Measure the level of ‘saturation’ of fats and oils using iodine, exploring the relation to obesity, with this lesson plan and experiment for 16–18 year olds

In this activity, students discuss which fats and oils are ‘healthy’, undertaking a class experiment that uses the addition of iodine across C=C double bonds to test saturation. Students then review case studies of obese people in connection with diet pills to decide who could be prescribed the medication orlistat or Xenical.

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

Before the lesson

Prior to the session, collect different fats and oils that are used in foods. See the ‘Sample results’ in the ‘Answers’ section below for suggestions.

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


  1. Introduce (or review) the C=C double bond, indicating that this structure is present in fats and oils.
  2. Pose questions to find out what students know about different fats and oils.
  3. List products or substances named, dividing these into ‘healthy’ and ‘unhealthy’. Keep the list to refer to at the end of the session.
  4. Ask what makes a fat or oil ‘healthy’.
  5. Introduce the concept of ‘saturation’ as it relates to fat or oil molecules.
  6. Indicate how this relates to the learning objectives.

What is a ‘healthy’ fat? experiment

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:

  1. Carry out the ’What is a healthy fat?’ experiment.
  2. Report their experimental results to a central pool.
  3. Agree on answers to the questions.
  4. Elect a spokesperson to report back.

Plenary 1

In a plenary:

  1. Review the results.
  2. Invite spokespersons to give answers to questions.
  3. Give each student the Fat information sheet.
  4. Ask them to check if the data support their experimental findings.
  5. Review students’ results and answers and give verbal feedback.

Activity: Who could be prescribed medication for obesity?

Introduce the next activity, treating obesity. Give each student the ’Who could be prescribed medication for obesity?’.  

Circulate and support as groups:

  1. Consider the case studies and calculate the Body Mass Index for each case.
  2. Suggest a course of ‘treatment’ for each person.
  3. Agree which case study (if any) should receive the medication Xenical as part of their treatment.
  4. Elect a spokesperson to feedback to the class.

Plenary 2

In a plenary:

  1. Invite spokespersons to give their group’s responses.
  2. Review learning.
  3. 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.


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.

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

Practical notes


For each group of students:

  • Eye protection
  • Measuring cylinder or pipette
  • Test tubes, one for each fat or oil
  • White card
  • Stopwatch
  • Beaker, 250 cm3
  • Test tube rack


For each group of students:

  • Samples of fat (melted) or oil, about 5 cm3 of each
  • Iodine 2% (HARMFUL), in potassium iodide solution
  • Hot water (at around 70–80 °C)

Health, safety and technical notes


What is a ‘healthy’ fat?

Sample results table

OilTime/ minutes

Extra virgin olive1




Cod liver3






Sample results notes

  1. Could be compared with non-extra virgin. Quite a dark coloured oil, so hard to determine the end‑point.
  2. A very light coloured oil, so the end‑point is easy to see.
  3. Short time – high degree of unsaturation.


  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 could be prescribed medication for obesity?

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.