BACKGROUND:
The majority of the Earth’s water
can be found in oceans. The water however is not pure. It has
other components, for example compounds, elements, or small particular
matter, that have been dissolved in the water over eons of time, as water
eroded the Earth’s surface. Young children do not realize that water
fills the low spots on the Earth’s surface. Islands are not floating
in water, but high mountains surrounded by very low valleys.
Salt water is not good for humans. The
salt in the water makes the liquid undrinkable to humans because our body
cannot take out the salt. Too much salt in our bodies is not good
for it. Salt and fresh water and very different.
The term salt water is misleading because
it contains more than just table salt (sodium plus chlorine). It
also has other compounds, including sodium plus iodine and bromine.
The term seawater is also used to refer the ocean’s water.
PROCEDURE:
- Explain the differences between
fresh and salty water. Some students may misunderstand the word "fresh"
and think it is naughty water. You may want to show students on a
globe or map that oceans are salty and lakes are fresh. Slowly say
the oceans, Atlantic, Pacific, Arctic, and Indian are salt water.
Point out the Great Lakes in the United States as fresh water. If
you have local bodies of fresh water, discuss them.
- It is very difficult for students
to "see" what is in water. In this demonstration the students will
learn that clear water may have something in it. Start off this pre
lab by discussing and describing water. Ask the students how they
can tell if they have salt water or fresh water.
- Mix salt and water, epsom salt and
water, and plain water. Make sure that all the salt and epsom salt
is dissolved by using warm water. Mix the ingredients before showing
the children. Have students predict which sample is clean, and which
samples with nothing visibly in it. Record their answers on
the board.
- Put some dishes out in a place where
the water can evaporate. Have students look at the dishes every day,
asking students the same questions. If you have a hot plate
or if it is a warm day, you should be able to do this quickly. Make
sure you make a thin layer of liquid in the dishes. Go back to their
predictions to see if they guessed right.
