CECsci.212


TITLE:  TEMPERATURE AND WATER DENSITY

AUTHOR:  Steve McFarland, Decker Lake Youth Center;
         Salt Lake City, UT

GRADE LEVEL:  Appropriate for grades 7-8.

OBJECTIVE(s): Students will be able to:
1.  Explain the density of water as it relates to
    temperature.
2.  Describe the dynamics of ocean currents as they
    relate to temperature and density.
3.  Identify the forces governing convection in liquids.

MATERIALS:
Teacher materials --- a pair of two-liter plastic soda
bottles, two plastic straws, a glue gun with glue sticks, a
hot plate, green and red food coloring, a pitcher in which
hot liquid can be poured, a bowl large enough to contain one
liter bottle, crushed ice, and a cutting instrument.

ACTIVITIES AND PROCEDURES:
Cut each two-liter bottle such that each is approximately
eight inches tall. Drill or punch one hole about two inches
from the bottom, and the other about six inches from the
bottom, in each of the bottles. The holes should be one
above the other, and just large enough for the plastic
straws to fit into snugly.

Place the straws into the holes horizontally, joining the
two bottles both at the two inch and six inch levels. With
the glue gun, secure the straws to each bottle opening such
that no liquid could leak from the holes.

Boil about two liters of water, and chill another two liters
with ice. Place one of the two-liter bottles in a bowl, and
add ice inside the bowl, but outside the two-liter bottle
(this will keep the water that you add to one bottle cold
during the demonstration).

Add red food coloring to the hot water, and green to the
chilled water.  Pour the hot water into one two-liter
bottle, and add cold water into the two-liter bottle that
you have prepared with ice.

Observe the cold green water travel through the lower straw,
while the hot water travels through the upper straw. In the
end, each bottle will have a distinct layer of hot red water
on top, and a cooler green layer of water on the bottom.

TYING IT TOGETHER:
Students have difficulty believing that warm and cold water
will actually separate, since in each case they are dealing
with just plain old water.  As the experiment progresses, it
becomes apparent that the colder water sinks, and the hotter
water rises to the top.  It also becomes clear that water
will move when temperature differences exist.  The student
no longer has trouble visualizing the concept of ocean
currents as they relate to temperature differences.  This
activity can also be used to help students add to their
understanding of migration patterns and habitats of sea
creatures.