Answers · Food tests

Testing foods — answer key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. FQ1 · Practice

Options as numbered on the pupil sheet: 1 — Iodine solution; 2 — Benedict's solution; 3 — Biuret solution; 4 — Ethanol emulsion test; 5 — DCPIP.

Model answer. Starch is detected with iodine solution, reducing sugar with Benedict's solution (heated in a water bath), protein with biuret solution, fats and oils with the ethanol emulsion test, and vitamin C with DCPIP.

A full answer includes

Starch: iodine solution.
Reducing sugar: Benedict's solution.
Protein: biuret solution.
Fat or oil: the ethanol emulsion test.
Vitamin C: DCPIP.

Also accept

starch: iodine; reducing sugar: Benedict's; protein: biuret; fat or oil: emulsion; vitamin C: DCPIP

Watch for

“Iodine tests for sugar.” — Iodine detects starch only; sugar needs the Benedict's test.

2. FQ2 · Practice

Options as numbered on the pupil sheet: 1 — Blue-black; 2 — Green to brick-red, with a precipitate; 3 — Purple; 4 — Evenly cloudy white; 5 — The blue colour disappears; 6 — Stays blue; 7 — Stays orange-brown.

Model answer. 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 a precipitate. Biuret solution turns from blue to purple. In the emulsion test the liquid turns evenly cloudy white. DCPIP loses its blue colour.

A full answer includes

Iodine: blue-black.
Benedict's: green, yellow, orange or brick-red with a precipitate, after heating.
Biuret: purple.
Emulsion: evenly cloudy white.
DCPIP: the blue colour disappears.

Also accept

iodine: blue-black; Benedict's: green to brick-red with precipitate; biuret: purple; emulsion: cloudy white; DCPIP: blue disappears

Watch for

“The blue biuret reagent is a positive result.” — Biuret is blue to start with; only purple is positive.
“DCPIP turning colourless means no vitamin C.” — The blue disappears because vitamin C is there: that is the positive result.

3. FQ3 · Practice

Model answer. b. Biuret solution is blue to begin with. The mixture stayed blue, like the control, so this is a negative result: no protein was detected.

A full answer includes

I chose b.

Also accept

Watch for

“The tube is blue, so there is protein.” — Blue is the colour of the reagent itself; only purple shows protein.

4. FQ4 · Practice

Model answer. b. The Benedict's test only works when the mixture is heated in a water bath at about 80 °C. In a cold tube nothing happens even though glucose, a reducing sugar, is there: a false negative.

A full answer includes

I chose b.

Also accept

Watch for

“Benedict's works straight away, like iodine.” — Benedict's must be heated in a water bath for the colour change to happen.

5. FQ5 · Practice

Model answer. (a) Put a few drops of the bread sample in a well of a white spotting tile, and water in the next well as the control. Add 2 drops of iodine solution to each. Blue-black shows starch. If no blue-black appears (the iodine usually stays orange-brown, like the control), starch is not detected. (b) Put 2 cm³ of the sample in one test tube and 2 cm³ of water in another as the control. Add 2 cm³ of biuret solution to each and shake; no heating is needed. Purple shows protein; if the mixture stays blue, like the control, protein is not detected.

A full answer includes

(a) A few drops of sample in a tile well, water in the next well; 2 drops of iodine in each; blue-black = starch; no blue-black (usually stays orange-brown, like the control) = no starch detected.
(b) 2 cm³ sample and 2 cm³ water in two tubes; 2 cm³ biuret solution in each; shake; purple = protein; stays blue, like the control = no protein detected.

Also accept

(a) sample and water control on a tile; iodine; blue-black positive, no blue-black negative; (b) sample and water control in tubes; biuret; purple positive, blue negative

Watch for

“Iodine tests for sugar.” — Iodine detects starch only.
“The tube is blue, so there is protein.” — Biuret is blue to start with; only purple shows protein.

6. FQ6 · Practice

Model answer. P is vitamin C solution: DCPIP lost its blue, a positive result. P also gave a positive Benedict's result (green in this model): vitamin C can reduce Benedict's solution too. Its iodine well showed no blue-black, so no starch. Q is egg-white solution: biuret turned purple. R is cooking oil: the emulsion test gave an even cloudy white. S is starch suspension: iodine turned blue-black. T is glucose solution: its Benedict's result was positive, and DCPIP was negative, so the reducing substance is a sugar, not vitamin C.

A full answer includes

P: vitamin C solution (DCPIP positive; its Benedict's positive can come from vitamin C).
Q: egg-white solution (biuret purple).
R: cooking oil (emulsion test evenly cloudy white).
S: starch suspension (iodine blue-black).
T: glucose solution (Benedict's positive and DCPIP negative).

Also accept

P vitamin C (DCPIP); Q egg white (biuret); R oil (emulsion); S starch (iodine); T glucose (Benedict's positive, DCPIP negative)

Watch for

“Any sample that changes Benedict's colour must be glucose.” — Vitamin C can also give a positive Benedict's result; DCPIP tells the two apart.
“The iodine faded in P, so P contains starch.” — Only blue-black shows starch; a fading brown is still a negative result.
“Q is blue in most tests, so Q contains protein in each of them.” — Blue is the colour of the biuret and Benedict's reagents themselves; only the purple biuret result shows protein.

7. FQ7 · Practice

Model answer. Both samples removed the colour from the same volume of the same DCPIP, so each supplied the same amount of vitamin C. The drops were the same size, and the vitamin C solution did the job in half as many drops, so it contains about 12 ÷ 6 = 2 times as much vitamin C per cm³ as the orange juice.

