In 1909 the American chemist Stanley Benedict published a recipe for a blue solution that changes colour when heated with certain sugars. Doctors used it to test urine for sugar, a sign of diabetes. Benedict's solution is still the test you use for sugar in GCSE Biology, alongside three other reagents.
This lesson covers the food tests required practical for AQA GCSE Biology Topic 2 (Organisation). It's also useful for Combined Science.
What the practical asks you to do
Use qualitative reagents to test for a range of carbohydrates, lipids and proteins. Qualitative means the test shows whether something is present, not exactly how much.
Preparing the food
- Grind a small amount of food with a pestle and mortar.
- Add distilled water and stir to mix.
- Filter the mixture to get a clear solution to test. For the lipid test, use a small piece of the food itself.
Test distilled water in the same way as a control, so you know what a negative result looks like.
The four tests
| Nutrient | Reagent | Starting colour | Positive result |
|---|---|---|---|
| Reducing sugars (e.g. glucose) | Benedict's, heated in a water bath | Blue | Green, yellow, orange or brick red |
| Starch | Iodine solution | Orange-brown | Blue-black |
| Protein | Biuret solution | Blue | Purple |
| Lipids | Ethanol, then water (emulsion test) | Clear | Cloudy white emulsion |
Benedict's test for sugars
Add Benedict's solution to the food solution and heat the tube in a water bath at 80°C or more for about 5 minutes. The more reducing sugar there is, the further the colour moves along: blue, green, yellow, orange, brick red.
Use a water bath rather than heating the tube directly. It heats evenly and at a controlled temperature, so the contents don't boil over or spit out.
Not every sugar gives a positive result. Glucose does. Sucrose, ordinary table sugar, does not.
Iodine test for starch
Add a few drops of iodine solution. If starch is present, the orange-brown colour turns blue-black. This test works at room temperature.
Biuret test for protein
Add Biuret A, then Biuret B, to the food solution and gently shake. If protein is present, the blue solution turns purple. Biuret contains sodium hydroxide, an irritant at school strength (corrosive when concentrated), so wear eye protection and wash off any spills.
Emulsion test for lipids
Shake a small amount of the food with ethanol, then pour the ethanol into a tube of water. If lipids are present, a cloudy white emulsion forms. Ethanol is highly flammable, so keep it well away from any flame, including the one heating the water bath.
Some schools use Sudan III instead. It stains lipids red, and a red oily layer forms on top.
Common exam mistakes
- Heating the wrong test. Only Benedict's needs heating.
- "Benedict's turns red with starch." Benedict's tests for reducing sugars. Iodine tests for starch.
- Mixing up the colours. Iodine goes blue-black; Biuret goes purple.
- Giving amounts. These are qualitative tests. Benedict's colour gives only a rough idea of how much sugar there is.
- Forgetting the control. Without distilled water to compare, a pale colour change is hard to judge.
Frequently asked questions
What colour does Benedict's turn with sugar?
It turns from blue to green, yellow, orange or brick red when heated, depending on how much reducing sugar there is.
What is the test for starch?
Add iodine solution. It turns from orange-brown to blue-black if starch is present.
What is the test for protein?
Add Biuret solution. It turns from blue to purple if protein is present.
How do you test for lipids?
Shake the food with ethanol, then pour it into water. A cloudy white emulsion means lipids are present. Sudan III, which stains lipids red, is an alternative.
Try it
Say each answer before you flip the card. Then put yourself in Benedict's place in 1909: your solution shows sugar, but not which sugar. What would you have tested next?
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