Launch practical
About 60 minutes
- Set up the bench with the 13 items of apparatus.
- Take the readings yourself, into the results table you draw first.
- Process your results.
- Answer exam questions on your own results, which you mark against the mark scheme.
Runs in your browser: a 3D bench with 2D close-ups of each instrument for exact readings. Nothing to install and no account.
What you measure, and how
One run takes 25 readings. You take every one yourself, the way you would in class.
- On the 3D bench (Fig. 1) you set out the apparatus and read each instrument where it stands.
- In the 2D close-up (Fig. 2) the same instrument is drawn large, from the same run, for an exact reading. It is also how the practical runs on a device that cannot show 3D.
- In your table you write what you read, to the instrument’s resolution, and the notebook checks it against the instrument.
How to read it. Biuret reagent is blue.
What it shows
Reducing sugar + Benedict’s (blue) → brick-red precipitate, when heated
Each reagent detects one kind of nutrient. Iodine solution turns blue-black with starch. Biuret reagent turns lilac with protein. Sudan III stains lipid red, and the stained oil floats as a layer. Benedict’s turns from blue through green, yellow and orange to brick red with reducing sugars, but only when heated: more sugar, further along.
Key equation for the exam: No equation: learn each test and its positive result..
No equation to learn. Benedict’s needs heating; iodine, Biuret and Sudan III do not.
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| Safety goggles | Protect your eyes from the reagents and hot liquid. | — |
| Three foods to test | The class’s foods, labelled A, B and C. | — |
| Pestle and mortar, distilled water and a stirring rod | Grind each solid food and stir it with water, so some of it dissolves. | — |
| Filter funnel, filter paper and small beakers | Filter the ground food to get as clear a solution as possible. | — |
| Known solutions: starch, egg albumin, cooking oil, glucose standards (0.1–1.0 %) and sucrose | Positive controls, to see what each positive result looks like, and the glucose colour series. | — |
| Iodine solution, biuret reagent, Sudan III stain and Benedict’s solution, in dropping bottles | One reagent for each nutrient. | — |
| Teat pipettes, labelled | Move each solution and reagent without mixing them. | — |
| Spotting tile | Small wells for the iodine test. | — |
| Test tubes in a rack | For the biuret and Sudan III tests. | — |
| Test tubes in a second rack, labelled | For the Benedict’s tests. | — |
| Water bath: a 1 dm³ beaker of water on a tripod and gauze, with a thermometer | Heats every Benedict’s tube to the same temperature, about 80 °C; the thermometer checks it. | 1 °C |
| Bunsen burner, tripod, gauze, heatproof mat and test-tube holder | Heat the water bath; the holder lifts hot tubes. | — |
| White tile | Held behind the tubes so their colours are easy to judge. | — |
Method
In our own words, as the Lab runs it. Your school's method sheet may differ in detail.
Which version are you doing?
Boards set this practical slightly differently. Your course setting picks one; you can change it before you start.
Why: Most of the bench is the same; the samples, the tests and the exam questions follow your board.
Make a solution of each solid food
Grind a small piece of the food with a pestle and mortar. Add about 20 cm³ of distilled water and stir, so some of the food dissolves. Milk and juice are used as they are.
Do: Filter it through filter paper to get a clear solution (AQA’s method).
Why: The tests work on a solution. A clear solution lets you see small colour changes.
Make the bench safeSafety
Before you light anything: put your goggles on, and stand the Bunsen on a heatproof mat, well away from the lipid test (it uses ethanol).
Why: Biuret reagent contains sodium hydroxide, which damages eyes. Ethanol is highly flammable. Hot water scalds.
Choose how to heat the Benedict’s tubes
Benedict’s test needs heat. Heat about 700 cm³ of water in a large beaker on a tripod and gauze to about 80 °C. Check it with a thermometer standing in it.
Do: A water bath at about 80 °C.
Why: Benedict’s solution only reacts with reducing sugars when it is hot (AQA accepts any temperature of 65 °C or more). A water bath heats every tube to the same temperature, safely.
Take the readings
Controls first, then foods A, B and C: iodine on a spotting tile, biuret, Sudan III, then Benedict’s in the water bath with glucose standards of 0, 0.1, 0.25, 0.5 and 1.0 % and a sucrose solution.
Repeats: Each test is done once: the results are colours, not measurements, so there is no mean. Repeat any test that gave an unclear or unexpected result with a fresh sample and a clean pipette, and compare it with the controls beside it.
