The shape of the lesson
By the end of the lesson, learners can
- Four reagents, five answers: Pick a sample and a test on the virtual bench, then run it.
- Benedict's as a measurement: On its own Benedict's is qualitative.
- Finding a non-reducing sugar: Sucrose has no free reducing group, because the glycosidic bond joins the reducing ends of glucose and fructose together.
- Benedict's: heat; blue → green, yellow, orange, brick red.
- Non-reducing sugar: negative Benedict's, boil with HCl, neutralise, retest.
- Iodine: orange-brown → blue-black. Biuret: blue → lilac.
- Emulsion: ethanol, then water; milky white means lipid.
One picture every idea returns to
Each test pairs a reagent with a colour change. Benedict's (heat) goes from blue to a brick-red precipitate with reducing sugars, through green, yellow and orange as the sugar concentration rises. A non-reducing sugar such as sucrose gives a negative Benedict's until it is hydrolysed by boiling with dilute acid and then neutralised. Iodine in potassium iodide turns blue-black with starch, the emulsion test gives a milky-white emulsion with lipids, and biuret turns lilac with proteins. Standardise the Benedict's test and compare with colour standards, and it becomes a way to estimate concentration.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Five tests, one colour each.. Before you can talk about what a molecule does, you need to know it is there. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Four reagents, five answers | Pick a sample and a test on the virtual bench, then run it. Project tab 7 of the lab (Biological molecules lab, virtual food tests) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Benedict's as a measurement | On its own Benedict's is qualitative. | Pairs |
| 21 to 30 min | Teach and check: Finding a non-reducing sugar | Sucrose has no free reducing group, because the glycosidic bond joins the reducing ends of glucose and fructose together. | Jigsaw |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 19 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Finding a non-reducing sugar, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Benedict's must be in excess for the semi-quantitative test. | If all the Cu 2+ is used up, every concentration above a certain value gives the same brick red, and the test can no longer tell them apart. |
| If the first Benedict's test is already positive, a colour after hydrolysis proves nothing on its own: you would need a deeper colour (more precipitate) after hydrolysis than before to show a non-reducing sugar was there as well. | Return to Finding a non-reducing sugar: ask a pair to state the correct version and show it on the lab before moving on. |
Testing for biological molecules: worksheet
6 questions, 19 marks. Written in the style of the papers; no past paper question is reproduced.
Describe how you would carry out the emulsion test for lipids and state the result for a positive test.
A solution gives a negative result with Benedict's solution. Describe how you would show that it contains a non-reducing sugar.
Describe how you would use Benedict's solution and a set of colour standards to estimate the concentration of glucose in a fruit juice.
A student times how long glucose solutions take to show the first colour change with Benedict's: 0.1% takes 180 s, 0.5% takes 60 s, 1.0% takes 25 s. An unknown takes 90 s. Estimate its concentration and explain one weakness of the method.
Explain why sucrose gives a negative result with Benedict's solution but glucose gives a positive result.
A food sample gives these results: Benedict's stays blue; iodine turns blue-black; biuret turns lilac; emulsion test stays clear. State which biological molecules are present and which are absent.
Answer key and marking notes
- Shake the sample with ethanol so that any lipid dissolves.
- Pour the ethanol into a test tube of water (or add water).
- A milky-white emulsion (cloudiness) shows lipid is present; a clear solution means no lipid.
- Boil a fresh sample with dilute hydrochloric acid to hydrolyse the glycosidic bonds.
- Cool, then neutralise with sodium hydrogencarbonate (test with pH paper) because Benedict's needs alkaline conditions.
- Add Benedict's solution and heat in a water bath.
- A colour change from blue to green, yellow, orange or brick red shows a non-reducing sugar was present.
- Make a dilution series of glucose solutions of known concentration.
- Add the same volume of each standard and of the juice to the same (excess) volume of Benedict's solution.
- Heat all tubes in the same water bath at the same temperature for the same time.
- Compare the colour of the juice with the standards; the concentration is that of the closest standard, or between two.
- The time lies between those for 0.1% and 0.5%, so the concentration is between 0.1% and 0.5% (about 0.3 to 0.4% read from a graph of time against concentration).
- Weakness: deciding the moment of the first colour change by eye is subjective, so the timings are imprecise.
- (Also accept: only three standards, so the graph is poorly defined in that range.)
- Glucose has a free reducing group (on carbon 1) that reduces Cu2+ to Cu+, forming a brick-red precipitate.
- In sucrose the reducing groups of glucose and fructose are both used in the glycosidic bond, so sucrose cannot reduce Cu2+.
- Present: starch (blue-black with iodine) and protein (lilac with biuret).
- Absent: reducing sugars (Benedict's stayed blue) and lipids (no emulsion).
- Non-reducing sugars cannot be ruled out without the acid hydrolysis test.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which one of these food tests requires the mixture to be heated in a water bath?
- Athe Benedict's test for reducing sugars
- Bthe biuret test for proteins
- Cthe iodine test for starch
- Dthe emulsion test for lipids
What is the brick-red precipitate that forms in a positive Benedict's test?
- Acopper(I) oxide, formed when Cu2+ ions are reduced
- Bcopper(II) oxide, formed when Cu+ ions are oxidised
- Ccopper(II) sulfate, which is precipitated by the sugar
- Da complex of copper ions with the glycosidic bonds
In the emulsion test, why is the sample shaken with ethanol before it is added to water?
- Alipids dissolve in ethanol, then come out of solution as tiny droplets when the ethanol mixes with water
- Bethanol reacts chemically with lipids to form a white precipitate
- Cethanol hydrolyses the ester bonds and the fatty acids released are white
- Dlipids dissolve in water, and ethanol is needed to make that solution cloudy
The table shows the results of four food tests on samples P, Q, R and S. Which sample contains a reducing sugar and a protein, but no starch and no lipid?
