In this make and take, students move their boats by varying the water's surface tension with soap.

The attraction of water molecules to each other (cohesion) can be weakened by the addition of soap/detergent. The attraction between water particles at the water's surface produces surface tension. The soap reduces the surface tension of the water around it.

You can use cohesion to make a boat powered by surface tension.

The placement of the soap in the small notch at the back of the boat means that the only way for the soap to disperse is by moving out the back, causing the boat to move across the water.

The boat moves by being pulled forward by water molecules rather than being pushed by soap molecules. Since the surface tension is greater at the front of the boat than at the rear, it pulls the boat forward.

Once the soap has spread out across the surface of the water, the boat will stop moving forwards.

### Objectives

• Describe the role of surface tension in keeping water molecules together.

### Materials

• Per Group:
clean and detergent-free container of water (or sink with a plug)
bottle of liquid detergent
dish

• Per Pair of Students:
a 2L milk carton (1 for every 8 boats)
scissors
dish soap
eyedropper

### Key Questions

• How does soap affect the water's surface tension?
• Is the boat pulled forward or pushed forward? How?

### What To Do

Preparation:

1. Empty liquid detergent into the dish.
2. Cut the 2L milk carton lengthwise along the 4 long folds.

Instructions:

1. Cut out boats from the milk carton using the template provided. Optional: pre-cut the cartons before the class.
2. Cut a small notch at the back of the boat in the middle of the rear edge.
3. Place the boat near the edge of the container or sink full of water.
4. Use an eyedropper to drop a small amount of detergent in the notch at the end of the boat. The boat should move forward.

### Extensions

• Try this with different kinds of liquids (i.e. milk, vinegar).
• Does the boat still move forward in different liquids?
• Try cutting out different shapes of boats. Does the shape of the boat affect its ability to move?
• Try cutting out different sizes of boats. How does the size of the boat affect its ability to move?
• Sprinkle pepper on the water before the launch so you can see the water move
• Draw a cartoon that shows the breakdown of the surface tension behind the boat and how the boat is pulled forward.

### Other Resources

Science World | Boat Template PDF

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