Summary

In this lesson, students investigate how the density and therefore the volume, of water changes when it freezes. Although many students know that water freezes at 0 °C, most do not realize other changes also take place at this temperature. This lesson is designed to help students understand a common winter phenomenon: the development of potholes.

Grade level

Elementary School

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

• 5-PS1-1: Develop a model to describe that matter is made of particles too small to be seen.
• 5-PS1-2: Measure and graph quantities to provide evidence that regardless of the type of change that occurs when heating, cooling, or mixing substances, the total weight of matter is conserved.
• 3-5-ETS1-1: Define a simple design problem reflecting a need or want that includes specified criteria for success and constraints on materials, time, or cost.
• Scientific and Engineering Practices:
  • Developing and using models
  • Planning and carrying out investigations
  • Analyzing and interpreting data
  • Constructing explanations and designing solutions
  • Engaging in argument from evidence

Objectives

By the end of this lesson, students should be able to:

• Recognize that water can exist in three states: solid, liquid, and gas.
• Compare the ways the three different states of water behave relative to the containers they are in.
• Describe the change in volume of water as it freezes.
• Explain that the change in state from liquid to solid can exert a force on the container.
• Apply the results of the experiment to understand the relationship between winter weather and pothole formation.

Chemistry Topics

This lesson supports students’ understanding of the following topics in chemistry:

• States of matter
• Density
• Freezing

Time

• Engage: 10-25 minutes
• Explore Activity 1: 10 minutes
• Explore Activity 2: 20 minutes
  
• Explore Activity 3: 25 minutes
• Explore Activity 4: 10 minutes
  
• Explain: 45 minutes

Materials

Engage

• Mrs. Armitage, Queen of the Road by Quentin Blake (optional)
• Photographs of potholes

Explore

Activity 1

• Ice cubes
• Hot water (to see steam) or photograph of water boiling and producing steam

Activity 2

• Water molecules handout
• Scissors
• Glue or tape

Activity 3

• Water molecule models created in Activity 2

Activity 4

• Glass bottle with screw-on cap (the bottle should have a rounded neck like that of an Orangina bottle)
• Pitcher of water
• 2 Freezer bags, gallon size
• Container with a deep rim to hold the frozen bottle of water, such as a stock pot or bucket
• Freezer
• Optional: second glass bottle, exactly the same as the first, with 2 additional gallon-sized freezer bags

Safety

• Do not allow students to handle the glass bottle without teacher supervision.
• Place the glass bottle in two freezer bags before putting it in the freezer.
• Do not allow the students to touch the remnants of the glass bottle after freezing.
• Carefully discard the glass in the waste receptacle.

Vocabulary Terms

• Atom
• Molecule
• Freezing point

Keywords

• water, molecule, states of matter, phase, expansion, freezing point,

Teacher Notes

Logistics and Tips:

• To get the most from these lessons, it is important that students discuss their results and how their findings relate to the real-life problems being investigated before they move on to the next experiment.

Science Background:

• Water molecules and motion
  • All water molecules, whether solid ice, liquid water, or water vapor (invisible gas), consist of two hydrogen atoms bonded to one oxygen atom. However, the motion of the water molecules and the attraction between molecules differ among the three states. In solid water, the molecules are arranged in a fixed network (a crystal lattice) and vibrate in place. This is why ice maintains its shape. 
  • In liquid water the molecules not only vibrate but also move past each other easily. They are attracted to each other, but they are not in a fixed structure and easily take the shape of their container. 
  • In water vapor (a gas), the molecules are far apart from each other. They bounce off one other and off the walls of their container; because of all the empty space between molecules, the molecules are able to compress or expand to take the shape and fill their container.

Expansion with Freezing

• When most substances change from liquid to solid, their molecules move closer together causing their density to increase. But unlike most other substances, as water freezes, the space between its molecules increases as the crystal network forms, resulting in a larger volume and a decrease in density. If there is no empty space for the water to expand into, the expanding water exerts a force on the container.

Potholes

• Potholes form on roads in part because of the constant freezing and thawing of water in the winter. Water seeps into cracks in the road, and when temperatures drop, the water freezes. If the water has no room to expand, it exerts a force on the materials that make up the road. This, combined with the force of cars driving on the road, causes the road material to break up, which results in potholes.

Lesson Overview

• Engage:
• To begin the investigation, tell students to begin to think about what water, winter, and potholes have in common. Begin engaging the students by having them turn and talk to answer the following questions: 
  • What is a pothole? Where do you find them? What happens to a car or bike when it runs over a pothole? How do you think potholes are formed? 
  • After the students have had the opportunity to talk about what a pothole is, show them some photographs of potholes.
• Discuss what students know about water by asking the following questions: 
  • What are some characteristics of water? What is water made of? What does it mean when something is frozen? [clear, colorless, and odorless liquid; hydrogen and oxygen; cold, particles do not move freely]. Is there a relationship between potholes, water, and winter? What might it be?
    
