BACKGROUND:
Water is a very versatile substance; it can
be a solid, liquid, or gas. It has the ability to change into all these states
of matter that make water ideal for making different types of weather.
Children do not understand that water is a common component of air and can be
"pulled" out.
Condensation which is what this sweat on a
glass is called, produces dew. Dew does not fall like rain. It is just water
vapor that becomes liquid on a solid surface that has been cooled below a
certain temperature. This cooling usually happens during a clear, cool night,
but disappears once the morning Sun evaporates the dew. Frost is formed like
dew, but at temperatures below freezing. The water vapor changes directly to
small, fine frost crystals without condensing into water drop first.
The dew point is the temperature at which
the atmosphere is saturated with water vapor. A given volume of air containing
higher amount of water vapor has a higher dew point than the same volume of
drier air. The dew point gives an indication of the humidity. In meteorology
the dew point is applied in the prediction of where and when clouds will form.
In this lab the students will take a
close look at what evaporation, condensation, and precipitation mean, by
actually creating the different phases. These words are best defined by
watching the phenomena. Otherwise, the terms mean very little. Although
students live through weather system after system, they rarely observe what is
actually occurring.
PROCEDURE:
1. Go over the Water Cycle and make sure
students understand the vocabulary words.
2. Read Snow Crystals and Flakes
and discuss that snow is frozen vapor. Snow crystals can be the same,
especially when they are small. However, snowflakes which are crystals
that adhere are probably not ever the same. At the end of the poem you
can see the pattern how snowflakes come from the clouds. The first
time snowflakes were photographed were in the early 1920's by Wilson A.
Bentley, known worldwide as the "Snowflake Man."
3. The lab helps the students understand
condensation and precipitation. If you do these examples slowly, students
will be able to see these phenomena occurring all around them, just by
observing!
Experiment 1.
Place some ice in a small
clear dish or watchglass. Pour very hot water (almost boiling) into a beaker.
Place the crucible on top of the beaker. In a little while the water will
condense on the upper side of the crucible and a few drops will fall as
"rain." This is an example of condensation.
Experiment 2.
Fill a beaker about 1/3 full
of water. Add 1 piece of ice and place the thermometer in the beaker. Wait a
few minutes. If there is no condensation on the outside of the beaker, add
another piece of ice. Continue until "dew" forms on the outside of
the glass. Record the temperature at this point, this is called the "dew
point."
Experiment 3.
Put two scoops of ice into a
beaker and one scoop of salt. When you start to see ice form on the outside of
the beaker, measure the temperature of the ice and salt mixture. This is the
temperature it would have to be on this day for frost to form. The salt is
added to quickly melt the ice into water vapor, frost is nothing more than
frozen water vapor. When you make ice cream, the salt is used in a similar
manner.
