Observations and results
Did the raft move forward, away from the end where the sponge piece was placed?
You should have seen that when you added a drop or two of detergent on the sponge piece of the raft, the raft should have quickly moved away from the side where the detergent was added, propelling it forward through the water.
When detergent is added to water, it decreases the surface tension of the water. Compounds that lower water’s surface tension are called surfactants, which work by separating the water molecules from one another. In this activity the surface tension is lowered in the area where the surfactant is added, and the higher surface tension in front of the raft pulls the raft forward. As the detergent spreads through the water, it decreases the surface tension throughout the water, and the raft stops moving eventually because there is no longer a difference in the surface tension. How do you think a surfactant such as dish soap help to clean dirty dishes?
More to explore
Introduction to Surface Tension, from Massachusetts Institute of Technology
Sticky Water, from Exploratorium
Fun, Science Activities for You and Your Family, from Science Buddies
Build a Raft Powered by Surface Tension, from Science Buddies