BACKGROUND:
Water remains a liquid over a very
wide temperature range, namely, a range of 0°C to 100?C, between
freezing and vaporization. This spans the temperatures of most
parts of the Earth where life can occur.
Water has a very high specific heat, which
means that it can absorb or lose much heat before its temperature changes.
This is important in maintaining body heat in mammals, such as ourselves.
It takes a lot of energy to start evaporation because water has a high
latent heat of vaporization. For this reason, water evaporates slowly
from ponds and lakes, where many life forms are dependent on it.
Water is less dense in its solid state than
in its liquid state, so that ice floats instead of sinking.
This property permits life to develop in polar regions and subpolar regions
where ice floats and allows life to continue living below the surface.
If ice were heavier than water, it would sink, and more ice would form
on top of it. As a result, all life in the waters would be trapped
in the ice in the many areas of the world where it gets cold enough to
freeze water.
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 exercise consists of two parts.
First, the students will discover how water flows and secondly, the
students will discover how many drops of water they can put on a penny.
In the first
exercise give each child
two cups. Have them measure 1 cup of water into one cup.
Let them hold both cups and ask them the following questions:
Which
cup contains a fluid?
Which cup contains melted ice?
Which
cup contains air?
What is the fluid?
- Next ask the students to pour the
liquid. Ask them to describe the liquid. The water is flowing;
it is a fluid called water.
In the second exercise give each student
an eyedropper and penny. First discuss surface tension by using the
following illustrations. Surface tension is what allows a water strider
(insect) on water not to sink and explains why heavier items in water,
float. In this exercise the students will experience surface tension
by seeing how many drops of water they can put on a penny with an
eyedropper. Ask students to predict how many drops can fit on a penny.
You are going to have to illustrate and demonstrate very carefully how
to use an eyedropper.
- Using an eyedropper, gently let water
drops fall into the center of the penny. Count how many you can put
on before the water runs off. (It will be between 20 and 34.)
Let the children try this. Their numbers will be lower than yours,
and will change depending on whether they are letting the drops fall into
the center of the penny or on the edge.
- You may want to extend this lab by
seeing how many drops they can place on a nickel. Go over their results.
You may also want to repeat the experiment with alcohol. It has a
much lower surface tension and will not be able to hold as much liquid.
- The key concept here is to have the
students experience surface tension and to be able to observe surface tension
when they leave the classroom.
