The Correct Answer and Explanation is:
Based on the principles of surface tension, here is the correct prediction and explanation for the lab.
Prediction: The penny smeared with detergent will hold significantly fewer drops of water than a clean penny, likely less than ten.
Explanation
The outcome of this experiment is governed by the properties of water, specifically cohesion and surface tension, and how detergent disrupts them.
Water molecules exhibit a strong property called cohesion, which is the attraction between similar molecules. Because water is a polar molecule, with slight positive and negative charges, its molecules form strong hydrogen bonds with each other. At the surface of the water, where it meets the air, there are fewer water molecules to bond with above. This causes the surface molecules to be pulled more strongly inwards and sideways by the molecules below and next to them. This inward pull creates a tight, elastic-like film on the water’s surface known as surface tension. On a clean penny, this high surface tension allows the water to bead up and form a large, stable dome, holding many drops before the force of gravity overcomes the cohesive forces and the water spills.
Detergent, however, is a surfactant. This means it is a substance that reduces the surface tension of a liquid. Detergent molecules have a unique structure with two distinct ends: a hydrophilic head that is attracted to water and a hydrophobic tail that is repelled by it. When detergent is introduced to water, the hydrophobic tails try to escape the water by moving to the surface. In doing so, they push their way between the water molecules, disrupting the strong hydrogen bonds that hold them together.
By breaking up these cohesive forces, the detergent drastically weakens the water’s surface tension. The tight, elastic film is broken. As you add drops of water to the detergent coated penny, the water can no longer hold its domed shape. Instead, it spreads out thinly and spills over the edge of the penny much more quickly. Consequently, the penny will only be able to hold a very small number of drops before overflowing.
