Biology · Simulation and virtual lab Free
Food tests: starch, sugars, protein, fats and vitamin C
Test real Rwandan foods with iodine, Benedict’s, Biuret, ethanol and DCPIP, and find out what an unknown food contains.
- You will learn to
- Carry out food tests for starch, reducing and non-reducing sugars, protein and lipids.
- Use a control and record observations correctly.
- Compare the vitamin C content of juices with DCPIP.
- You will measure
- Colour changes; volume of juice (cm³) to decolourise DCPIP; vitamin C in mg per 100 cm³.
- Time needed
- About 35–45 min
- Equipment
- Nothing extra. Real version: test tubes, iodine, Benedict’s, Biuret reagents, ethanol, DCPIP, water bath, food samples.
- Price
- Free: no payment, no account.
Virtual lab
Follow the steps with the simulation above (go back to the simulation ↑). Each time you tap Record, a row is added to the table and a point to the graph.
Aim
To identify starch, reducing and non-reducing sugars, proteins, lipids and vitamin C in food samples, and to compare the vitamin C content of fruit juices.
What to do
- Choose Maize flour and the iodine test. Add iodine, observe and record. Repeat with distilled water as a control.
- Choose Glucose and the Benedict’s test. Add Benedict’s solution, heat for 5 minutes and record the colour.
- Test table sugar with Benedict’s. Then use the non-reducing sugar test: boil with acid, neutralise, add Benedict’s and heat.
- Test egg white with the Biuret test and cooking oil with the ethanol emulsion test.
- Use the DCPIP test on the vitamin C standard, then on orange, lemon and tomato juice. Record the volume needed each time.
- Do all the tests on Unknown food X and write what it contains.
Results
📏 Read the instruments carefully
| Instrument | Smallest reading | Uncertainty |
|---|
| Dropper / pipette | 1 drop (≈ 0.05 cm³) | ± 1 drop |
| Syringe (DCPIP test) | 0.1 cm³ | ± 0.05 cm³ |
| Water bath thermometer | 1 °C | ± 0.5 °C; keep it near boiling |
| Colour judgement | — | compare with a control tube and a white background |
Write every reading with its unit, and with the same number of decimal places as the instrument. Use the 🔍 Magnifier on the picture to read small scales at eye level.
⚠️ Avoid these mistakes
- Not heating the Benedict’s test: the colour stays blue even if sugar is present.
- Forgetting to neutralise after boiling with acid in the non-reducing sugar test.
- Heating ethanol over a flame: it is flammable. Use a water bath and keep flames away.
✍️ Conclusion
Write your own conclusion first: what do your results and graph show, and which law or principle do they support?
Check your conclusion
Each nutrient gives its own colour change: starch turns iodine blue-black, reducing sugars turn hot Benedict’s solution green to brick red, protein turns Biuret purple, lipids give a milky emulsion, and vitamin C decolourises DCPIP. The smaller the volume of juice needed to decolourise DCPIP, the more vitamin C it contains.
Questions
What is the colour of a positive starch test?
Iodine changes from orange-brown to blue-black.
Why must Benedict’s test be heated?
The reducing sugar only reduces the copper(II) ions in Benedict’s solution when heated in a boiling water bath; without heating the colour stays blue.
Why does sucrose give a negative Benedict’s test at first?
Sucrose is a non-reducing sugar. It must first be hydrolysed (broken down) into glucose and fructose by boiling with acid.
Why must the acid be neutralised before adding Benedict’s solution?
Benedict’s test only works in alkaline conditions.
Why is distilled water tested as well?
It is a control: it shows the colour of a negative result, so we can compare.
Juice A decolourises DCPIP with 0.5 cm³ and juice B with 1.5 cm³. Which has more vitamin C?
Juice A, because a smaller volume contains enough vitamin C to decolourise the same amount of DCPIP.
