The detergent lowers the surface tension of the water so it is not strong enough to stay in a dome and it will run off the five cent coin. Get the students to repeat this with another liquid: milk/lemonade/oil/orange juice and come back and combine the results.
Questions to ask them:
What is surface tension?
Do you think that the surface tensions of the liquids are the same?
Why do you think a bug can stay on top of the water?
Why can the five cent coin hold more drops of some liquids than of others?
How does surface tension and density differ?
Were your predictions correct? If not why do you think it was different?
Can you think any ways you have experienced surface tension?
Give each student a Petri dish with milk in it. Put drops of red, yellow and blue food colouring in the milk to make a triangle. Ask the students to predict what will happen when detergent is added to the middle of the Petri dish.
Get them to put 4-5 drops of detergent into the middle and record what they see. They can draw a picture in their notebooks or on the worksheet.