62 What is a 'healthy' fat? Post 16 Working in groupsSelf assessmentPeer assessmentSharing objectives and criteriaQuestioningUsing feedbackUsing tests

The addition of iodine across C=C double bonds is used as a means of showing degree of saturation of fat and oil molecules. In this activity, students discuss which fats / oils are ‘healthy’, undertake a class experiment and review case studies of obese people to decide who should get the ‘fat’ pill.

Learning objectives

Students will understand that:

  • the number of C=C double bonds present in a fat / oil molecule indicates the degree of ‘saturation’
  • iodine molecules react with C=C double bonds, showing the degree of ‘saturation’
  • the iodine number can be calculated, giving a quantitative comparison.

Sequence of activities

Prior to the session, collect different fats and oils that are used in foods. See Resources for suggestions.

Tell the students that they may bring samples of about 25 cm3 from home in screw-top containers.

Introduce (or review) the C=C double bond, indicating that this structure is present in fats and oils.
  • Pose questions to find out what students know about different fats and oils.
  • List products or substances named, dividing these into ‘healthy’ and ‘unhealthy’. &
  • Keep the list to refer to at the end of the session.
  • Ask what makes a fat or oil ‘healthy’.

Introduce the concept of ‘saturation’ as it relates to fat or oil molecules.

Indicate how this relates to the learning objectives.

Give each student a copy of What is a ‘healthy’ fat? to read through.

Explain that they will carry out the experiment to find out which fats and oils are ‘healthy’.

Organise students into groups of two to four. Supervise them as they:
  • carry out the What is a ‘healthy’ fat? experiment
  • report their experimental results to a central pool
  • agree on answers to the questions
  • elect a spokesperson to report back.
In a plenary:
  • review the results
  • invite spokespersons to give answers to questions
  • give each student the Fat information sheet
  • ask them to check if the data support their experimental findings
  • review students’ results and answers and give verbal feedback.

Introduce the next activity, treating obesity.

Give each student the sheet Who should get the ‘fat’ pill?

Circulate and support as groups:
  • consider the case studies and calculate the Body Mass Index for each case
  • suggest a course of ‘treatment’ for each person
  • agree which case study (if any) should receive Xenical, the fat pill, as part of the treatment
  • elect a spokesperson to feedback to the class.
In a plenary:
  • invite spokespersons to give their group’s responses
  • review learning
  • ask the students to write a summary of their learning about fats and oils and fat in the diet.
Take in the summaries and give written feedback based on the quality of students’ thinking.


This topic needs to be handled sensitively and teachers will need to pay particular attention to student behaviour during the group discussions about Xenical.

Assessment for learning commentary

The group discussion promotes listening skills and develops coherence in thinking. Peer review, through hearing the group responses to answers, provides students with opportunities to accept or challenge alternative viewpoints. Discussing the case studies and needing to reach agreement promotes active debate.

An assessment of how well individuals understand the learning objectives is given in the written feedback.



For each student

Download Word Download PDF What is a healthy fat?
Download Word Download PDF Fat information sheet
Download Word Download PDF Who should get the ‘fat’ pill?


For each group of students

  • About 5 cm3 of each fat (melted) or oil
  • Measuring cylinder or pipette
  • One test-tube for each fat or oil
  • Iodine 2% (Harmful) in potassium iodide solution
  • White card
  • Stopwatch
  • 250 cm3 beaker
  • Hot water (at around 70‑80 °C)
  • Test-tube rack
  • Eye protection.

Safety note

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


Investigating fats and oils

Sample results

Oil Time
/ minutes
Extra virgin olive 3.5 Could be compared with non-extra virgin. Quite a dark coloured oil, so hard to determine the end‑point.
Peanut 3.0 A very light coloured oil, so the end‑point is easy to see.
Cod liver 1.5 Short time ‑ high degree of unsaturation
Soya 1.5
Sunflower 1.0


  1. See results table.
  2. See results table.
  3. Those with the shortest times
  4. Change cooking oils to sunflower / other highly unsaturated oil; eat food with a higher proportion of unsaturated oils
  5. Find out the number of calories in each fat.

Who should get the fat pill?

BMI values:

Leanne 85.3 / (1.682) = 30.2
Fiona 78.8 / (1.752) = 32.0
Mark 95.3 / (1.752) = 31.1

All are classified as obese. Leanne is the best candidate for Xenical as there is some evidence that this drug is most beneficial for diabetics.


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