Lesson focus: Food tests use controlled chemical observations to identify nutrient groups, but careful planning is needed for valid comparisons.
Plan safe tests for starch, reducing sugarA type of sugar, such as glucose, that turns Benedict's solution from blue to orange or brick red when heated., protein and lipids, including control experiments and comparable samples.
Build on earlier learning
NutrientsThe previous lesson linked nutrients to functions. Food tests now provide chemical evidence for some of those nutrients.
Practical
Planning food tests
- Starch: add iodine solution; blue-black shows starch.
- Reducing sugar: add Benedict’s solution and heat in a water bath; green to brick-red shows sugar.
- Protein: add Biuret solution; lilac shows protein.
- Use the same volume of food sample in each test and include a control experiment with water instead of food.
Record: A results table with a positive and negative result for each test.
Safety: Wear eye protection; use a water bath, not a flame, to heat Benedict’s tests.
You will be successful when you can…
- Select the correct reagent or method for each nutrient.
- Describe positive and negative observations.
- Include positive and negative control experiments.
- Plan comparable quantities and safe heating.
Words that carry the science
- reagent
- A chemical used to produce an observable change in a test.
- positive control
- A sample known to contain the target substance.
- negative control
- A sample known not to contain the target substance.
- reducing sugar
- A sugar that reduces Benedict’s reagent when heated, including glucose.
- emulsion
- Tiny droplets of one liquid dispersed through another, making a cloudy mixture.
- qualitative
- Describing a type or category rather than an exact amount.
Plan four tests as controlled comparisons
01Each test has a specific observation
Iodine solution changes from orange-brown to blue-black with starchA carbohydrate made of many glucose molecules joined together, which plants use to store energy; it turns iodine solution blue-black.. Benedict’s reagentA chemical used in a test to show whether a substance is there, usually by changing colour. changes from blue toward green, yellow, orange or brick-red with increasing reducing sugarA type of sugar, such as glucose, that turns Benedict's solution from blue to orange or brick red when heated. after heating in a water bath.
Biuret reagent changes from blue to lilac or purple with proteinA nutrient made of chains of amino acids, used for growth and repair of cells; found in meat, fish, eggs and beans.. In the ethanol emulsionA cloudy mixture of tiny drops of one liquid spread through another, like milk. test, lipid produces a cloudy white emulsion after ethanol extract is mixed with water.
02Control experiments show whether the test worked
A known positive sample demonstrates the expected observation. A negative controlA test sample you know does not contain the substance, used to check what a negative result looks like. experiment, such as distilled water, shows the baseline and can reveal contaminated reagents.
Food samples should be ground with equal volumes of water and filtered if necessary. Equal sample mass, reagent volume, heating time and temperature make comparisons more meaningful.
03Safety belongs inside the method
Use a hot-water bath for Benedict’s test rather than heating a test tube directly. Wear eye protection and use holders for hot tubes. Ethanol is flammable, so keep it away from flames and use small quantities with ventilation.
Do not taste laboratory food samples. Treat unknown materials and reagents as laboratory substances, even if the original food is edible.
Plan a starch test with control experiments
Plan how to test bread suspension for starch.
- Place equal volumes of bread suspension, starch solution and distilled water into separate wells.
- Add the same number of iodine drops to each sample.
- Record the colour immediately against a white background.
- Expect starch solution to turn blue-black and distilled water to remain orange-brown before interpreting the bread result.
How to make the reasoning visible: Control experiments verify the reagent response and provide a baseline for judging the unknown sample.
Complete the planning matrix
The observations below use freshly prepared samples and suitable control experiments.
| Target | Method | Positive observation | Key control experiment or safety action |
|---|---|---|---|
| Starch | Add iodine solution | Blue-black | Control experiments: known starch and water |
| Reducing sugar | Add Benedict’s and heat in water bath | Blue toward green/yellow/orange/brick-red | Equal heating; eye protection |
| Protein | Add Biuret reagent | Lilac or purple | Control experiments: known protein and water |
| Lipid | Ethanol extract, then add water | Cloudy white emulsion | No flames; use ethanol safely |
Why should each food suspension use the same mass of food and volume of water?
Why are both positive and negative control experiments useful?
Check your understanding
Answer from memory, then check against the suggested answer.
Which reagent tests for starch?
Which food test requires heating?
What indicates lipid in an ethanol emulsion test?
A student heats Benedict’s reagent over a naked flame. Identify the problem and improve the method.
Explain why colour charts can support only an approximate comparison of reducing sugar.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student wants to find out whether an unknown food contains starch, reducing sugar and protein.
Describe how to prepare the food sample.
(1)Describe the three tests and the positive result for each.
(3)Describe one way to make the results trustworthy and one safety precaution.
(2)Total for this question: 6 marks
Command word: For planA description of what you will do in an investigation and why., write a reproducible method with quantities kept comparable, not just reagent names.