Biology/Chemistry
Changing
the Freezing Point of Water with Salt
Shirley
Hatcher |
Daniel Hale
Williams School |
2710 South
Dearborn |
|
CHICAGO IL
60616 |
|
|
(773) 534-9226 |
Objective(s):
To
teach first grade students that salt water freezes at a lower
temperature than
plain water
To
determine the temperature at which water freezes
To
understand why salt water freezes at a lower temperature than plain
water
To
understand why the string sticks to the ice cube when salt is placed on
the
cube
To
compare student hypotheses with the actual results of the experiment
Materials:
Classroom
graph, glass, string, water, ice cube, salt and thermometers
Strategy:
Each
student will receive a cup with ice and a piece of string.
Float the ice cube on water filled to near
the top of the cup. Place the string
across the ice cube, and then give each student a teaspoon of salt to
pour on
the ice cube and string. Each student
will determine if the string freezes, sinks, does nothing or disappears.
Give
each student a cup containing ice and water with about 2/3 ice and 1/3
water. Then gently stir with a
thermometer and record the temperature every 2 minutes for about 6-8
minutes. Then add one teaspoon of salt and
repeat the
process. Add one more teaspoon of salt
and repeat the process one final time.
Performance
Assessment:
Have
twenty students read and discuss material about the weather,
particularly cold
weather, snow and ice. Give students a
pretest asking them to predict the outcome of what will happen to the
strings
when salt is placed on the ice cubes and what will happen when the salt
is
added to the ice-water mixture. Next
the students do the two experiments.
Give a post-test to the students after the experiments are
finished. Chart all students’ results from
the second
activity on a class graph. Discuss the
graph with the class focusing on mathematical concepts.
Conclusions:
The
majority of the students should understand the concept of water
freezing at 32
degrees Fahrenheit (0 degrees C). When
salt is sprinkled on the ice cube, it lowers the freezing point to
below 32
F. Since the ice cube cannot get any
colder than it already is the surface starts to melt.
The string lies in the puddle of melted salt water.
As the salt water is diluted by further
melting of the ice cube, the freezing point rises.
The water refreezes attaching the string to the ice cube. This
“depression” by salt of the freezing point of water is also seen in the
second
activity. A mixture of plain water and
ice is in “equilibrium” at 32 degrees F, but addition of salt lowers
the
equilibrium temperature (as seen with the ice cube experiment). Within limits the more salt is added to the
ice-water mixture, the lower the equilibrium temperature. This
principle
underlies the use of salt to melt ice on roads during the winter and
also the
use of ice and salt in an old-fashioned ice cream maker.
References:
Greydanus,
Rose, Now I Know Changing Seasons 1983 Troll Associates
Palazzo,
Janet, Now I Know What Makes The Weather 1982 Troll Associates
Science Experiments/Volume 2 1991 Evan-Moor Corp.