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WATER AND ICE: Density and molecular structure

WATER AND ICE: Density and molecular structure


The target age range for this lesson is middle school and up. For an elementary-appropriate version, see “Water and Ice: Investigating density through melting and freezing.”

Part I (optional): Students investigate the difference in density between water and ice by observing the change in water level in a glass of ice water before and after the ice has melted.

Part II: Students investigate the molecular basis of the lower density of ice than of liquid water by constructing an ice crystal lattice structure out of candy and toothpicks.

Recommended age/grade range: Middle school and up


References for step 6: Molecular geometry of ice crystal lattice 


Included in kits unless otherwise noted

Part I

● ice cube trays 

● clear plastic cups

● dry erase markers

● Teacher needs access to a water source for filling students’ cups. A pitcher may be helpful also.

● Teacher needs access to a freezer to make ice cubes.

Part II

● Molymod ice molecular model kit

● Toothpicks

● Gumdrops to represent Oxygen atoms*

● Mini marshmallows to represent Hydrogen atoms*

*Other objects may be substituted as available.


Part I: Investigating differences in ice and liquid water density through observation

1. Activity set up

At least one day before the activity, make enough ice cubes so that there is at least one (up to two) ice cube per student. Aim to make the ice cubes as similar in size as possible.

You may pre-fill the cups with water before class or have students fill them (a pitcher or large water bottle may be helpful) in class. 

2. Introduction to water and ice

Introduce or review phases of matter (solid, liquid, gas). 

Review the phases of water (solid=ice, liquid=liquid water, gas=water vapor). 

Introduce or review phase transitions. 

  • Ice-liquid water = melting (add heat)
  • Liquid water-vapor = evaporation (add heat)
  • Liquid water-ice = freezing (remove heat)
  • Vapor-liquid water = condensation (remove heat)
  • Ice-vapor = sublimation (add heat)
  • Vapor-ice = deposition (term is rarely used) (remove heat)

The differences in the physical form of these substances and the ways that they function are related to their microscopic, molecular structure. 

3. Optional: Water & ice density experiment

For detailed step-by-step instructions, see the lesson plan “Water and Ice: Investigating density through melting and freezing.”

Pass out clear cups and fill about half-way with water, and add an ice cube or two to each.

Have the students mark a horizontal line on the outside of the clear cup to indicate the starting water level. (You can help the younger students do this. Dry erase markers are included in the materials kit so that the lines can be rubbed off and the cups reused for the experiment in the future.) Use the same color marker for all of the cups so that you will be able to distinguish the starting water level from the ending water level.

Ask the students to notice where the ice cube is in the glass. Is it toward the top, or did it sink to the bottom? (Toward the top)

Ask them if they have any predictions about what will happen as/when the ice melts. Specifically, you can ask them if they think the water level in the cup will get higher or lower. 

Using a different color marker than they used to mark the starting water level, have the students mark a horizontal line on the outside of the clear cup to indicate the ending water level after the ice has completely melted. 

4. Discussion of results

Ask students to share their observations. Everyone should have noticed that the ending water level was lower than the starting water level.

Were students surprised by the results? If they made a prediction ahead of time, did the results match their prediction? 

What happened to cause the water level to go down? Which substance took up more space (had a greater volume): the ice cube, or the liquid that it melted into? (The ice cube)

Define/explain the concept of density. Density describes and measures how much material (matter) is in a given amount of space (volume)

H20 is different from most substances on Earth in that the solid form, ice, is less dense than the liquid form. This is because of the “lattice structure” that is formed by the arrangement of the bonds between the molecules.

Part II: The molecular structure of ice and water

5. Density and molecular structure

Display the assembled Molymod molecular model of the ice crystal lattice structure. 

  • Explain (or ask students to deduce) which model components represent Hydrogen atoms and which represent Oxygen atoms. The chemical symbol for water is H20, which is an abbreviation for two Hydrogen atoms and one Oxygen atom. The model shows how ice is made up of a structure where the bonds between the atoms are in a definite and repetitive arrangement called a lattice. 
  • Ask for ideas about how the atoms might be arranged differently in liquid water. Then demonstrate by dissembling the molecular model into its component H20 molecules. Illustrate that when water is in the liquid form, the molecules are free to move around and shift positions with respect to each other, and that they can be much closer together than in the ice lattice. 
  • Draw connections between the concept of density and the differences between the molecular structure of liquid water and ice.

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6. Geometry of water molecules and hydrogen bonds in the ice lattice

Challenge the kids to build their own ice molecular lattice using gumdrops for Oxygen atoms, mini marshmallows for Hydrogen atoms, and toothpicks for Hydrogen bonds. (Other objects can be used as available.)

Use the Molymod model as an example. Coach students to identify which of their materials match which parts of the model.

This activity will probably be more educationally beneficial and more relevant if the students have learned some molecular chemistry first (e.g. atoms and the periodic table, atomic weights, protons, neutrons, electrons, valence electrons, valence levels, covalent and ionic bonds, polar and nonpolar molecules, hydrogen bonds).

Detailed instructions for making the candy & toothpick model need to be written/provided. 

Key elements of the resulting structure are:

● An individual H20 molecule consists of 2 Hydrogen atoms covalently bonded to 1 Oxygen atom

Diagram, schematic

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● In an individual H20 molecule, the angle formed by the two Hydrogen atoms is 104.45 degrees. For the purposes of the student-made models, the angle can be approximate as long as they support the structure. 

Diagram, schematic

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● Tetrahedral shape of 4 Hydrogen atoms around each Oxygen atom, 2 H covalently bonded and 2 H hydrogen-bonded to each O, repeating structure


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● Molecular basis of hexagonal symmetry will be visible: Ring of 6 H20 molecules that is “dimpled” or “folded” (Refer to model.)

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Optional questions for additional discussion

● Discuss “real life” observations of what happens when liquid water freezes and expands and vice-versa. (E.g. polygonal patterns on the tundra form due to water that freezes in cracks in the ground and pushes the surrounding earth up. Pingos form when underground water freezes and expands, pushing the surrounding earth up. When frozen water in the soil – permafrost – thaws out, the liquid water takes up less space, so we get sinkholes and our buildings settle into the ground.)

● Discuss the consequences for life on Earth if solid water (ice) were more dense than the liquid form, as is true for most other substances. (E.g. rivers would freeze from the bottom up, and fish wouldn’t be able to overwinter; we couldn’t ice skate etc. until a pond or river was frozen solid throughout its whole depth)