Cohesion and
Adhesion
https://water.usgs.gov/edu/adhesion.html
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.
http://vhmsscience.weebly.com/lab-write-up.html