch
- The sample contains protein
- The sample contains reducing sugar
Reagent and colour
Select ALL THREE reagent-and-result pairings that are CORRECT.
- Iodine → blue-black shows starch
- Biuret → purple shows protein
- Benedict's → brick-red shows reducing sugar
- Iodine → purple shows protein
- Biuret → brick-red shows starch
- Benedict's → blue-black shows fat
The Benedict's test method
An interactive activity.
Reading the results 🔬
A **colour change** is a **positive** result — the nutrient is present. **No change** is a **negative** result — it is absent. Test one nutrient at a time on a fresh portion of sample. Stay safe: the Benedict's test needs a hot water bath, and **ethanol is flammable**, so keep it well away from any flames.
Food test summary
To test for starch, add _____ solution — it turns blue-black. To test for reducing sugars, add Benedict's solution and _____ it, turning brick-red. _____ reagent turns purple with protein. The ethanol _____ test gives a cloudy white result with fats.