• Optional: After the discussion about potholes, water, and winter, read the book Mrs. Armitage, Queen of the Road by Quentin Blake to the students. This book will give students a clearer understanding of what happens to cars when they drive on an extremely bumpy road, full of potholes.

• Explore:
  • Activity 1: Three States of Matter: H₂O (discussion) Students begin their exploration by thinking about the three states in which water can be found: solid, liquid, and gas. 
    • Introduce or review the three states of matter in which students witness water existing. You normally think of water as a liquid, but can water change forms (states of matter)? [yes, it can be solid, liquid, or gas, and it can change from one to another]
    • Show the students some ice cubes and ask the students to touch them. How does the ice feel? What other substances does it remind you of? What state of matter are ice cubes? [cold, hard; crystals; solid]
    • Have students imagine that they put the ice cubes in a pot on the stove and turned on the heat. What would happen to the ice? What state of matter would result? [ice cubes would melt; liquid] What would happen to the water if you turned up the heat on the pot? What state of matter would result? [water would boil and become water vapor, a gas]
    • Show the students a photograph of water boiling and highlight the steam. Explain that water vapor, a gas, is invisible. However, when water vapor comes in contact with cooler air, some of it condenses, or turns back into liquid water. Steam is actually made of liquid water droplets. Steam is evidence that some of the water in the pot turned into vapor, invisible gas. (You may also point out that the bubbles that form in boiling or near-boiling water are bubbles of water vapor, and that as you let a pot of water boil, the amount of water reduces. The liquid water does not disappear; it changes into water vapor/steam.) What is the one thing you were adding to change each state of matter from solid to liquid, liquid to gas? [thermal energy (commonly referred to as heat)]
  • Activity 2: Water Molecule Models (hands-on): Students will compare different ways water can be described (as a formula or as a drawing). They will make models of water molecules to gain a better understanding of what water is and looks like on the microscopic level.
    • Ask students what they think of when they think of water. Where do we find water? What do we do with water? Students may think about drinking, bathing, swimming, and rain. Go on to ask: 
      
    • What is water actually made of? What does the tiniest “piece” of water look like? Explain that a droplet of water contains trillions of microscopic water molecules. Show the students a diagram of one water molecule, such as the one shown to the right. What is this? What do the circles represent? [water molecule; atoms or elements] 

• Next to the diagram, write on a white board or chalkboard the formula for water: H₂O. Then ask the students to look at both representations of water.
• What does the “H” represent? [hydrogen] What does the “O” represent? [oxygen] What does the number 2 mean? [there are two atoms of hydrogen in the molecule] What do you think the one larger circle represents? [an oxygen atom] The two smaller circles? [hydrogen atoms]
  
• Explain that a water molecule is made of two hydrogen atoms and one oxygen atom. A molecule of water is the smallest piece of water there can be.
  
• Give each student the water molecule handout and ask them to cut out the hydrogen and oxygen atoms. NOTE: If students cut out circles using their own paper, the colors and sizes for each atom of each element should remain consistent (hydrogen atoms should all look the same; oxygen atoms should look the same as each other, but different from hydrogen).
  
• Have students make their own water molecule by attaching two hydrogens to one oxygen. Set them aside—their water molecules will be used in Activity #3.
• Activity 3: H₂O Molecules in Solids, Liquids, and Gases (discussion and physical activity): Students will use the water molecules they made to examine how they move relative to one another in three different states: solid, liquid, and gas.
  • Students will use the water molecules they made to examine how theymove relative to one another in three different states: solid, liquid, and gas.
  • Remind students that they know that a sample of water is made up of many water molecules, which consist of two hydrogen atoms and one oxygen atom. They also know that water can take the form of a solid, liquid, or gas, and (thermal) energy is needed to make the transitions between the states of matter.
  • Are water molecules in solid ice the same or different from molecules in liquid water or water vapor? [They are the same.] If they are the same, why are ice, liquid water, and water vapor so different from each other? [Students may or may not know that the difference is a result of how the molecules are arranged relative to each other and how they move relative to each other.]
  • Introduce or review the idea that all molecules, in all states of matter, are in constant motion.
  • In what ways do you think molecules move? [guide students to understand that they vibrate, or shake back and forth; slide past each other; and bounce around]
  • Molecules can vibrate, slide past one another, and bounce around in all directions. Do molecules in ice do all of these things? What about molecules in liquid? What about the molecules in water vapor?
  • Guide students to understand that:
    • Molecules are in constant motion, regardless of what state the substance is in.
    • Molecules in ice vibrate. They don’t move around relative to each other, but they do vibrate in place.
    • Molecules in ice are strongly attracted to one another in a rigid framework. This is why solid ice keeps it shape and does not flow.
    • Molecules in liquid not only vibrate, but also slide past one another. This is why water flows and why it takes the shape of its container.
    • Molecules in water vapor are far apart and are constantly moving in all directions relative to each other. They bounce off each other and off the walls of the container they are in. This is why gas fills the entire space of its container.
      
