is defined as the force that acts on the surface of a liquid and
tends to minimize the surface area. Water will bead up because
the surface tension is pulling it into a spherical shape, which
has less surface area than other shapes. This is why rain drops
and water drops are round.
on the right shows how surface tension pulls water into a round,
or spherical shape. The red arrows show how the water is pulled
in toward the middle of the water drop.
Tension Pulls Water into a Spherical Shape.
Surface of Water
creates a thin elastic film on the surface of water. This film will
support small objects so that they will sit on top of the water
without sinking. It is possible to float paper clips and sewing
needles on the top of the water if it is done carefully. This is
not easy to do, however. One trick that works well is to take a
small piece of tissue paper and lay it on the surface of the water.
Put the pin, paper clip, or needle on top of the floating tissue.
Lay them sideways on the tissue. The tissue will soak up water and
sink leaving the pin, paper clip, or needle floating on the surface.
image shows a paper clip floating on water. The film caused by surface
tension can be seen along the edges of the paper clip.
Tension at the Molecular Level
A water molecule
has one oxygen atom (O) and two hydrogen atoms (H). There is a partial
negative charge on the oxygen atom end and a partial positive charge
on the hydrogen atom end as shown in the diagram to the right. Positive
and negative charges attract each other and try to pull together.
The negative end of the water molecule is attracted to the positive
end of other water molecules. Under the surface of water these attractions
balance out, but on the surface they are unbalanced. This is because
the molecules on the surface have one surface exposed to the air
and no other water molecules to balance out the charges. So, the
surface molecules are pulled inward and surface tension is created.
To learn more about how this works click the button below. It opens
an animation that shows how surface tension works at the molecular
level. The animation will open in a new window.
Up Surface Tension
can be broken by adding soap to water. Soap disrupts the surface
tension causing the thin film to split and fly apart. The button
below opens a short video that shows how soap affects surface tension.
The video shows pepper floating on the surface of water in a bowl.
The tip of a tootpick is dipped in liquid dish soap and touched
to the surface of the water. The pepper moves quickly away from
the toothpick and goes to the edges of the bowl. The video is short,
but may take a few minutes to load on a 56 K modem.