Make safe and clear upSafety
Turn off the Bunsen at the gas tap. The tripod and gauze stay hot for a while. Lift the hot tubes out with a test-tube holder and leave them in the rack to cool. Pour the solutions down the sink with plenty of water. Rinse the spotting tile, test tubes and pipettes. Wipe the bench and wash your hands.
While you take readings
Pipettes for the solutions
Use a teat pipette to put each solution into its well or tube.
Do: A clean, labelled pipette for each solution.
Why: A pipette that has been in one solution carries a little of it into the next.
How much reagent to add
Each test uses a set amount of reagent: 2 drops of iodine, an equal volume (2 cm³) of biuret or Benedict’s, 3 drops of Sudan III. The emulsion test uses 2 cm³ of ethanol.
Do: The amounts in the method.
Why: Too much coloured reagent hides a small colour change; too little gives a colour too pale to see.
Judge each colour against a white tile
Hold a white tile behind the tubes, and look at them side by side.
Why: A pale colour is hard to judge against the dark bench. White behind the tube shows its true colour.
Let the Sudan III tubes stand
Shake each tube gently, then leave it in the rack for a minute before you look.
Why: Stained fat is less dense than water. Given time, it floats up and collects as a red layer on top.
How long to leave the tubes in the water bath
Stand the Benedict’s tubes in the water bath, then look at the colours.
Do: 5 minutes.
Why: The tubes take a little while to heat up, and the colour keeps changing for several minutes.
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| Biuret reagent (contains sodium hydroxide, corrosive, and copper sulfate) | Burns the skin and can damage the eyes; harmful if swallowed | Wear goggles; add it with a dropping pipette; wash any splash off at once with plenty of water. |
| Hot water bath (80 °C), Bunsen flame, hot tripod and gauze | Scalds and burns | Heat the tubes in the water bath, not straight in the flame; lift hot tubes out with a test-tube holder; keep the Bunsen on a heatproof mat; leave the tripod and gauze to cool before moving them. |
| Hot Benedict’s solution spitting out of the tube | Scalds to the face and damage to the eyes | Heat the tubes in a water bath rather than in the flame; wear eye protection; point tubes away from people. |
| Sudan III stain (dissolved in ethanol, highly flammable) | Catches fire near a flame; stains skin and clothes | Do the Sudan III test away from the Bunsen; keep the bottle closed; wash off any splash. |
| Iodine solution (low hazard at this concentration) | Irritates the eyes and stains the skin | Wear goggles; add it from a dropping bottle. |
| Foods (allergens such as milk; nuts are not used) | An allergic reaction | Never taste anything in the lab; check allergies before the lesson; wash your hands after. |
Variables
- Independent: you change
- Concentration of glucose (the standards) in %
- Dependent: you measure
- Colour of the Benedict’s solution after heating
- Control: you keep the same
- Volume of solution tested in cm³. 2 cm³ in each tube, measured with a pipette.
- Volume of Benedict’s solution in cm³. 2 cm³ added to every tube.
- Temperature of the water bath in °C. About 80 °C, checked with the thermometer.
- Time in the water bath in minutes. 5 minutes for every tube.
Results from a specimen run
| Solution | Colour with iodine solution | Colour with biuret reagent | Sudan III result |
|---|---|---|---|
| Positive controls: starch; egg albumin; oil | Turns blue-black | Turns lilac or purple | A red-stained oily layer floats on top |
| Distilled water (control) | Stays orange-brown | Stays blue | No oily layer: the stain stays mixed in |
| Cheese | Stays orange-brown | Turns lilac or purple | A red-stained oily layer floats on top |
| Honey | Stays orange-brown | Stays blue | No oily layer: the stain stays mixed in |
| Potato | Turns blue-black | Stays blue | No oily layer: the stain stays mixed in |
| Reading | Temperature of the water bath in °C |
|---|---|
| Before the Benedict’s tubes go in | 81 |
| Solution | Concentration of glucose in % | Colour after heating |
|---|---|---|
| Distilled water (control) | 0.00 | Stays blue |
| Glucose solution | 0.10 | Green |
| Glucose solution | 0.25 | Yellow |
| Glucose solution | 0.50 | Orange |
| Glucose solution | 1.00 | Brick-red precipitate |
| 1 % sucrose solution | 0.00 | Stays blue |
| Food | Colour after heating |
|---|---|
| Cheese | Stays blue |
| Honey | Brick-red precipitate |
| Potato | Green |
Processing the data
- Which nutrient tests gave a positive result?
- 12 of 12 match a careful test
- How much reducing sugar? (semi-quantitative)
- About 0.1 %
- Why sucrose stays blue
- Sucrose is a non-reducing sugar, and Benedict’s only reacts with reducing sugars.
