Testing foods — questions

1. identify

Match each nutrient to the food test that detects it.

Write the number of the matching option.

1. Iodine solution
2. Benedict's solution
3. Biuret solution
4. Ethanol emulsion test
5. DCPIP

• Starch: ______
• Reducing sugar: ______
• Protein: ______
• Fat or oil: ______
• Vitamin C: ______

Nudge (only if you are stuck): Each nutrient has its own reagent. Only one test uses ethanol.

2. identify

Match each food test to what you see when the result is positive.

Write the number of the matching option.

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

• Iodine solution: ______
• Benedict's solution: ______
• Biuret solution: ______
• Ethanol emulsion test: ______
• DCPIP: ______

Nudge (only if you are stuck): A positive result is a change from the reagent's own colour. Two of the options are what the reagent looks like before it changes.

3. interpret

A student adds biuret solution to a sample of fruit squash. After shaking, the mixture is blue, the same as the water control. What does this show?

Circle one answer.

• The squash contains protein
• No protein was detected in the squash
• The tube needs heating before the colour changes
• The squash contains reducing sugar

Nudge (only if you are stuck): What colour is biuret solution before anything is added?

4. explain

A student adds 2 cm³ of Benedict's solution to 2 cm³ of glucose solution and leaves the tube in the rack. After 5 minutes it is still blue. Which statement explains the result?

Circle one answer.

• Glucose is not a reducing sugar
• The tube was not heated in a water bath, and Benedict's solution only changes colour when heated
• Benedict's solution only detects starch
• The glucose solution had turned into starch

Nudge (only if you are stuck): Which food test needs the water bath?

5. describe

Describe how you would test a sample of ground-up bread for (a) starch, using iodine solution, and (b) protein, using biuret solution. For each test, say what you use as the control, and what you would see for a positive and a negative result.

(a) Starch test with iodine

(b) Protein test with biuret

Nudge (only if you are stuck): Iodine works on a spotting tile; biuret is mixed in test tubes with no heating.

6. identify / explain

Samples P to T are each one of these: glucose solution, starch suspension, egg-white solution, cooking oil, vitamin C solution. The table shows the results of all five tests, done beside a water control. Treat each result as positive or negative. Name each sample and give the result or results that show it.

P is … because …

Q is … because …

R is … because …

S is … because …

T is … because …

Nudge (only if you are stuck): Compare each row with the water control and mark each result positive or negative. Two samples give a positive Benedict's result: which other test tells them apart?

7. calculate

In this module's model experiment (teaching-model data), samples are added drop by drop from the same pipette to 1 cm³ of the same DCPIP solution. The blue disappears after 6 drops of vitamin C solution and after 12 drops of orange juice. The vitamin C solution has more vitamin C in each cm³ than the orange juice. How many times as much?

Number of times as much (no unit): ______________ unit ________

Nudge (only if you are stuck): With the same DCPIP and the same drop size, fewer drops means more vitamin C per cm³. Divide the larger number of drops by the smaller.

8. identify

In the emulsion test, the clear ethanol from a food sample is poured into a tube of water. Which description shows that the food contains fat?

Circle one answer.

• A white solid settles at the bottom of the tube
• A layer of oil floats on top of clear water
• The liquid turns evenly cloudy white all the way through
• The liquid stays clear

Nudge (only if you are stuck): An emulsion is made of tiny droplets spread through the water.

9. describe

Describe how to use DCPIP to find out whether a fruit drink contains vitamin C. Include the control, when you would stop adding the drink, and the result that shows vitamin C.

Nudge (only if you are stuck): Which liquid goes in the tube first, and which is added drop by drop?

10. sequence

Three solutions, P, Q and R, were made by dissolving different amounts of glucose in water, with nothing else added. Each was tested with Benedict's solution in the same way: 2 cm³ of solution and 2 cm³ of Benedict's, 5 minutes at 80 °C. P turned green, Q turned brick-red and R stayed blue. Put the solutions in order from the least glucose to the most.

Write the number of each step in order.

1. Solution R (stayed blue)
2. Solution P (green)
3. Solution Q (brick-red)

• First: ______
• Second: ______
• Last: ______

Nudge (only if you are stuck): Look at the picture of the five Benedict's tubes in the reading.

11. interpret / explain

Milk was tested with all five food tests, each beside a water control. The results are in the table. (a) Which nutrients were detected in the milk? (b) A student says: "The biuret test is the test for milk." Explain why the student is wrong.

(a) Nutrients detected

(b) Why the student is wrong

Nudge (only if you are stuck): (a) Find every result that is different from the control. (b) Does a test detect a food, or a substance?

12. explain

A student tests honey with Benedict's solution in a water bath, and the tube turns green. A classmate uses the same bottle of Benedict's with water instead of honey, and her tube also turns green. (a) What does the classmate's tube show? (b) Explain why every food test needs a control like hers.

(a) What the green control shows

(b) Why a control is needed

Nudge (only if you are stuck): What colour should Benedict's solution stay with water only?

13. explain

Freshly squeezed orange juice makes the blue of 1 cm³ of DCPIP disappear after 8 drops. The same juice, left in an open glass for two days, needs 16 drops. The same DCPIP solution and the same pipette were used both times. Explain what these results show about the vitamin C in the juice.

Nudge (only if you are stuck): Which juice needed more drops to do the same job? What does that tell you about how much vitamin C each drop brings?