Apple juice is produced by chopping up apples and crushing them. If apple tissue is cut and crushed, its colour may deepen and it can turn brown.
Some students studied how vitamin C affected colour change in apples after they were cut and crushed.
The students carried out these steps:
• Put a label on a large test-tube and pour in 5 cm3 of vitamin C solution.
• Put a label on a second large test-tube and pour in 5 cm3 of distilled water.
• Put a label on a third large test-tube and leave it without any liquid.
• Prepare three apple cubes, each measuring 1 cm × 1 cm × 1 cm. Exclude all skin and core.
• Slice each cube into many small pieces. Keep the pieces from each cube separate.
• Place the pieces from one cube into the test-tube containing vitamin C solution.
• Place the pieces from the second cube into the test-tube containing distilled water.
• Place the pieces from the third cube into the test-tube with no liquid added.
• With a glass rod, carefully crush the apple pieces at the bottom of each test-tube.
• Wash the glass rod between uses in the different test-tubes.
• Begin timing.
At the start of timing, the students observed that the apple tissue in each test-tube was white.
They checked the colour of the apple tissue in each test-tube after 5, 10 and 15 minutes and wrote down their observations.
Fig. 1.1 shows part of one notebook:
vitamin C: white → white → white
distilled water: white → pale yellow → dark yellow
no added liquid: yellow → dark yellow → brown
(a(i))[2]
Describe the most suitable apparatus for measuring the necessary volumes of vitamin C solution and distilled water.
(a(ii))[1]
Explain why the students were told to clean the glass rod.
(a(iii))[3]
Complete the headings in Table 1.1 and use the observations in Fig. 1.1 to complete the table.
(a(iv))[1]
Using your observations in Table 1.1, state what can be concluded about the effect of vitamin C on the colour change when apple is cut up and crushed.
(a(v))[2]
Using your observations in Table 1.1, state what can be concluded about the effect of distilled water on the colour change when apple is cut up and crushed.
(a(vi))[2]
The students knew that the method may have had sources of error that influenced their observations. Suggest one possible source of error and suggest how it could be overcome.
(b(i))[2]
The students then measured the pH of the vitamin C solution and the pH of the distilled water using universal indicator paper, with these results: distilled water: pH 6; vitamin C solution: pH 4. Describe how you would use universal indicator paper to determine the pH of a liquid.
(b(ii))[2]
When apple tissue is cut up and crushed, an enzyme comes into contact with oxygen in the air. In the presence of oxygen, the enzyme catalyses the reaction of a colourless molecule in the cells of the apple tissue to cause the change in colour. Suggest how this might explain the effect of: distilled water; vitamin C.