BACKGROUND:
Water is a transparent, odorless,
and tasteless liquid. It illustrates the three states of matter in
normal conditions on Earth including
solid (ice), gas (steam), and liquid (water). The form it takes depends
upon the temperature. At low temperatures, the molecules do not move
around as much and form a crystalline structure that is rigid (ice).
In the liquid state, water molecules move more freely. Water molecules
in the form of steam are moving very fast with large spaces between the
molecules. Although ice is crystalline, it tends to have the molecules
in a rigid structure that is spaced farther than the molecules of liquid
water and this is quite important, for if ice were denser, it would sink
in water. Imagine what would happen if icebergs grew from the bottom
of the ocean instead of floating on the surface.
Another chemical quality of water is that
water has a very large heat capacity, meaning that it can absorb a great
deal of heat without itself becoming extremely hot. This fact
makes the oceans large reservoirs of heat that greatly affect the overall
weather and climate patterns of the world.
Water's surface tension (the ability of a
substance to stick to itself) makes it an excellent substance to float
heavy objects upon. Water not only sticks to itself, but also to
other surfaces, and this allows it to move against gravity, which is very
important to plants when transporting water form the soil to their leaves.
This upward motion is known as capillarity or capillary movement.
PROCEDURE:
This lab consists of two parts.
In the first experiment, the students will experiment with "floating" a
pin on water by increasing the surface tension of the water. Experiment
II has students trying to discover why capillary action works.
- Discuss that surface tension is an
important property of water. The molecules within the bulk of a liquid
are attracted equally in all directions by the surrounding molecules.
However, the molecules on the surface of a liquid are attracted only inward
and sideways. This unbalanced molecular attraction pulls some of
the surface molecules into the bulk of the liquid and a condition of equilibrium
is reached when the surface area is reduced to a minimum. The surface
of a liquid therefore behaves as if it were under a strain or tension.
This force is called surface tension. We may define surface tension
as the force which causes the surface of a liquid to contract. A
liquid surface acts as if it were a stretched membrane. A steel needle
carefully placed on water will float.
- EXPERIMENT I.
Can a pin float?
Give the students a pin, a glass of water, and a piece of paper tissue.
Have them experiment to see if the pin will float. Don't give them
too many clues, except the definition of surface tension. After
a few minutes, illustrate how to float the pin by putting the tissue on
the surface of the water, then laying the pin gently on top, and then allowing
the tissue to sink to the bottom of the glass. The pin will float as the
tissue drops to the bottom. The students have to be careful not to
break the surface tension or else the pin will sink.
- EXPERIMENT II.
One of the reasons
why water rises in thin capillary tubes is due to surface tension.
Water is brought up to the surface of the soil and to the roots of plants
by this action. The thinner the tube the higher it will rise.
You may want to use food coloring to help see the rise easier.
- After the students complete the experiments they
will realize that capillary action is caused by surface tension and
that in a thin capillary tube, the water molecules climb up the sides of
the tube because of surface tension.