- Why the water controls
- It shows what a negative result looks like, to compare every other tube with.
| Quantity | Specimen run | Accepted |
|---|---|---|
| Water bath temperature | 81 °C | 80 °C |
Common mistakes and the fix
Mistakes to check in this practical, and what to do instead. In a write-up or an exam answer, the mark goes to naming the step and saying what it does to the readings.
Method: Use it straight from the mortar, cloudy with bits of food
Bits of food make the solution cloudy, so a faint lilac in the biuret test is hidden: a food with a little protein can look as if it has none.
The fix: Make a solution of each solid food. The tests work on a solution. A clear solution lets you see small colour changes.
Method: Leave it as it is
No effect on your results, but a risk of eye damage, burns and fire: flagged in the safety review.
The fix: Make the bench safe. Biuret reagent contains sodium hydroxide, which damages eyes. Ethanol is highly flammable. Hot water scalds.
Method: Hold each tube straight in the Bunsen flame
Each tube boils for a different time, so the colours cannot be compared fairly. Boiling Benedict’s can spit out of the tube into your face or eyes (a safety risk AQA names).
The fix: Choose how to heat the Benedict’s tubes. Benedict’s solution only reacts with reducing sugars when it is hot (AQA accepts any temperature of 65 °C or more). A water bath heats every tube to the same temperature, safely.
Iodine: starch solution: Recorded “Stays orange-brown”
The fix: Look again: the well has turned a very dark blue-black.
Iodine: starch solution: Recorded “Turns purple”
The fix: Purple is the biuret result for protein. Iodine turns blue-black with starch.
Exam answer: Leaving out: Biuret (reagent) (allow copper sulfate and sodium hydroxide)
The fix: Name the reagent: biuret reagent.
Exam answer: Leaving out: (From blue) to lilac / purple / mauve (allow pink)
The fix: Give the colour change: from blue to lilac (purple). Brick red is the Benedict’s result; blue-black is iodine’s.
Worked exam-style questions
Written for Study Hack in the style of exam questions, on the specimen run above. They are not taken from any exam board's papers. Try each one before you open the worked answer.
- Question 1Test for protein · Knowledge[2 marks]
A student wants to find out if a food contains protein.
Name the reagent the student should use. Give the colour change if protein is present.
Worked answer and how the marks are given
A full-mark answer
Biuret reagent. It turns from blue to lilac (purple).
Mark scheme
- Biuret (reagent) (allow copper sulfate and sodium hydroxide)
- (From blue) to lilac / purple / mauve (allow pink) Do not accept “from lilac / purple / mauve / violet / pink to / into blue”. Only with the mark for: Biuret (reagent) (allow copper sulfate and sodium hydroxide).
Where marks are usually lost
- Missing: Biuret (reagent) (allow copper sulfate and sodium hydroxide). Name the reagent: biuret reagent.
- Missing: (From blue) to lilac / purple / mauve (allow pink). Give the colour change: from blue to lilac (purple). Brick red is the Benedict’s result; blue-black is iodine’s.
- Question 2Making the standards · Applying knowledge[2 marks]
A technician made 25 cm³ of 0.2 % glucose solution by diluting a 1.0 % glucose solution with distilled water.
Calculate the volume of 1.0 % glucose solution the technician used.
Volume = cm³
Worked answer and how the marks are given
A full-mark answer
volume = 0.2 ÷ 1.0 × 25 = 5 cm³
Mark scheme
- 0.2 ÷ 1.0 (= 0.2) × 25 (method)
- 5 cm³
- Allow answers from 4.950 to 5.050 cm³ (the marker's tolerance; correct rounding of the student's own working is also accepted).
- Allow the unit written as cm3, ml.
- A correct answer with no working scores full marks.
Where marks are usually lost
- Answer 20 cm³ (gains 1 mark). That is the volume of water to add. The question asks for the volume of 1.0 % glucose: 5 cm³.
- Answer 125 cm³ (no marks). Upside down: the stock is stronger, so you need less of it than the total. 0.2 ÷ 1.0 × 25.
- Missing: 0.2 ÷ 1.0 (= 0.2) × 25. The diluted solution is a fraction of the stock’s concentration: wanted ÷ stock.
- Question 3Planning food tests · Knowledge[6 marks]
A student wants to find out which nutrients are in a sample of milk.
Describe how the student could test the milk for protein, starch and sugar. Give the result of each test if the nutrient is present.
