BACKGROUND:
Salt water is made during the movement
of water on land over time. The components of rocks are eroded and
become part of a "salty" solution. Remind students that the word
"salt" refers not just to table salt, but to many chemicals that
are classified as salts. Salt water or seawater has characteristics
similar to fresh water with some noticeable differences because of the
salts that are dissolved in water. The viscosity (i.e., internal
resistance to flow) of seawater, for example, is higher than that
of fresh water because of its higher salinity. The density of seawater
also is higher. Seawater's freezing point is lower than that of pure
water and its boiling point is higher.
It is easier to float objects in seawater
than in fresh water. Many humans can “float” in seawater but not
fresh water. The reason is that water with salt in it, is more dense.
So objects that cannot float in fresh water may be able to float in seawater.
Floating refers to having a substance rise to the top of another substance.
For example, a balloon with helium will float in air because it is lighter
(or less dense). If you forced an object to go under the water and
let it go, it will float to the top.
PROCEDURE:
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Use the diagram below to
discuss how "salts" are eroded from land. Ask the students
if clouds contain salts. The students should be able to realize that clouds
do not have salts in them because only fresh water can evaporate into a
gas.
-
Water can evaporate from a "salty"
water solution by leaving the salts behind. Ask the students if there
are any other bodies of water besides the oceans that are made up of salts.
The Great Salt Lake in Utah and the Caspian Sea in Russia both are salty.
-
Discuss with students the different
properties of pure water and salt water. Salt water is denser, conducts
electricity because of its ionic content, and chemically has more ingredients.
-
Demonstrate with your students how
the density of water can be changed by using salt. Place a fresh
egg in a glass of fresh water and the egg will sink to the bottom.
Stir salt into the glass of water (about 100 ml to 250 ml of water).
Replace the egg and this time it will float because you have increased
the density of the water.
-
Ask the students if it is easier
to float in sea water or fresh water. Salt water can make any object
float easier because it is denser than fresh water, therefore it can support
bigger objects.
