Optical Activity of Sugar

This demonstration shows that sugar solutions are optically active and can rotate plane-polarized light, while water cannot. Using polarizing filters and an overhead projector, the optical activity of sugar is made visible.

1. Cut two square holes in each cardboard sheet, spaced evenly apart.
2. Tape polaroid films over the holes, ensuring one sheet has the polarizing direction at 90° to the other.
3. Dissolve as much sugar as possible in 200 mL water in one beaker, heating to make a concentrated syrup. Leave the other beaker with plain water.
4. Place one polaroid sheet on the overhead projector and set the beakers on top, each over one of the polarized light squares.
5. Observe both solutions as light passes through.
6. Place the second polaroid sheet above the beakers aligned with the first sheet at 90°.
7. Observe that light only passes through the sugar solution, showing optical rotation.

Optical Rotation Demonstration Polaroid and Glucose - Farnborough Chemistry:

📄 Optical activity of sugar (Demonstration 17, page 36) - Dr Magdalena Wajrak and Mr Tim Harrison: https://www.ecu.edu.au/__data/assets/pdf_file/0003/910443/Chemical_Demonstration_Booklet_Interactive_Final_14.2.21.pdf

• Use glucose or fructose instead of sucrose and compare the degree of rotation.
• Try different concentrations of sugar to see how the brightness changes.
• Replace the overhead projector with a laser pointer and polarizing lenses for a smaller-scale version.

• Handle the hot plate carefully to avoid burns.
• Prevent sugar solution from boiling over onto the hot plate.
• Allow hot sugar solution to cool before disposal.
• Wear safety glasses to protect against splashes.

• Why does light pass through the sugar solution but not through plain water? (Because sugar is optically active and rotates polarized light, while water is not optically active.)
• What property of sugar molecules makes them optically active? (Their chirality – sugar molecules lack symmetry and exist in enantiomeric forms.)
• How would the demonstration change if a racemic mixture of sugar is used? (There would be no net rotation of polarized light, so no light would pass through.)
• What happens if you increase the concentration of the sugar solution? (The rotation of light increases, making the transmitted light brighter or more visible.)