Worked answer and how the marks are given
A full-mark answer
Protein:
1. Put 2 cm³ of milk in a test tube.
2. Add 2 cm³ of biuret reagent and shake gently.
3. If it turns from blue to lilac (purple), the milk contains protein.
Starch:
4. Put a separate sample of milk on a spotting tile and add a few drops of iodine solution.
5. If it turns from orange-brown to blue-black, starch is present.
Sugar:
6. Put 2 cm³ of milk in a test tube and add 2 cm³ of Benedict’s solution.
7. Heat it in a water bath at about 80 °C for 5 minutes.
8. If the blue changes to green, yellow, orange or brick red, sugar is present.
Compare each tube with a water control against a white tile.Mark scheme
Indicative content:
- Add biuret reagent (allow copper sulfate and sodium hydroxide)
- Biuret turns from blue to lilac / purple if protein is present
- Add iodine solution (to a sample)
- Iodine turns from orange-brown to blue-black if starch is present
- Add Benedict’s solution (to a separate sample)
- Heat with Benedict’s (in a water bath at about 80 °C, at least 65 °C) for a few minutes
- Benedict’s turns from blue to green / yellow / orange / brick red if sugar is present
- Put a separate (equal) sample in a test tube for each test
- Compare with a control (water) / a white tile behind the tubes
Levels of response Level Marks What the answer does 3 5–6 A coherent method that would give valid results: all the important steps are there, in a sensible order. 2 3–4 A method that might not give valid results: most of the important steps are there, but some are missing or out of order. 1 1–2 A method that would not give valid results: some relevant steps are given, but they are not linked into a workable method. In Study Hack a six-mark answer is not given an automatic level: you tick the points your answer makes, then choose the level that fits it.
- No relevant content: 0 marks.
Evaluate
| Source of error | Type | Effect | Improvement |
|---|---|---|---|
| Judging colours by eye | Random | The judgement is subjective: a green–yellow or a pale lilac can be read either way, so two people may record different colours. | Compare each tube with the controls and the glucose standards side by side against a white tile, or measure the colour with a colorimeter. |
| Benedict’s not heated, or not heated for long enough | Systematic | The tubes stay blue, or every colour comes out behind (green instead of orange): sugar is missed or underestimated. | Heat every tube in a water bath at about 80 °C for 5 minutes. |
| Heating the tubes straight in a Bunsen flame | Random | Each tube boils for a different time, so the colours cannot be compared fairly; the boiling liquid can spit out. | Use a water bath: every tube reaches the same temperature, safely. |
| One pipette used for every solution | Systematic | A little of each solution is carried into the next: a water control or a sugar-free tube changes colour. | Use a clean, labelled pipette for each solution, and always run a water control. |
| The wrong amount of reagent | Systematic | Too much blue reagent hides a faint lilac and holds Benedict’s back; one drop gives colours too pale to see. | Add the amount in the method: an equal volume of biuret or Benedict’s, 2 drops of iodine, 3 drops of Sudan III. |
| A cloudy (unfiltered) food solution | Systematic | Bits of food hide a faint colour change, especially the lilac of a little protein. | Filter the ground food to get a clear solution. |
| Reading the Sudan III tubes straight after shaking | Random | The stained fat is still spread as droplets, so the layer has not formed yet. | Shake gently and leave the tubes to stand for a minute before looking. |
| A tube put into the water bath late (or taken out early) | Random | It is heated for less time than the others, so its colour is behind. | Put all the tubes in together and time 5 minutes. Repeat any tube that is out of order. |
| Colours judged against the dark bench, with no white tile behind the tubes | Systematic | A pale lilac or a pale green looks blue against the bench, so small amounts of protein and sugar are missed. | Hold a white tile behind the tubes and look at them side by side. |
Repeatable. When you repeat a test on the same food solution with the same method and equipment, you see the same colour.
Reproducible. Another student, or a different method such as a colorimeter, gets the same result for the same food.
Other exam boards
How this practical lines up with other GCSE boards' required or core practicals. Codes only; check your own specification for the exact activity.
| Board | Practical | Match | What is different |
|---|---|---|---|
| AQA | Required practical 4 | This page | — |
| Edexcel | 1.13B | Similar, with differences | Tests for fats with the ethanol emulsion test; this lab uses Sudan III. |
Print it
- Blank write-up: the nine-step lab book with the correct table headings, ready to print for a real lesson.
- Specimen lab book: this page's run written up in full, with its exam answers ticked against the mark scheme.
- Printable method card: the practical on one index card. Practise with the cards in Study Hack.
- Practical workbook: a full simulated data set to process on paper, with exam questions and answers.
- Teacher and technician pack: apparatus lists, technician notes and marking points.