For teachers: expected results and how the model works
Benedict’s colours: blue (none), green (trace), yellow-orange (moderate), brick red (much reducing sugar). Heating takes 10 s of real time for 5 simulated minutes. Sucrose is negative until acid hydrolysis AND neutralisation. DCPIP: 1 cm³ is decolourised by 0.25 mg of vitamin C (standard 0.10 mg/cm³ needs 2.5 cm³; orange 0.5 cm³ ≈ 50 mg/100 cm³; lemon ≈ 45; tomato ≈ 15). Unknown X is random each time it is chosen.
Quick check 3 questions · scored
Choose one answer for each question. You see at once if it is right, why, and your score at the end.
1. Which reagent tests for starch?
Iodine turns blue-black with starch.
2. After heating with Benedict’s solution a sample turns brick red. What does it contain?
A brick-red precipitate shows a lot of reducing sugar.
3. Juice A decolourises DCPIP with 0.4 cm³, juice B with 1.2 cm³. Which has more vitamin C?
A needs a smaller volume, so it is more concentrated in vitamin C (3 times more).
Your score:
Exam practice Exam set
National-exam style questions on this simulation (13 marks). Try each one on paper first, then open the marking scheme.
Describe how you would test a sample of milk for protein, giving the reagents and the positive result. (3 marks)
Marking scheme: Add sodium hydroxide solution to the milk (1), then a few drops of dilute copper(II) sulfate (Biuret test) (1); a purple/violet colour shows protein (1).
A student tests a food: iodine stays orange-brown; Benedict’s gives brick red; Biuret stays blue; the emulsion test gives a milky layer. State what the food contains and what it does not contain. (4 marks)
Marking scheme: Contains reducing sugar (1) and lipid (1). No starch (1) and no protein (1).
1 cm³ of DCPIP is decolourised by 2.5 cm³ of a vitamin C solution of 0.10 mg/cm³ and by 0.50 cm³ of orange juice. Calculate the vitamin C content of the juice in mg per 100 cm³. (4 marks)
Marking scheme: Vitamin C needed = 2.5 × 0.10 = 0.25 mg (1). Juice: 0.25 mg in 0.50 cm³ (1) = 0.50 mg/cm³ (1) = 50 mg per 100 cm³ (1).
Give two safety precautions when carrying out food tests. (2 marks)
Marking scheme: Wear eye protection / point test tubes away from people when heating (1); use a water bath, not a direct flame, for ethanol; handle sodium hydroxide and acid with care (1).
Donat Sciences and MathsBiology · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
Food tests: starch, sugars, protein, fats and vitamin C
Aim: To identify starch, reducing and non-reducing sugars, proteins, lipids and vitamin C in food samples, and to compare the vitamin C content of fruit juices.
Apparatus (real laboratory): test tubes, iodine, Benedict’s, Biuret reagents, ethanol, DCPIP, water bath, food samples.
What to do (with the simulation on www.donatstem.co.rw)
- Choose Maize flour and the iodine test. Add iodine, observe and record. Repeat with distilled water as a control.
- Choose Glucose and the Benedict’s test. Add Benedict’s solution, heat for 5 minutes and record the colour.
- Test table sugar with Benedict’s. Then use the non-reducing sugar test: boil with acid, neutralise, add Benedict’s and heat.
- Test egg white with the Biuret test and cooking oil with the ethanol emulsion test.
- Use the DCPIP test on the vitamin C standard, then on orange, lemon and tomato juice. Record the volume needed each time.
- Do all the tests on Unknown food X and write what it contains.
Results
| # | Food | Test | Observation | Result (−, +, ++, +++) | Conclusion |
|---|
| 1 | | | | | |
| 2 | | | | | |
| 3 | | | | | |
| 4 | | | | | |
| 5 | | | | | |
| 6 | | | | | |
| 7 | | | | | |
| 8 | | | | | |
| 9 | | | | | |
| 10 | | | | | |
| 11 | | | | | |
| 12 | | | | | |
Graph
No graph: make a table of foods (rows) against nutrients (columns) and fill it with ✓ or ✗.
