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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.)

A picture containing indoor

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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)



Students capture falling snowflakes, observe with hand lenses/cell phone microscopes, draw/describe snowflake types and hexagonal symmetry, and preserve snowflakes by casting in superglue. Ideally, this activity will be done when it is snowing out. If that is not possible, observations can be based on snow on the ground, if the latest snow is relatively recent and if it has not been significantly changed by melting, rain on snow, strong winds, or other conditions that would destroy the snow crystal structure.


Ken Libbrecht – Snowflake Science: A Snowflake Primer – 

Ken Libbrecht – Types of Snowflakes chart – 

Ken Libbrecht – The Snow Crystal Morphology diagram (explanation of the Nakaya Diagram):


● Black velvet snow catcher sheets

● magnifying glasses

● cell phone microscopes

● Nakaya diagram handouts – laminated

● Snow crystal types charts – laminated

● Small paintbrushes

● Superglue (thin, liquid type – not gel)

● Microscope slides and cover slips


1. Activity set-up

At least 30 minutes prior to beginning the activity, place snow catcher sheets, paintbrushes, and slides and cover slips outside. This will allow them to cool down so that snowflakes don’t melt upon contact with them.

2. Introduction: What are snowflakes?

Definition of precipitation

What types of precipitation have you experienced or heard about? 

What types of frozen precipitation – forms (snow, sleet, freezing rain, graupel, hail)

Snow is special in that it is formed from water vapor that freezes in the clouds without first passing through its liquid phase

In order for this to happen, there must be a tiny speck of dust, bacteria, or some other type of tiny particle in the cloud. Water that is in its vapor form in the clouds starts to freeze onto the particle. (The particle around which a snow crystal forms is called a condensation nucleus.) The water vapor freezes in a special, orderly pattern, called a crystal. Once a snow crystal forms, it continues to grow as it moves around in the cloud, eventually falling from the sky and landing on the ground.

Watch Ken Libbrecht falling/growing snow crystal animations. 

When we see these crystals falling from the sky, we usually call them snowflakes. Snowflakes can be individual snow crystals, or they can be clumps of snow crystals stuck together.

Snow crystals can be different shapes and sizes, but they all have six sides or arms. (Sometimes when we find them they are broken, so we can’t see all of the arms.)

Look at snow crystal types chart. 


We are going outside now to observe snowflakes. We’ll do this by catching them on velvet and looking at them through magnifying glasses and cell phone microscopes so we can see all the details. 

When you are looking at the snowflakes that you catch, pay special attention to these things:

● Is it a single snow crystal or a clump of crystals stuck together?

● If it’s a crystal, what shape is it? How would you describe it/what does it remind you of? Which of the snow crystal types on the chart does it look most like?

● Can you see/count 6 sides or 6 arms, depending on the shape of the crystal?

Observing crystals.

3. Outdoor Session – catching and observing snowflakes

Pass out velvet snow-catcher cards and magnifying glasses (cell phone microscopes optional)

Place snow crystal types cards in accessible location

4. Explain procedures for making snowflake “fossils”

Refer to Ken Libbrecht activity sheet/web page: 

Preserving crystals using superglue.

5. Snow crystal stories: Messages from the sky

Snow crystals aren’t just beautiful, but they provide information about the conditions under which they formed in the clouds. Certain types of snow crystals form under different conditions. 

What types of snow crystals did you observe during our observation session? Refer to snow crystal types chart: 

Ukichiro Nakaya was a Japanese scientist who lived during the 1900s. He studied how different temperature and humidity (moisture) conditions in the clouds influence the types of snow crystals that form. He created a diagram to explain this relationship, and it is referred to by scientists today as the “Nakaya Diagram” in his honor.

Pass out copies or display on overhead the Nakaya Diagram from Ken Libbrecht:

Provide an orientation to diagram axes (temperature and humidity) and explain what it shows. (Note temperature scale is given in both Celsius and Fahrenheit.) Read an explanation here:

Ask students to find the snow crystal on the diagram that best matches the type they observed.  Coach them through the process of determining what the temperature and humidity conditions were like in the cloud where and when this snow crystal formed.

EARTH’S FRESHWATER RESOURCES: Snow as a water source


Students observe various forms of water (clouds, Greenland ice sheet, oceans) in a photo of the Earth from space and watch a teacher-led demonstration illustrating the distribution of Earth’s water resources, emphasizing the very small amount of Earth’s water that is fresh water and available to humans and ecosystems. 

This activity is an abbreviated version of several related lessons from the National Oceanic and Atmospheric Administration (NOAA) addressing water resources, water use, and water conservation.


