Summer Research Program for Science Teachers
Why Does Ice Float?
Savitri Singh
Health Opportunities High School, Bronx, NY
PREREQUISITE:  Covalent bonding, dipole-dipole attraction, hydrogen bonding, the molecular structure of water, polarity of chemical bonds.
PRIOR  HW:  (1)Use the internet to access information on the following: Input "Why does ice float" on the AOL Netfind.  Click on the hot links such as
"Why does ice float", "Properties of water", "Molecular ice cubes", "Salt and ice", "About ice and water", "How to make six sided snow flakes" and any other
interesting links. [9-12 Content Standard A- Identify questions and concepts that guide inquiry]
 (2)  Place a can of soda in a freezer for a night. Describe the condition of the can the next morning.
DO  NOW:  Pairs of students will be given a 500 ml beaker half filled with water and a plastic cup with at least four cubes of ice.  Students will be
required to drop the cubes of ice one at a time in the 500 ml beaker.  Each cube is dropped at a different speed and then observed to see if it will float
or sink.
MOTIVATION:  Have you ever wondered how fish survive in the winter or in the north pole?  Why is the soda can broken after being in the freezer for a
1)  How is the electron cloud distributed about the O---H  bond in the water
2)  Why is the water molecule polar?
3) How would two water molecules align themselves next to each other?
4)  How many electrons, protons and neutrons does a hydrogen atom have?
[9-12 Content Standard B- Structure and properties of matter]
The water molecule is polar.
The attractive force between water molecules is a form of dipole-dipole attraction.  Since the hydrogen atom has only 1 proton and 1 electron,
partially pulling away its only electron basically leaves a proton behind.   A fairly large positive charge is thus developed about the hydrogen end in the
water molecule.  This form of dipole-dipole attraction  present between water molecules is stronger than a regular dipole-dipole attraction.  It is known as

5)  Using small molecular models, arrange 6 water molecules in a pattern they will form in the liquid phase.
6)  Do another arrangement with six water molecules in a pattern they will form in the solid phase.
[Content Standard Unifying Concepts- Models]
(Students are reminded that the  particles of a solid are arranged in a regular pattern to form a rigid structure.  Further the force of attraction
between these particles  is stronger than in a liquid.)
7) What is in the center of the hexagon formed by water molecules in the solid phase? 
8) In what phase of matter will the six water molecules occupy more space?
9) What is the mass of six water molecules? (solid or liquid)
10) In which phase are H2O molecules less dense? [9-12 Content Standard B- Structure and properties of matter]
11)  For a solid to float on a liquid, how should their densities compare?
12) How do fish survive in the winter and in the north pole?
Six water molecules in the solid  state can orient themselves to form a regular hexagon.  These molecules will form a rigid structure like molecules
do in solids.  They will bond to each other by hydrogen bonding.  However, in the liquid state the molecules will not form a regular, rigid hexagon.  They
will bond to each other by hydrogen bonding and pack together.
The center of the hexagon has empty space.  Water molecules in the solid phase occupy more space.  i.e  water(liquid) expands when it freezes and can break
its container if there is no room available for the frozen water to expand.
Solid H2O (ice) is less dense than liquid H2O (water).  Ice will therefore float on water.  Rivers never become completely frozen because as soon as
water freezes, it floats up to the surface.  Fish live in the unfrozen, denser liquid water below the layer of ice.

To supplement this lesson, students will be shown a five minutes clip entitled "Why does ice float?" from a laser disc.
Due to hydrogen bonding:
(1)  water expands when it freezes,
(2)  ice floats on water and
(3)  fish can stay alive during winter in rivers, lakes and  seas.
Return to Chemistry Lesson Plans Menu