Adhesives make objects cohere (or adhere) together. Cohesion is the word for this sticking force. Cohesion occurs on lots of everyday objects – rain to car windows, chewing gum on the carpet, dirt to you when you have been in the garden. This experiment demonstrates cohesion (and does not leave a mess like the chewing gum!)
What you will need
- Spice (sage, chopped parsley, thyme or any other spice that sprinkles on water and floats)
- Dishwashing detergent
How to do it
- Fill a dish with water.
- Sprinkle spice on top of the water.
- Observe how the spice distributes itself over the water.
- Now drop several drops of dishwashing detergent in the centre of the water.
What happens to the spice?
Notice how the effect only occurs in the middle of the water.
The spice stays scattered all about the water because the water is pulling on the spice evenly from all directions. When you drop the detergent into the water, it reduces the cohesiveness between the water and the spice. In other words, it reduces the pulling action on the spice and the spice appears to run away from the detergent. But the water around the edges (untouched by the detergent) still has its full pulling strength.