Checked automatically

Number of times as much (no unit): 2 (within ±0.05)
- Common wrong answer 0.5 : Fewer drops means more vitamin C, so the vitamin C solution is more concentrated: the answer is bigger than 1. Divide 12 by 6.
- Common wrong answer 6 : That is the difference in drops, 12 − 6. To compare, divide: 12 ÷ 6.

A full answer includes

12 ÷ 6 = 2: for these solutions (same DCPIP, same drop size), the vitamin C solution has about 2 times as much vitamin C in each cm³.

Also accept

Watch for

“Fewer drops means less vitamin C.” — With the same DCPIP and drop size, fewer drops means more vitamin C per cm³.

8. FQ8 · Practice

Model answer. c. The fat comes out of the ethanol as millions of tiny droplets spread evenly through the water. The droplets scatter light, so the whole liquid looks evenly cloudy white.

A full answer includes

I chose c.

Also accept

Watch for

“A positive emulsion test is a white solid at the bottom, or a layer of oil on top.” — The emulsion is tiny droplets spread evenly through the whole liquid.

9. FQ9 · Practice

Model answer. Put 1 cm³ of DCPIP in a test tube. Add the fruit drink drop by drop from a pipette, shaking after each drop, and count the drops. If the blue colour disappears, vitamin C is present (with a coloured drink, the tube ends the colour of the drink rather than colourless). If the tube is still blue after 20 drops, stop: vitamin C is not detected. As the control, add water to 1 cm³ of DCPIP in the same way: it stays blue.

A full answer includes

1 cm³ DCPIP in a test tube; add the drink drop by drop with a pipette, shaking after each drop, counting the drops.
Vitamin C is present if the blue colour disappears; if still blue after 20 drops, stop: vitamin C not detected.
Control: water added to 1 cm³ DCPIP in the same way stays blue.

Also accept

1 cm³ DCPIP, drink added drop by drop, shaking, counting; blue disappears = vitamin C; still blue after 20 drops = not detected; water control stays blue

Watch for

“DCPIP went colourless, so there is no vitamin C.” — The blue disappears because vitamin C is there.

10. FQ10 · Practice

Options as numbered on the pupil sheet: 1 — Solution R (stayed blue); 2 — Solution P (green); 3 — Solution Q (brick-red).

Model answer. R stayed blue, so no glucose was detected: it has the least. P turned green, which shows a little glucose. Q turned brick-red, which shows the most. The colours can be compared because glucose is the only substance in these solutions that changes Benedict's solution, and the volumes, temperature and time were the same for all three. Drinks such as fruit juice could not be ranked like this from the colours alone: vitamin C can also turn Benedict's green.

A full answer includes

R (blue) → P (green) → Q (brick-red).

Also accept

R, P, Q

Watch for

“Green is the strongest colour, so it means the most sugar.” — The series goes blue, green, yellow, orange, brick-red as the amount of reducing sugar increases.
“A green Benedict's result always means a little sugar.” — Only when sugar is the only substance that can change Benedict's, as in these glucose solutions. Vitamin C can turn it green too.

11. FT1 · Apply

Model answer. (a) Milk contains reducing sugar (Benedict's turned orange with a precipitate), protein (biuret turned purple) and fat (the emulsion test went cloudy white). Starch and vitamin C were not detected. (b) Each food test detects a substance, not a food. Biuret detects protein, which is in milk but also in egg white, meat and beans; and milk contains other nutrients too, so it gives several positive results.

A full answer includes

(a) Reducing sugar (Benedict's orange), protein (biuret purple) and fat (emulsion cloudy white); no starch or vitamin C detected.
(b) A test detects one substance, not a food; milk contains several nutrients, so it gives several positive results, and other foods also give a positive biuret result.

Also accept

(a) reducing sugar, protein, fat; (b) tests detect substances, not foods; milk contains several; biuret detects protein in many foods

Watch for

“A test names a food.” — Each test detects one kind of substance; one food can contain many.
“The emulsion test shows milk, because milk is cloudy white.” — The test shows fat; milk is cloudy white because it is itself an emulsion of fat droplets.

12. FT2 · Apply

Model answer. (a) Water contains no reducing sugar, so the control should have stayed blue. Turning green shows that the Benedict's solution, or the glassware or pipette, is contaminated. So the green in the honey tube may not come from the honey, and the test must be repeated with fresh reagent and clean tubes. (b) A control is the same test on water, done at the same time. It shows what a negative result looks like under exactly the same conditions, so a food result only counts as positive if it is different from the control, and it shows up contamination like this.

A full answer includes

(a) The Benedict's (or the tube or pipette) is contaminated with a reducing substance, so the green in the honey tube cannot be trusted.
(b) The control shows the negative result under the same conditions; a result only counts as positive if it differs from the control; it also reveals contamination.

Also accept

(a) contamination of reagent or glassware; honey result unreliable; (b) control shows the negative under the same conditions, positive only if different, detects contamination

Watch for

“You do not need a control, because you already know what the reagent looks like.” — Only the control shows that the reagent and glassware behave correctly in this run.

13. FT3 · Apply

Model answer. Both juices removed the colour from the same volume of the same DCPIP, so each supplied the same amount of vitamin C, and the drops were the same size. The old juice needed twice as many drops (16 instead of 8), so each drop carried half as much vitamin C: the concentration of vitamin C in the juice fell to about half while it stood for two days.

A full answer includes

The old juice needed more drops (16 instead of 8) to remove the colour from the same DCPIP, so its vitamin C concentration is lower.
About half (8 ÷ 16): the concentration of vitamin C fell to about half while the juice stood.
The comparison is fair only because the DCPIP volume and concentration and the drop size were the same.

Also accept

old juice needs more drops so has a lower vitamin C concentration; about half; fair because same DCPIP and same drop size

Watch for

“More drops means more vitamin C.” — More drops means a lower concentration of vitamin C: a bigger volume of juice was needed to do the same job.