    • Have students think about the concept of state change, changes between solid, liquid, and gas, and how this relates to the addition or removal of energy.
      
    • Remember: how can you change ice from being solid to being liquid? [heat it up] How can you change liquid so it becomes a gas? [heat it up] So, think about how the molecules in a liquid move differently from the molecules in a solid. What do you think adding energy does to molecules? [it makes them move faster; it allows them to move past each other] What happens to the molecules in a liquid when you remove energy (cool off) a liquid? [they slow down; they can’t move apart or past one another as easily]
      
    • It may be instructive for students to use their bodies to model the motion of water molecules in ice, then liquid, then gas. Each student acts as a single water molecule. This is best done outside where there is a lot of room to move around:
      • Solid: Students stand side by side to form a solid cube. Their bodies should be in contact and each student should be shaking to simulate vibration.
      • Liquid: Students should be touching each other, but can move sideways to demonstrate flow. Each student should be in contact with at least one other student at all times.
      • Gas: Students should not be in constant contact, but should be bouncing around (carefully) off each other and any obstacles like walls. They can be apart from each other and should slowly get farther and farther apart.
    • You can reinforce students’ understanding of phase changes by having them change phase several times.
    • Finally, have students demonstrate their understanding of the relative locations of water molecules in the three states. Draw three large empty beakers on a wipe board or chalkboard and label them solid, liquid, and gas. Ask the students to come to the beakers to attach their water molecules in each one, thinking about how the molecules relate to each other.
• Activity 4: The Power of Water: The Pothole Model Part One (demonstration): In this activity, students bring together what they have learned to explore the effects of freezing on water in a container. This experiment will take two days to complete.
  • Show the class an empty bottle with screw cap lid. With close supervision, allow the students to touch and feel the bottle and its weight. What is this bottle made of? Do you think it is strong?
    
  • Show the class a pitcher of water. Do you think water is strong enough to break this glass bottle?
    
  • Fill the bottle with water until it is overflowing and screw the cap on as tight as you can. What happened to the water when it was poured into the bottle? [it filled the bottle, took the shape of its container] Why did it take the shape of the container? [the molecules were able to slide past one another]
    
  • Dry the outside of the bottle and flip it over to show that water is not leaking and there is very little air inside (maybe only a tiny bubble). Did the water break the bottle? Which do you think is stronger, the bottle or the water?
    
  • Tell the students that you are going to place this bottle in the freezer overnight. If you have a second, identical bottle, explain that you will put it in the freezer overnight as well. It will not have any water in it and will serve as a control to separate the effects of the temperature change on the bottle from the effects of the freezing water. Predict what you are going to see when you examine the bottle(s) tomorrow.
    
  • Place the water-filled bottle into a freezer bag, zip it closed, place that bag into another freezer bag and zip it closed. NOTE: The bottle needs to be placed in the freezer with the cap facing up. The bottle should not be on its side. Do the same for the empty bottle.
    
  • Note that the class will examine the results the following day, as described in the Explain section below.
    
• Explain: In this part of the lesson, students will bring together what they have learned to explain why potholes form during winter months.
  • Examine the results of the Activity 4: Power of Water experiment:
  • Before the class sees the results, review how you set up the experiment.
  • Carefully remove the water-filled bottle from the bag, place it in a wide rimmed container and show the glass bottle to the class. Did anything happen to the bottle? How did the glass crack? Water did not break the glass when it was filled yesterday, what was the variable or the one thing that was changed? [the glass cracked; water was placed in freezing conditions] If you have also put an empty bottle in the freezer, show it as well.

