44 Which fruit contains the most vitamin C? What conditions affect the amounts of vitamin C? 14-16 Working in groupsSelf assessmentPeer assessmentSharing objectives and criteriaQuestioningUsing feedbackUsing tests

In this practical activity, students compare the amounts of vitamin C present in a variety of fresh fruit and fruit juices. In groups they analyse different samples and then compile a full set of class results.

This activity is a companion to Do vitamin pills really work?

Learning objectives

Students will be able to:

  • carry out a simple titration
  • find out differences in the amounts of vitamin C levels present in fruit
  • find out how vitamin C levels are affected by different conditions.

Sequence of activities

Prior to this lesson, suggest that students bring a piece of fresh fruit or juice sample for testing for vitamin C. It is best to bring the juice in the original packaging.
Invite suggestions about where vitamin C comes from, together with ideas about how much vitamin C is present in the source suggested.

Organise suggestions in a primitive table – lots, some and little vitamin C.

Introduce the practical, to test different fruits and juices for their vitamin C levels, and the learning objectives. Explain that the test will confirm or change some of the suggestions made.

Organise students into groups of three or four.
  • Give each student a copy of Which fruit contains the most vitamin C?
  • Assign each group to a set of equipment.
  • Demonstrate the technique for testing the samples.
  • Ensure there is a minimum of four samples to test in each group.
  • Check for overlap – the same fruit or juice may be tested more than once by different groups, giving multiple results for reliability.
  • Circulate and supervise the groups as they extract juice from fresh fruit and test the juice samples.

Collate the results.

When the groups have completed their tests, review their results.

In a class discussion, review all the results.

Use any duplicate results to help students understand about the reliability of test results.

Refer to juice packaging materials to give background information.

Introduce the next activity in which students plan an investigation to explore how vitamin C levels are affected by different conditions.

Give each group a different investigation to plan, using the basic test as a starting point.

Suggested investigations:

  • How does temperature affect the amount of vitamin C?
  • How does light affect the amount of vitamin C?
  • How does packaging and storage surroundings affect the amount of vitamin C?
  • How does a ‘slow release’ vitamin C tablet work?
  • Do fruits of different varieties (eg mandarin oranges, navel, clementines, satsumas, etc.) contain different amounts of vitamin C?
  • Does frozen, concentrated orange juice contain as much vitamin C as fresh juice?
  • Which fruit contains the most vitamin C per 100g? Make a thorough comparison of as wide a range of fruit varieties as possible.

Agree criteria for peer assessment of investigation plans.

Give each group access to information sources and presentation materials.

Circulate and support as students:

  • plan their investigations
  • prepare a presentation of their plan
  • elect a spokesperson to feedback to the class.
In a plenary:
  • hear each presentation
  • apply the agreed criteria
  • give feedback based on the quality of the proposed investigations and their scientific content
  • ensure that each student makes a written record of the planned investigation.
The investigations could be carried out during a later session or in an after school club.
Give written feedback that reinforces the good points about the investigations, giving suggestions for improvements.

Assessment for learning commentary

Working together on a simple titration gives a sound basis from which groups work in teams to plan an investigation. The suggested investigations permit differentiation by task and outcome.

Agreeing the criteria and then assessing each other’s investigation plans stimulates students to perform well. The presentations also give an opportunity to evaluate activities across the class, adding coherence and depth to scientific understanding of the issue.

Teacher feedback after the practical work can point out the key scientific features for the investigation plans. Feedback on the presentations will reinforce the peer assessments. Feedback on written work supports development of scientific thinking.

Resources

Props

For each student

Download Word Download PDF Which fruit contains the most vitamin C?

Equipment

For each student

  • Eye protection.

For each group of students

  • Access to the three prepared solutions
  • About 100 cm3 water
  • Fruit juices to test, with packaging if the fruit is not fresh
  • Droppers or plastic pipettes, one for each juice
  • Test-tubes, one per test, allowing six per group
  • Test-tube rack
  • 10 x 10 cm white card for background
  • Juice extractor and beakers for juice if fresh fruit is used.
  • Materials for creating a presentation such as a large sheet of paper and marker pens, OHTs and OHT pens, networked computer or writeable CD ROM and IWB access.

For the whole class

  • Access to a solution of 0.05 mol dm-3 iodine (Harmful) in potassium iodide prepared using 1.27 g iodine and 1.5 g potassium iodide made up to 100 cm3 distilled water (see CLEAPSS recipe cards for full details)
  • 0.1% starch solution
  • Vitamin C solution prepared by dissolving a 100 mg tablet in 100 cm3 water (giving a 1 mg cm-3 solution).

Safety notes

It is the responsibility of the teacher to carry out an appropriate risk assessment.

  • Eye protection should be worn.

Answers

Results examples

Fruit juice / vitamin tablet Number of drops needed to react with all the iodine
1000 mg vitamin C tablet in 100 cm3 15
Lemon juice 30
Lime 35
Orange juice from 1 day open carton 40
Kiwi 15
Melon 20
Mango 30

Note that the kiwi fruit contains the most vitamin C.

Questions

  • See results
  • See results
  • To ensure the end-point is not missed
  • The amount of oxidation ‑ vitamin C is lost by oxidation when the juice is exposed to air
  • See results and packaging

Acknowledgements

V. Kind, Contemporary chemistry for schools and colleges. London: Royal Society of Chemistry, 2004.