You can observe the surface tension of water through simple experiments that demonstrate the cohesive forces between water molecules.
Here's a practical way to observe surface tension:
The Coin Drop Experiment
This experiment clearly shows how surface tension allows water to resist external forces and hold its shape.
- Gather Materials: You'll need a coin (a penny or nickel works well), water, and a pipette or dropper.
- Prepare the Coin: Place the coin on a flat, level surface. Make sure it's clean and dry.
- Add Water Drop by Drop: Fill the pipette with water. Slowly and carefully squeeze out the water, one drop at a time, onto the surface of the coin.
- Observe the Water's Shape: Notice how the water initially forms a tiny droplet. As you add more drops, the droplet gets bigger and more rounded. The water molecules are attracted to each other (cohesion), creating a "skin" on the surface.
- Continue Adding Drops: Keep adding drops until the water finally spills over the edge of the coin. You'll likely be surprised by how many drops the coin can hold before the surface tension is overcome.
- Explanation: The water molecules at the surface are more strongly attracted to other water molecules below and beside them than to the air above. This creates a net inward force that minimizes the surface area and forms the rounded droplet. The surface tension allows the water to hold together, creating a bulge that extends beyond the coin's edges before finally breaking.
Other Ways to Observe Surface Tension:
- Floating a Needle: Carefully place a small needle flat on the surface of still water. If done gently, the surface tension will support the needle, allowing it to float even though it's denser than water.
- Water Striders: Observe water striders (insects) walking on the surface of a pond or lake. Their weight is supported by the water's surface tension.
- Soap Bubbles: The thin film of soapy water that forms a bubble is held together by surface tension. The soap reduces the surface tension of the water, allowing it to stretch more easily.
These simple experiments and observations illustrate the power of surface tension and the cohesive forces that govern the behavior of water.