Freshwater Availability lesson plan. No date. Precipitation Education, Global Precipitation Measurement Mission, NASA.  

Earth’s Water lesson plan. No date. Precipitation Education, Global Precipitation Measurement Mission, NASA. 
Where Is Earth’s Water? 2021. USGS Water Science School, U.S. Geological Survey.


Included in kits unless otherwise noted

● NASA photo of Earth from space, laminated

● measuring cups (1 x 2 cup measure, 1 x ½ cup measure)

● ice cube trays (2) – packaged with ice and water activity materials

● optional: simple water cycle diagram poster

● 5 gallon bucket full of water.

● Optional: School can provide a globe.


1. Engage/activate prior knowledge

Ask students to think about the lesson topic by posing questions such as “What are some of the ways that you use water everyday?” and “Where does the water that you use come from?” (from NASA Freshwater Availability classroom activity)

2. Observing water on planet Earth

Display Earth “blue marble” photo. (Photo source: NASA Freshwater Availability classroom activity)

Ask students to make observations and share what they notice with the group (when called upon, in an orderly fashion!). Draw their focus to WATER in the image. What are the blue areas? (Oceans, which are made up of water.) 

One of Earth’s nicknames is the “Blue Planet.” Why do you think this is? What makes it the blue planet? (About 70% of Earth’s surface is covered by water, which appears blue.)

But not all water appears blue in this photo. What are the white swirled areas? (Clouds, which contain water droplets or ice particles.) What about the solid white area in the upper right portion of the photo (Snow on the Greenland ice sheet – water in its solid forms.)

Water is one thing that makes the Earth so special among all of the planets. All living things need water to survive. As far as scientists know, water is not present on most planets.

3. Introduction to freshwater as a limited resource

Referring to the photo, ask if we would want to drink the water in the oceans? Why or why not? (No. It is salt water, and it does not help our bodies the way that fresh water does. Most living things can’t use salt water for survival. Similarly, farmers can’t use salt water to water crops.)

Introduce the idea that even though there is a lot of water on Earth, most of that is salt water that is in the oceans. 

It turns out that even though there is a lot of water on Earth, it’s mostly (97%) salt water. The amount of fresh water is only a small portion of the total water on Earth (3%), and that is the only water that people and land-based ecosystems can use.

4. Visually represent the distribution of Earth’s water resources

The following demonstration is adapted slightly from the NASA Global Precipitation Measurement Mission lesson plan, “Earth’s Water.”

The demonstration starts with a 5-gallon bucket filled with water. Tell the students that this represents all of the water (100%) on Earth.

We just learned that most of the water on Earth is salt water; the rest is called freshwater. We are going to focus on freshwater. Remove two cups of water from the bucket (in e.g. a liquid measure). These two cups represent all of the freshwater on Earth, which makes up 3% of Earth’s total water supply. Move the bucket representing the oceans out of the way, since we are only considering freshwater from now on. 

Ask students to share ideas about where/in what forms freshwater can be found on Earth. Suggestions might include lakes, rivers, glaciers, snow fields/ice, permafrost, aquifers/underground, as water vapor in the atmosphere/in clouds. Guide them toward identifying polar ice caps/glaciers as containing water in its frozen form. 

Remove ½ cup of water from the 2 cups of water representing all freshwater. Pour the remaining 1 ½ cups of water into an ice cube tray. The 1 ½ cups of water in the ice cube tray represent the amount of freshwater that is stored as ice in glaciers and polar ice caps (and therefore not available for use by humans). 

Focus on the remaining ½ cup of water. This represents the liquid freshwater that is in the ground, surface waters (rivers & lakes), and water vapor in the atmosphere. So if the 5 gallon bucket represents all of the water on Earth, this half cup shows how much of that total (1%) is freshwater that is available to use.

Optional: Explain that not all of the ½ cup of water is clean and usable by humans (for consumption). Use an eye dropper to remove one drop of water from the ½ cup. This drop represents the amount of freshwater that is clean and available for humans to use.

5. The sources of freshwater

We have learned that living things need freshwater to survive, and that besides the freshwater that we need to drink, cook, and wash, humans use freshwater for growing the food that we eat, generating electricity (hydroelectric power), making just about anything that we buy and use from paper to clothing to cars (Isaiah, 2014).

We have also learned that only a small amount of Earth’s total water can be used by humans for these things.

So where does the water that we use come from? (Lakes, streams, groundwater) And where does that water come from? (Rain, snow)

Header image: Courtesy NOAA

Snow: Tiny Crystals, Global Impact Museum Exhibit Walkthrough