Cohesion refers to the attraction between molecules of the same substance. In water, this is due to the hydrogen bonds formed between the slightly positive hydrogen atoms of one water molecule and the slightly negative oxygen atom of another. These strong bonds hold water molecules together, creating a strong surface tension, allowing things like insects to walk on water.
Adhesion refers to the attraction between molecules of different substances. Water molecules have a strong affinity for other polar molecules, like those found in glass or wood. This is again due to the polar nature of the water molecule and the ability to form hydrogen bonds with these other substances.
Here's a breakdown of how these forces work in everyday examples:
* Cohesion:
* Surface tension: The strong cohesive forces between water molecules create a surface tension that allows objects like a paperclip to float on water.
* Water droplets: The tendency of water molecules to stick together forms spherical droplets, minimizing their surface area due to cohesion.
* Adhesion:
* Capillary action: The attraction between water molecules and the walls of a narrow tube (like a plant's xylem) causes the water to rise against gravity, due to adhesion and cohesion working together.
* Wetting: Water molecules readily adhere to polar surfaces like glass, making it "wet." This contrasts with non-polar surfaces like wax, which repel water.
In summary:
* Cohesion: Water molecules stick to each other because of strong hydrogen bonds.
* Adhesion: Water molecules stick to other polar substances, also due to hydrogen bond formation.
These forces, combined with the unique properties of water, are essential for life on Earth, facilitating processes like transportation of nutrients in plants, regulating temperature, and supporting aquatic life.