- Asample P
- Bsample Q
- Csample R
- Dsample S
A student adds Benedict's solution to a fruit juice and leaves the tube in a rack at room temperature. After five minutes the mixture is still blue. What is the correct conclusion?
- Ano valid conclusion can be drawn, because the mixture was not heated
- Bthe juice contains no reducing sugar
- Cthe juice contains a non-reducing sugar instead of a reducing sugar
- Dthe juice contains starch, which masks the colour change
A student needs 10 cm3 of a 0.25% glucose solution to make a colour standard, using a 1.0% glucose stock solution and distilled water. Which volumes should be mixed?
- A2.5 cm3 of stock and 7.5 cm3 of water
- B2.5 cm3 of stock and 10.0 cm3 of water
- C7.5 cm3 of stock and 2.5 cm3 of water
- D0.25 cm3 of stock and 9.75 cm3 of water
A sample gives an orange colour with Benedict's. The precipitate is filtered, dried and weighed: 0.12 g. A second, identical sample is boiled with dilute hydrochloric acid, neutralised and tested in the same way, giving 0.31 g of precipitate. What does the sample contain?
- Aboth a reducing sugar and a non-reducing sugar
- Ba reducing sugar only, because the first test was already positive
- Ca non-reducing sugar only, because the second test gave more precipitate
- Da reducing sugar only, which the acid converted into more reducing sugar
After a semi-quantitative Benedict's test, the precipitate is filtered off and the blue filtrate is placed in a colorimeter. Benedict's solution is in excess in every tube. How does the absorbance of the filtrate change as the glucose concentration of the samples increases, and why?
- Ait decreases, because more Cu2+ is reduced and removed as precipitate, leaving a paler blue filtrate
- Bit increases, because more brick-red precipitate forms in the tube
- Cit stays the same, because Benedict's solution is in excess in every tube
- Dit increases, because the filtrate turns from blue to red as glucose rises
Quiz key
Benedict's solution must be heated (about 80 °C or above) for a reducing sugar to reduce the blue Cu2+ ions to a brick-red precipitate. The biuret test also uses copper(II) ions, which is why it is often confused with Benedict's, but it works at room temperature: the lilac colour appears as soon as the reagents are mixed. Iodine in potassium iodide turns blue-black with starch without heating (heating actually makes the colour fade). The emulsion test only needs shaking with ethanol and then adding water.
A reducing sugar donates electrons to the blue Cu2+ ions, reducing them to Cu+, which forms insoluble red copper(I) oxide. Copper(II) oxide would need the copper to be oxidised, the opposite of what a reducing sugar does (and it is black). Copper(II) sulfate is soluble and blue; it is a component of reagents, not the precipitate. A copper complex with bonds in the molecule is how biuret works (with peptide bonds), not Benedict's.
Lipids are soluble in ethanol but insoluble in water. When the ethanol solution is mixed with water, the lipid can no longer stay dissolved and forms a suspension of tiny droplets that scatter light, seen as a milky-white emulsion. No chemical reaction takes place, so there is no precipitate. Ethanol does not hydrolyse ester bonds; that needs water and an enzyme (lipase) or acid or alkali. Lipids do not dissolve in water at all; that is exactly why the emulsion forms.
P gives brick red with Benedict's (reducing sugar), lilac with biuret (protein), and negative results with iodine (stays orange-brown) and the emulsion test (clear). Q has no reducing sugar (Benedict's stays blue) and does contain starch (blue-black). R contains a reducing sugar and lipid (milky white) but no protein, because biuret stayed blue. S contains a reducing sugar (orange is still a positive Benedict's result) and starch, but biuret stayed blue so it has no protein.
Benedict's solution must be heated in a water bath for the reduction of Cu2+ to happen at a useful rate, so a blue result without heating does not show that reducing sugar is absent. Concluding that there is no reducing sugar ignores the faulty method. A non-reducing sugar can only be shown by a negative heated Benedict's test followed by acid hydrolysis and a positive repeat. Starch does not mask the Benedict's result; it is detected separately with iodine.
0.25% ÷ 1.0% = 0.25, so 0.25 × 10 = 2.5 cm3 of stock is needed, made up to 10 cm3 with 7.5 cm3 of water. Adding 10.0 cm3 of water gives a total of 12.5 cm3 and a concentration of only 0.20%. 7.5 cm3 of stock gives a 0.75% solution, the proportions reversed. 0.25 cm3 of stock treats the percentage as a volume and gives only 0.025%.
The 0.12 g from the untreated sample shows a reducing sugar is present. Acid hydrolysis breaks glycosidic bonds in non-reducing sugars such as sucrose, releasing monosaccharides that are reducing; the extra 0.19 g of precipitate shows a non-reducing sugar was present too. Saying reducing sugar only ignores the increase, which needs an explanation. Saying non-reducing sugar only ignores the positive first test. Hydrolysis of a reducing monosaccharide such as glucose does not create extra reducing groups; the increase must come from a sugar that was not reducing before.
The blue colour of the filtrate comes from the Cu2+ ions that were not reduced. More glucose reduces more Cu2+ to copper(I) oxide, which is filtered off, so the filtrate is paler and absorbs less light. The precipitate is no longer in the cuvette, so it cannot raise the absorbance. Excess Benedict's means some Cu2+ is always left, but the amount left still depends on the glucose concentration. The red colour is the insoluble precipitate; the filtrate stays blue, only becoming paler.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.