The shape of the lesson
By the end of the lesson, learners can
- The five food tests: Pick a food and a test, then press Do the test.
- Comparing vitamin C with DCPIP: Put 1 cm 3 of blue DCPIP in a tube, choose a juice, and press Add one drop again and again until the DCPIP status reads colourless.
- The structure of DNA: Extended.
- Starch: iodine, orange-brown to blue-black. Protein: biuret, blue to purple.
- Reducing sugar: Benedict's, heat in a water bath, blue to green, yellow, orange or brick red.
- Fat: shake with ethanol, pour into water, cloudy white emulsion. Vitamin C: DCPIP, blue to colourless; fewer drops means more vitamin C.
- Extended DNA: two strands, double helix, bases, bonds between base pairs, A with T and C with G.
One picture every idea returns to
Iodine solution turns from orange-brown to blue-black with starch. Benedict's solution, heated in a water bath, turns from blue to green, yellow, orange or brick red with reducing sugars. Biuret turns from blue to purple with protein. Shaking a food with ethanol and pouring it into water gives a cloudy white emulsion if fat is present. Vitamin C turns blue DCPIP colourless. Extended: DNA is two strands coiled into a double helix; each strand carries bases, bonds between pairs of bases hold the strands together, and the bases always pair A with T and C with G.
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, five colours.. Each group of biological molecules has its own chemical test, and each test ends in a colour change you can see. 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: The five food tests | Pick a food and a test, then press Do the test. Project tab 3 of the lab (Food tests bench: ten foods, five tests) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Comparing vitamin C with DCPIP | Put 1 cm 3 of blue DCPIP in a tube, choose a juice, and press Add one drop again and again until the DCPIP status reads colourless. Project tab 4 of the lab (Counting drops of juice into DCPIP) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: The structure of DNA | Extended. Project tab 5 of the lab (Build and unzip a DNA double helix) and let a student drive it while the class predicts. | Connect-Extend-Challenge |
| 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, 21 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 The structure of DNA, 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.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
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 |
|---|---|
| State both colours: the colour of the reagent at the start and the colour it turns. "Iodine turns blue" instead of blue-black, or a Benedict's result given without heating, are classic lost marks. | Return to The five food tests: ask a pair to state the correct version and show it on the lab before moving on. |
| Do not read the number of drops as the amount of vitamin C. | More drops means less vitamin C in each drop of juice. |
| Pair the bases A with T and C with G, never A with G or C with T. | And it is the bonds between pairs of bases that hold the two strands together. |
Food tests and the structure of DNA: worksheet
6 questions, 21 marks. Written in the style of the papers; no past paper question is reproduced.
Describe how you would test a sample of fruit juice for reducing sugar, and state the result you would see if a lot of reducing sugar were present.
Describe the ethanol emulsion test for fats and oils and state a positive result. Give one safety precaution.
A student tested three foods. The table shows the results.
| Food | Iodine | Benedict's (heated) | Biuret | Ethanol emulsion |
|---|---|---|---|---|
| X | blue-black | blue | blue | clear |
| Y | orange-brown | brick red | blue | clear |
| Z | orange-brown | yellow | purple | cloudy white |
State which nutrients each food contains and suggest which food could be milk.
A 0.1% vitamin C solution decolourised 1 cm3 of DCPIP after 6 drops. A sample of pineapple juice decolourised 1 cm3 of the same DCPIP after 15 drops. Calculate the vitamin C concentration of the pineapple juice.
A student found that freshly squeezed orange juice decolourised DCPIP after 6 drops, but the same juice after boiling for ten minutes needed 18 drops. Explain this result and state two variables the student should keep the same.
Extended. (a) Describe the structure of a DNA molecule. (b) In a sample of DNA, 30% of the bases are A. State the percentage of the bases that are T and the percentage that are C.
Answer key and marking notes
- Add an equal volume of Benedict's solution (blue) to the juice in a test tube.
- Heat the tube in a hot water bath (about 80 °C) for a few minutes.
- A lot of reducing sugar gives a brick red colour (precipitate).
- Smaller amounts give green, yellow or orange; with no reducing sugar it stays blue.
- Shake (or crush) the food with ethanol so that any fat dissolves.
- Pour the ethanol into water (or add water to it): a cloudy white emulsion shows that fat is present.
- Ethanol is flammable, so keep it away from any flame (and wear eye protection).
- X contains starch only (blue-black with iodine; all other tests negative).
- Y contains a lot of reducing sugar only (brick red with Benedict's).
- Z contains some reducing sugar (yellow), protein (purple) and fat (cloudy white emulsion), but no starch.
- Z could be milk, which contains a sugar, protein and fat.
- Concentration of juice = 0.1% × (drops of standard ÷ drops of juice).
- = 0.1 × 6 ÷ 15
- = 0.04%.
