Lesson
Purpose of the work:
- To determine the presence of unsaturated bonds in vegetable oils using a qualitative reaction with iodine.
Expected results:
After completing the work, students can:
- develop teamwork skills
- be able to analyze and summarize the information received
- draw logical conclusions
Teacher’s Guide:
- The task is performed in pairs.
- Before starting laboratory work, please read the safety rules by following the link:
- To download the worksheet, follow the link:
Theory
Vegetable oils are esters of glycerol and fatty acids. Fatty acids can be saturated (without double bonds) or unsaturated (with one or more double bonds).
The presence of double bonds can be detected by their reaction with halogens (e.g., iodine solution). If unsaturated bonds are present, the iodine solution becomes decolorized.
In industry and laboratories, this reaction is used to determine the iodine value of vegetable oils: the more unsaturated bonds present, the stronger the decolorization and the higher the iodine value.
Practical part
Step 1. Put on gloves. Pour 3-4 ml of water into three test tubes.
Step 2. Add several drops of iodine solution to each test tube and gently mix.
Step 3. To the first test tube, add 2 drops of vegetable oil and shake it. You may notice that iodine and oil do not mix because they belong to different phases.
Step 4. To make them mix, add a drop of detergent. Close and shake vigorously for 5–10 seconds.
Step 5. For control, to the second and third test tubes, add 2 drops of ethanol and 2 drops of glycerol respectively. Mix slightly.
Step 6. Observe the changes and after 5–10 minutes compare the degree of decolorization.
The reaction of vegetable oil with iodine shows that it contains fatty acids with unsaturated C=C double bonds. Like alkenes, such acids can undergo addition reactions at the double bond, decolorizing iodine or potassium permanganate solution. Ethanol and glycerol do not have such bonds and therefore do not cause decolorization.
Conclusion
During this work, students learned to perform a qualitative test for unsaturation using vegetable oil and iodine. They concluded that organic compounds with double bonds react with iodine, leading to decolorization.