Here is an activity that surprises most people at first. But the resolution of the surprise can provide a meaningful understanding of Beer's law.
It is directly related to the student module 2204 Beer's law, and is demonstrated near the end of the video in that module.
- Use two identical 100 mL graduated cylinders (or any identical, parallel-sided high containers, such as pasta tubes).
- Measure 10 mL of a solution of food colouring (such as red wine or cordial) into each. [You may need to experiment with concentrations to maximize the effectiveness of the activity.] Looking side-on to the cylinders, the intensities of colour should look indentical.
- Add water to one of them,to make the volume up to 100 mL.
- Compare the intensities of colour looking side on.
- Compare the intensities of colour looking down from above the cylinders, placed side by side - preferably after asking the students to commit to a prediction.
- Engage the students in developing an explanation for the observations, with reference to Beer's law.
The self-check questions in Module 2204: Beer's law may be helpful