- Boiling destroys (breaks down) vitamin C, so each drop of boiled juice contains less vitamin C and more drops are needed.
- Keep the same volume of DCPIP (1 cm3) and the same concentration of DCPIP.
- Also acceptable: the same size of drop (same pipette), shaking after each drop, the same end point (blue just disappears).
- (a) Two strands coiled together to form a double helix; each strand contains bases.
- Bonds between pairs of bases hold the two strands together; A always pairs with T and C with G.
- (b) Every A is paired with a T, so T = 30%.
- That leaves 100 − 60 = 40% for C and G, which are equal in number, so C = 20%.
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 of these food tests must be heated in a water bath to give a result?
- ABenedict's test for reducing sugar
- Bbiuret test for protein
- Ciodine test for starch
- DDCPIP test for vitamin C
A food is shaken with ethanol and the ethanol is then poured into water. Which observation shows that the food contains fat?
- Aa cloudy white emulsion forms
- Bthe mixture turns purple
- Cthe mixture turns blue-black
- Da brick red precipitate forms
A drop of iodine solution on a cut potato turns blue-black. A drop on a glucose solution stays orange-brown. What can be concluded?
- AThe potato contains starch; the glucose solution does not
- BThe potato contains glucose; the glucose solution contains starch
- CBoth contain starch, but the potato contains more
- DNeither contains starch, because iodine tests for sugar
The bar chart shows how many drops of four juices were needed to decolourise 1 cm3 of the same DCPIP solution. Which statement is correct?
- AJuice B has the highest vitamin C concentration, four times that of juice D
- BJuice D has the highest vitamin C concentration, four times that of juice B
- CJuice B has the highest vitamin C concentration, one quarter that of juice D
- DJuice A has twice the vitamin C concentration of juice C
A student tests a breakfast cereal solution for reducing sugar with Benedict's solution. Which control would best show that any colour change is caused by the cereal?
- ARepeat the test with water in place of the cereal solution
- BRepeat the test using more cereal and no Benedict's solution
- CHeat the same tube for twice as long
- DTest the cereal solution with iodine as well
A 0.1% vitamin C solution decolourises 1 cm3 of DCPIP after 9 drops. A fruit juice decolourises the same volume of DCPIP after 6 drops. What is the vitamin C concentration of the juice?
- A0.15%
- B0.067%
- C0.54%
- D1.5%
Extended: what holds the two strands of a DNA molecule together?
- Abonds between pairs of bases, one on each strand
- Bbonds between bases next to each other on the same strand
- Ca chain of amino acids wound around both strands
- Dbonds between A on one strand and G on the other
Extended: a DNA molecule contains 1000 bases in total. 180 of the bases are G. How many of the bases are A?
- A320
- B180
- C640
- D820
Quiz key
Benedict's solution must be heated (about 80 °C) for the blue colour to change to green, yellow, orange or brick red. Biuret, iodine and DCPIP all work at room temperature, so heating them is not part of the test.
Fat dissolved in ethanol forms tiny droplets when the ethanol meets water, giving a cloudy white emulsion. Purple is the positive biuret result for protein, blue-black is iodine with starch and a brick red precipitate is Benedict's with reducing sugar.
Iodine solution turns from orange-brown to blue-black only with starch, so the potato contains starch and the glucose solution does not. Iodine does not detect glucose (Benedict's does), and a solution that stays orange-brown contains no starch at all.
B needs only 5 drops and D needs 20, so each drop of B contains four times as much vitamin C: B has the highest concentration. Reading the tallest bar as the most vitamin C reverses the relationship. A needs 10 drops and C 15, so A has 1.5 times, not twice, the concentration of C.
Using water instead of the cereal, with the same Benedict's solution and heating, shows the result when no sugar is present; if it stays blue, any change in the cereal tube is due to the cereal. Leaving out Benedict's removes the test itself, heating longer changes another variable, and iodine tests for starch, not sugar.
Concentration = 0.1% × (drops of standard ÷ drops of juice) = 0.1 × 9 ÷ 6 = 0.15%. Using 6 ÷ 9 gives 0.067%, which wrongly suggests the juice is weaker than the standard although it needed fewer drops. 0.54% multiplies 0.1 by both counts, and 1.5% misplaces the decimal point.
Each base on one strand is bonded to its partner base on the other strand, and these base pairs hold the strands together like the rungs of a ladder. Bases next to each other on one strand do not join the two strands, DNA is not wrapped in an amino acid chain at this level, and A pairs with T, not G.
G pairs with C, so there are 180 C as well, making 360 bases. That leaves 1000 − 360 = 640 for A and T, which pair with each other and so are equal: 320 A. 180 confuses A with C, 640 is A and T together, and 820 is simply 1000 − 180.
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.