• Place the bottle back in the bags and allow the ice to melt at room temperature in the wide rimmed container. Once melted, carefully, pour the contents into the wide rimmed container and allow the class to make observations. What happened to the glass? Why did the glass break?
• Students may initially think that the glass broke because it got too cold. Guide students to understand that the water broke the glass by exerting a force on it. The question is: Why or how did the water exert a force on the glass? If you have used a control, challenge students to explain the importance of the control. [It got just as cold as the water-filled bottle, but did not break because there was not water inside it to expand.]
• Explain that unlike most other substances, water expands when it freezes. Make sure that they understand that by “expand” you mean that the water takes up more space when it is solid than when it is liquid (at room temperature).
  • Why does ice take up more space than liquid water? Guide students to think about what this might tell them about how water molecules are arranged within ice as opposed to within liquid. The details could be beyond the grade level, but students should be able to understand the concept that there is more empty space between molecules when they are arranged to form crystals of ice. Although they are freer to move about in liquid form, there is less space between the molecules when in liquid form.
  • Make a connection between the density of ice and its behavior in a glass of water. What other observation about ice and water indicates that ice is less dense (takes up more space) than liquid water? Guide students to think about what happens when you put ice in a cup of water. [It floats.] Why does it float? [Because it is less dense (takes up more space) than liquid water.]
• Have students apply what they have learned to explain how potholes form.
  • Remind the students that their original goal was to determine the relationship between water, winter, and potholes. They learned from the experiment and through modeling that water expands when it freezes. The water molecules create an intricate structure when frozen. Since there was nowhere for the ice to expand into because water filled the entire space within the bottle, as the water became ice, the ice put pressure on the glass and cracked it. How was this glass bottle a model for a pothole?
  • Encourage students to construct a scientific explanation to explain pothole formation. Their explanation should include a claim (how potholes form), backed up by evidence (from the experiment and other observations), and reasoning (e.g., it gets cold in the winter; it makes sense that water seeps into cracks in the road and then freezes).
  • Have students prepare their explanation in a form that you or they prefer. This could include, for example, a poster that describes the process, a cartoon or graphic story, or a multimedia presentation.
    
• Elaborate: Students can extend their understanding of expansion with freezing with additional activities.
  • Have students determine the extent to which water expands as it freezes by measuring the volume of a number of samples, first as water and then as ice. One method is to add water to a plastic 50 mL graduated cylinder up to 40 mL. Then put the cylinder in the freezer. The ice should rise to about 43 mL.
    
  • They can calculate the absolute increase in volume and then the percentage increase in volume. What is the average increase for all of the samples together?
    
  • Another implication of the fact that water expands as it freezes is that ice therefore floats on water. Have students research how this property is important to living things in lakes in the winter. What would happen if ice did not float?
    
  • Have students research the relationship between temperature and density of water. At what temperature is water the densest? (about 4°C) What happens to cold water in the ocean? (it sinks) Why would a lake have ice on the top, but not deep in the lake? (as the water gets colder than 4°C, it gets less dense, so it floats)
    
  • Freezing water in a narrow neck glass bottle caused it to shatter. Have students find out what will happen if water is frozen in a freezer bag or plastic water bottle (to its maximum capacity)? Why is the result the same or different from when using a glass bottle?
    
• Evaluate: Assess the quality of the students’ responses to tasks you assigned in the Explain sections. Further assessment can be made using any of the following items.

Multiple Choice Questions:

1. When water changes from liquid to solid, it is—
   1. dissolving.
   2. getting denser.
   3. changing state.*
   4. undergoing a chemical reaction.
      
2. Andres places a sealed container of water in the freezer. When the water freezes, which of the following changes?
   1. The volume of water*
   2. The weight of the container
   3. The types of atoms that the water is made of
   4. The number of atoms that the water is made of
      
3. A designer wants to create an ice mold in the shape of a pony. She wants to design the mold so that it can be filled with water, frozen, opened up to remove the ice pony, and then use it again. She needs to decide what material to use to make the mold. Which property or properties are most important for the mold to have?
   1. It should be magnetic.
   2. It should be less dense than water or ice.
   3. It should be smooth and brightly colored.
   4. It should be flexible and able to withstand changes in temperature.*

Constructed Response Question:

Keili makes a claim about states of matter. She writes: Substances can be solids, liquids, and gases. Water turns solid when the temperature is below the freezing point, which is 0 °C. Freezing is a physical change. The only thing that happens when something freezes is that it turns from liquid to solid.

Evaluate Keili’s statement:

1. What is correct about what she wrote? [Keili is correct when she says that substances can be solids, liquids, or gases and that liquids turn solid when the temperature is below the freezing point.]
2. What is incorrect and why is it incorrect? [She says that the only change that happens when something freezes is that it turns from liquid to solid. It can also change density, like water does.]
3. How should she revise her statement to make it completely correct? [Substances can be solids, liquids, and gases. Water turns solid when the temperature is below the freezing point, which is 0 °C. Freezing is a physical change. One thing that happens when something freezes is that it turns from liquid to solid. It can also expand or contract.]
   

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