The Penny’s Unexpected Water Capacity
A simple experiment reveals a surprising phenomenon: a small penny can hold far more water than expected. Instead of overflowing after just a few drops, water often piles up on the penny’s surface, forming a distinct, rounded dome. This challenges the intuitive understanding of how liquids behave.
During this experiment, water gradually accumulates on the penny, creating a convex, almost bubble-like shape that rises above the coin’s edge. This dome can continue to grow, holding a significant volume of water, before finally spilling over. While the exact number varies depending on factors like the specific penny’s condition, the water temperature, and the method of adding drops, a typical penny can often support anywhere from 20 to 40 drops of water.
The Science Behind the Phenomenon
The penny’s ability to hold a domed mass of water is primarily due to surface tension. This arises from strong attractive forces that exist between water molecules. Inside the bulk of a water droplet, molecules are surrounded and pulled in all directions by neighboring molecules. However, at the surface, water molecules are only pulled inward and sideways by other water molecules, as there are no water molecules above them to exert an upward pull. This inward pull creates a net force that minimizes the surface area of the liquid, effectively forming a taut, elastic-like “skin” on the water’s surface.
These attractive forces between like molecules are called cohesive forces. In the case of water, hydrogen bonds create particularly strong cohesive forces, allowing water molecules to cling tightly to each other. Adhesion, which is the attraction between water molecules and the surface of the penny, also plays a role, though cohesion is the dominant force in forming the dome. The penny’s relatively flat and non-absorbent surface provides a stable base, allowing these strong cohesive forces to resist gravity and maintain the dome shape until the weight of the water finally overcomes the surface tension.
Observing Surface Tension in Everyday Life
The principle of surface tension is at work in many everyday observations. One example involves water striders, insects that walk across pond surfaces. These insects possess specialized, water-repellent legs that distribute their weight over a large enough area, allowing the surface tension of the water to support them without breaking through. The water’s “skin” acts as a temporary, flexible platform.
Another instance involves water beading up on waxy surfaces, like a freshly waxed car or plant leaves. On these hydrophobic surfaces, the cohesive forces between water molecules are much stronger than their adhesive forces to the surface. This causes the water to pull itself into spherical droplets, minimizing its contact area with the non-wetting surface. Similarly, the effectiveness of soap in cleaning dishes illustrates how surface tension can be altered. Soap molecules disrupt water’s cohesive forces, reducing surface tension and allowing water to spread more easily to penetrate grease and grime.