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.
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 density or 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.
This lesson supports students’ understanding of the following topics in chemistry:
- States of matter
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
- Mrs. Armitage, Queen of the Road by Quentin Blake (optional)
- Photographs of potholes
- Ice cubes
- Hot water (to see steam) or photograph of water boiling and producing steam
- Water molecules handout
- Glue or tape
- Water molecule models created in Activity 2
- 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
- Optional: second glass bottle, exactly the same as the first, with 2 additional gallon-sized freezer bags
- 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.
- Freezing point
water, molecule, states of matter, phase, expansion, freezing point,
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.
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 thus are easily moved by gravity, taking the shape of their container. In water vapor, the molecules move quickly and 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 into the shape of a container to take the shape of their container.
Expansion with Freezing
Unlike most other substances, water decreases in density when it turns from liquid to solid. As water freezes, the space between molecules increases as the crystal network forms, and the volume of water increases. If there is no empty space for the water to expand into, the expanding water exerts a force on the container. A more complete explanation can be found at: http://www.iapws.org/faq1/freeze.html.
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.
- To begin the investigation, tell students they will 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.
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)]