Frightened run-away pepper
We all know how glue or tape can be used to stick two objects together. These adhesives cause the two objects to cohere (or adhere) together. Cohesion is the word for this sticking force. Cohesion also occurs on common everyday objects – rain to a car’s windows, dust to the ceiling fan, chewing gum on Mom’s carpet. This experiment demonstrates cohesion in an interesting situation – and Mom’s carpet stays looking as good as new!
- Fill a dish with water.
- Sprinkle some pepper on top of the water.
- Now drop several drops of dishwashing detergent in the center of the water.
Pretty cool, huh. What happens to the pepper? Notice how the effect only occurs in the middle of the water.
The pepper stays scattered all about the water because the water is pulling on the pepper evenly from all directions. When you drop the detergent into the water, it reduces the cohesiveness between the water and the pepper. In other words, it reduces the pulling action on the pepper and the pepper appears to run away from the detergent. But the water around the edges (untouched by the detergent) still has its full pulling strength.
Cohesion is the force that holds a material together. It results from the attraction that atoms and molecules have for one another. This attraction decreases as the distance between particles increases. Thus, with few exceptions, cohesion is highest in solids. Liquids are less cohesive than solids, and gases are practically non-cohesive. Powders can also exhibit cohesion, especially if they consist of fine particles packed together. Packed dirt, for example, can be a solid driving surface because of its cohesive properties.