Cohesion and Adhesion                    adhesion


    To describe Cohesion and Adhesion simply: cohesion is water's attractiveness to itself, adhesion is water's attractiveness to other materials such as a pine needle. Cohesion exists because of the polarity of water. The water has a dipole that causes it to act like a magnet, attracting other water molecules to it. Adhesion is the attractive forces that cause water to "stick" to a surface other than its own. A good example of this is when you try to pour water from one glass to another. When you tilt the glass with the water in it the water will fall down the side of the glass instead of into the other glass. This is caused by adhesion and gravity combined, if you were to tilt the glass to a greater degree the water would no longer get suck to the side of the glass anymore because gravity is making the water go down into the other glass.
     There exists an experiment for students to see cohesion and surface tension first hand. Students drop water drops onto a penny one by one until the water can't hold itself together and surface tension fails and the water spills over the side of the penny. The number of water drops that can be placed on the penny is a factor of the water's cohesion and surface tension.
     The salt water has a much lower cohesion than plain water so it's attractive forces are less than plain water. The surface tension does increase when the salt is added to the water so that means that the penny drop experiment is mostly affected by the water's cohesive forces.



Cover Page
Phase Changes
Water & Light