Biology/Chemistry

 

Strong Ice 

 

Barbara Pawela

Retired

5730 S Kensington

 

COUNTRYSIDE IL 60525

 

708-482-7908

 

 

Objective(s):

 

Grades 6-8:  To have the students realize that, although almost all solids and liquids expand when the temperature is raised, water is one of the few substances that expands as it freezes.  A quantity of water will occupy more space as a solid than it will as a liquid.  The expansion of water as it freezes into ice occurs because hydrogen bonding pulls the molecules into an open crystalline structure that occupies more space than liquid water.  The release of heat in the change of state causes a transfer of energy.  If the system expands, it is doing work on the surroundings and energy is being transferred from the system.

 

 Materials:

 

Activity 1:  freezer, 4 cans of diet soda and 4 cans of regular soda, 4(quart-size) Ziploc bags,

                   small plastic pot with cover, small tin with lid, 3 pencils, pop top, sticky tape

Activity 2:  a balloon, 5 plastic glasses, warm water, 5 INSTANTS expandable capsules

Activity 3:  small glass vial or bottle with screw on cap, empty can, salt, water, ice

 

 

Strategy:

 

Activity 1

Two days prior to lesson put each can of pop into a Ziploc bag and place into a freezer.  Fill the plastic pot to the brim with water, put the cover on, and place into a freezer.  Fill a tin to the brim with cold water.  Press on the lid as hard as you can, without spilling the water.  Put a bottle top on the lid.  Put two pencils under the tin and one pencil on the bottle top.  Wind sticky tape around the two bottom pencils on both ends looping it around the top pencil. (See illustration on following page)  Carefully place the tin into a freezer.  

Begin the presentation by having the students examine the soda pop cans in their Ziploc bags.   

         

 

 The contents of the cans should freeze and alter the shape of the cans.  Some of the cans may even have burst (diet Pepsi often does).  The cans can be left frozen, or they can be thawed and the liquid poured out of the cans that may have burst.  Display the cans in their Ziploc bags in the class and have the students examine them.  Tell the students to describe what they see.  Ask them to hypothesize as to what could have caused the cans to alter their shape or even split some cans.   Tell the students that the cans had been frozen.  Discuss the relationship of matter, energy, and the giving up of heat in the change of phase.  Discuss and conclude that there was a transfer of energy and forces from the system were at work.  If the system expands it is doing work on the surroundings and energy is being transferred.  Examine the plastic pot.  (When the water freezes, it should lift the cover.)  Examine the tin and the pencils. (The lid should have been pushed up by the ice and the pencil may have broken.)

 

Activity 2

Divide the students into five groups.  Fill the plastic glasses or cups ¾ full with warm water and

give each group one INSTANTS capsule.  Drop the capsule into the cup.

The capsule will expand after about twenty seconds and will continue to expand for a while.  Observe, discuss and, conclude that the capsule expanded or, in other words, its volume increased.

Have a student blow up a balloon.  Again discuss and conclude that the balloon expanded.

 

Activity 3.

Fill an empty can with cold water, ice, and a teaspoon of salt.  Fill a small vial or bottle to the brim with water.  Carefully screw on the cap tightly.  Place the vial or the bottle into the can with the water, ice, and salt.  After about ten or fifteen minutes, either the vial or bottle will crack, or the plastic cap will crack.  (Only the teacher should handle this part of the demonstration.)  Have the students observe.  Teacher and students discuss and conclude that putting salt in water lowered the freezing point. 

Ask the students why the water got bigger and pushed outward as it froze.   After their answers, explain that the reason water expands when cooled below 4 degrees Celsius is due to hydrogen bonding.  Hydrogen bonding forms a weak link between hydrogen atoms and oxygen atoms of adjacent water molecules.  At higher temperatures hydrogen bonding does not affect water because the water molecules are too far apart for any interaction to occur.  Once the temperature drops to 4 degrees Celsius, the molecules have less kinetic energy and are closer together.  Water is polar..  The positive ends of some water molecules attract the negative ends of other molecules.  When water reaches 4 degrees Celsius the molecules have been pushed as close to one another as they will be.  Below 4 degrees the water molecules begin to align themselves into a crystal lattice structure of ice.  This requires the water molecule to widen the angle between the Oxygen-Hydrogen single bonds from the usual 104.5degrees.  In order for the water molecules to make the maximum number of hydrogen bonds the molecules move farther apart causing the ice to have a greater volume.  As a result ice expands. 

 

Performance Assessment:

 

The students’ participation and responses during the activities and follow-up discussions.

 

References:

 

Amery, Heather; The Know How Book of Experiments , Tulsa: EDC  PUBLISHING (1989)  p.25

Smith, Richard and Smoot, Robert C., Chemistry  A Modern Course,  Merrill Publishing Company,  Columbus (1990) pp.344-345. 

Shug, Ken