Water molecules are made up of two hydrogen atoms and an oxygen atom, hence the chemical designation of H2O. Water molecules in close proximity to one another will tend to cohere together, a result of a slight charge imbalance or polarity. When molecules of the same substance are attracted to one another, that is called “cohesion;” when molecules of different substances attract, that is called “adhesion.”
In a water droplet, every part of the droplet is subjected to the same distribution of forces -- they are equally pulled in all directions - except at the surface! Since there’s no water beyond the surface to pull the water molecules outward, the molecules on the water’s surface are just pulled to the side and down by other water molecules. This phenomenon connects the surface molecules together, that is, they resist separating. This accounts for the surface tension that allows insects to rest on the surface of a body of water rather than sink even though they’re denser than the water. To push through the surface they would have to push apart the water molecules, which would require additional force. It also explains why it’s possible to float a paper clip – or even a coin - on the surface of water.
At a molecular level, atoms and molecules are configured based on the interplay and balance of attractive and repulsive forces that arise from how atomic bonds are configured, how electrons are distributed, and other aspects that characterize matter. These small-scale forces and atomic-scale effects are instantiated in observable, macroscopic behaviors like cohesion and adhesion, one of many examples of the connection between chemistry and physics.
On Earth, gravitational forces can overpower smaller-scale forces like molecular attraction or repulsion. For example, water droplets get “flattened” by gravitational effects as they fall to the ground. But, on board the International Space Station where gravitational effects are not nearly as prominent, it’s possible to directly observe more nuanced effects of surface tension and other intermolecular effects.
Sometimes forces between atoms are visualized as springs, with the spring constant – or level of tension in the spring – representing how flexible or “springy” a molecular bond might be. In this analogy, different types of bonds – ionic and covalent bonding, van der Waals, etc. – correspond to different qualities of the springs. The idea of using springs to represent bonds has some merit, because atomic bonds can store or release energy and can be configured in different ways, just like a spring. On the other hand, the spring is a model or analog; it’s not a description of the actual physical characteristics of molecular bonds. Scientists often use models, representations, analogs, and other conceptual bridges to explain or understand phenomena.
Have you ever noticed that you can fill a water glass so the water bulges over the top of the glass? How is that possible?
Imagine water droplets on board the International Space Station. How do you think they’d behave differently than water droplets on Earth?
What’s the difference between cohesion and adhesion?
Describe what happens when the water droplets cohere together.
What are the main forces that determine the water’s behavior?
Why does the water cover the astronaut’s hand like a glove? Can you think of a useful application of this effect?
What is it about water’s molecular structure that makes it exhibit cohesive and adhesive properties?
What would happen if you tried the space-based demonstrations on Earth?
How would you predict whether or not a substance would adhere to water?
Bonus Question: What would happen if the demonstrations were done using soapy water instead of plain water? What about if they were done with salt water? Would the results be different? Why or why not?