Questions
- What is the colour of a positive starch test? [2]
- Why must Benedict’s test be heated? [2]
- Why does sucrose give a negative Benedict’s test at first? [2]
- Why must the acid be neutralised before adding Benedict’s solution? [2]
- Why is distilled water tested as well? [2]
- Juice A decolourises DCPIP with 0.5 cm³ and juice B with 1.5 cm³. Which has more vitamin C? [2]
- Describe how you would test a sample of milk for protein, giving the reagents and the positive result. [3]
- A student tests a food: iodine stays orange-brown; Benedict’s gives brick red; Biuret stays blue; the emulsion test gives a milky layer. State what the food contains and what it does not contain. [4]
- 1 cm³ of DCPIP is decolourised by 2.5 cm³ of a vitamin C solution of 0.10 mg/cm³ and by 0.50 cm³ of orange juice. Calculate the vitamin C content of the juice in mg per 100 cm³. [4]
- Give two safety precautions when carrying out food tests. [2]
Donat Sciences and MathsBiology · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
Marking scheme: Food tests: starch, sugars, protein, fats and vitamin C
Expected results
Benedict’s colours: blue (none), green (trace), yellow-orange (moderate), brick red (much reducing sugar). Heating takes 10 s of real time for 5 simulated minutes. Sucrose is negative until acid hydrolysis AND neutralisation. DCPIP: 1 cm³ is decolourised by 0.25 mg of vitamin C (standard 0.10 mg/cm³ needs 2.5 cm³; orange 0.5 cm³ ≈ 50 mg/100 cm³; lemon ≈ 45; tomato ≈ 15). Unknown X is random each time it is chosen.
Answers
- What is the colour of a positive starch test? [2]
Iodine changes from orange-brown to blue-black.
- Why must Benedict’s test be heated? [2]
The reducing sugar only reduces the copper(II) ions in Benedict’s solution when heated in a boiling water bath; without heating the colour stays blue.
- Why does sucrose give a negative Benedict’s test at first? [2]
Sucrose is a non-reducing sugar. It must first be hydrolysed (broken down) into glucose and fructose by boiling with acid.
- Why must the acid be neutralised before adding Benedict’s solution? [2]
Benedict’s test only works in alkaline conditions.
- Why is distilled water tested as well? [2]
It is a control: it shows the colour of a negative result, so we can compare.
- Juice A decolourises DCPIP with 0.5 cm³ and juice B with 1.5 cm³. Which has more vitamin C? [2]
Juice A, because a smaller volume contains enough vitamin C to decolourise the same amount of DCPIP.
- Describe how you would test a sample of milk for protein, giving the reagents and the positive result. [3]
Add sodium hydroxide solution to the milk (1), then a few drops of dilute copper(II) sulfate (Biuret test) (1); a purple/violet colour shows protein (1).
- A student tests a food: iodine stays orange-brown; Benedict’s gives brick red; Biuret stays blue; the emulsion test gives a milky layer. State what the food contains and what it does not contain. [4]
Contains reducing sugar (1) and lipid (1). No starch (1) and no protein (1).
- 1 cm³ of DCPIP is decolourised by 2.5 cm³ of a vitamin C solution of 0.10 mg/cm³ and by 0.50 cm³ of orange juice. Calculate the vitamin C content of the juice in mg per 100 cm³. [4]
Vitamin C needed = 2.5 × 0.10 = 0.25 mg (1). Juice: 0.25 mg in 0.50 cm³ (1) = 0.50 mg/cm³ (1) = 50 mg per 100 cm³ (1).
- Give two safety precautions when carrying out food tests. [2]
Wear eye protection / point test tubes away from people when heating (1); use a water bath, not a direct flame, for ethanol; handle sodium hydroxide and acid with care (1).