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Download and ShareWater 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.
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Bob Thirsk, Astronaut, Canada Space Agency: “Hi, I’m Bob Thirsk, an Expedition 20/21 flight engineer aboard the International Space Station, and I’m speaking to you from the Columbus Laboratory Module. The topic for today is surface tension of water and intermolecular forces. Molecules of liquid attract and repel each other through intermolecular forces. These forces are responsible for cohesion, which means a liquid sticking to itself, and adhesion, which means a liquid sticking to another material. Molecules at the surface of a liquid experience unbalanced molecular forces. Since they are on the boundary, the molecules are attracted from the inside, but not the outside. This causes the surface to behave like a stretched elastic membrane.
Here in space, surface tension is critically important, even for something as simple as getting a drink of water. If I poured out this drink of water on the Earth, gravity would smush it flat. But, here in orbit, it floats in mid-air. It makes a perfect sphere. Surface tension arises because tiny forces act in between individual molecules of water. These intermolecular forces are like tiny springs, connecting all the H2O molecules. If this sphere of water is stretched slightly in one direction, the tiny springs pull it back to the original direction. Stretched…and back to a sphere. The water always tries to be in a sphere, because this way the little springs are all stretched the same amount; none have to work harder than another. To share the work evenly, the water takes the shape of the least surface area, which for a freely-floating object, is a sphere.
Intermolecular forces cause water to be attracted to itself, which is called cohesion. For example, here are two spheres of water. When the spheres are several centimeters apart, there are no attractive forces between them. But, when I bring them close enough together to attract, they merge through their attractive forces.
Intermolecular forces can also cause water to be attracted to other materials. This is called adhesion. For example, water can stick to a straw. When I move the straw close to the sphere, the water sticks. Water sticks to many other materials as well. Think about all the things on Earth that you see little water drops on: windows, tree leaves, even your own skin.
Let me show you with my hand, the molecules in my own skin will pull hard enough to make the water stick. The water tries to find a balance between the cohesion with the water and the adhesion to my hand. Eventually, it settles on an unusual shape that fills the space between my fingers. If I continue by adding water, I can eventually build an entire glove of water that will stay fixed to my hand. It sticks to my hand by intermolecular forces. How cool is that?
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