To understand the relationship between molecular motion and the slippery properties of ice.
This lesson helps to demonstrate the application of the concept that different states of matter exhibit differences in its molecular motion. Students should understand that temperature affects the motion of molecules and plays an important role in what state a particular type of matter is found. As temperature increases, particle movement increases. When the temperature reaches a certain level, a solid substance is able to change phase and become a liquid. When this continues, the substance will once again change phase and become a gas or vapor.
By the end of the 8th grade, students should have sufficient grasp of the general idea that a wide variety of phenomena can be explained by alternative arrangements of vast numbers of invisibly tiny, moving parts. Students should see a great many examples of reactions between substances that produce new substances very different from the reactants. Then, they can begin to absorb the rudiments of atomic/molecular theory, being helped to see that the value of the notion of atoms lies in the explanations it provides for a wide variety of behavior of matter. Each new aspect of the theory should be developed as an explanation for some observed phenomenon and grasped fairly well before going on to the next. (Benchmarks for Science Literacy, pp. 77-78.)
Students should become familiar with the characteristics of different states of matter and transitions between them. Some students may have difficulty with this concept because they lack an appreciation of the very small size of particles or may attribute macroscopic properties to particles. Students might also believe that there must be something in the space between particles. Finally, students may have difficulty in appreciating the intrinsic motion of particles in solids, liquids, and gases; and have problems in conceptualizing forces between particles (Benchmarks for Science Literacy, p. 336.)
In this lesson, students will explore a website to learn that what happens at the surface of ice to give it its slippery nature can be explained by molecular motion. They will also see that this molecular motion is a unique attribute of ice that has recently been investigated by Professor Somorjai and his colleagues.
Before this lesson, students should have been introduced to the idea that matter may go through different phase changes. Students should be familiar with solids, liquids, and gases. They should also understand that heating and cooling a system can impact the phase of that matter. In addition, students should have some exposure to the concept of molecular motion at the atomic level. Prior to working on this lesson, it would be helpful for students to understand the ideas presented in the Science NetLinks lessons: A Matter of State and Temperature Changes Everything.
To prepare for this lesson review the article: Why is ice so slippery? Mysteries of the "invisible" ice surface.
Before exploring the website introducing students to the concept of the relationship between molecular motion and the properties of ice, conduct a general discussion about the physical and molecular properties of ice.
- Discuss the other states of matter in which ice can exist. (Ice can be a liquid (water) or a gas.)
- How does the molecular motion differ between a solid, liquid, and a gas? (In solids, the atoms or molecules are closely locked in position and can only vibrate. In liquids, the molecules are more loosely connected and can slide past one another. In gases, the atoms or molecules have more energy and are free of one another except during occasional collisions.)
- Describe the physical characteristics of ice. (Ice is cold, hard, wet, smooth, and slippery.)
- Can you think of any other solids that are as slippery as ice? How does the slipperiness of ice compare to solids such as wood or cement? (Other solids are not as slippery as ice.)
- What do you think makes ice slippery? (Allow students to freely express their opinions.)
Begin this part of the lesson by telling students that they will explore a website to learn more about the molecular nature of ice and what makes it slippery. Have students review the The Science of Hockey, on The Exploratorium website. Instruct students to only read the first section, The Ice, and page 1 of the section on Skating. They should also click the audio sections to listen to more information about the topic.
Distribute the Putting the Ice in Hockey student sheet. Ask students to take notes related to the questions on the activity sheet as they explore the website. Students can use their notes during the class discussion.
Lead students through a discussion about the relationship between molecular motion and the properties of ice by asking the following questions:
- Describe the difference between "fast ice" and "slow ice." (Fast ice is harder and colder with a smoother surface, while slow ice is warm and soft and may have a rough surface. "Fast ice" is less "chippy" and there is less "snow.")
- Which kind of ice do hockey players seem to prefer? Why? (Players seem to prefer ice that is "fast" because the passing and skating are easier.)
- Where are the hockey arenas located where the ice is better? Why? (Hockey arenas in Canadian cities such as Edmonton are known to have better quality ice. Less heat and humidity get into the arena. This keeps the ice cold, which is one of the keys to maintaining fast ice.)
- How does the ice differ for hockey and figure skating? (For hockey, the temperature of the ice is kept at 16° Fahrenheit [-9° Centigrade] and for figure skating it is 22° Fahrenheit [-5.5° Centigrade]. Figure skaters prefer softer ice for their landings.)
- In the past, what did scientists think caused ice to be slippery? (In the past, scientists believed that either pressure or friction melted the ice, creating a water lubricant that allows skates and pucks to slide.)
- According to Professor Somorjai and his colleagues, why is the previous theory about why ice is slippery incorrect? (Skates and pucks do not generate enough pressure to instantly liquefy ice. Professor Somorjai's findings indicate that ice itself is slippery. You don't need to melt the ice to skate on it, or need a layer of water as a lubricant to help slide along the ice.)
- According to Professor Somorjai, what may account for the difference between "fast ice" and "slow ice?" (The "quasi-fluid" or "water-like" layer on the surface of the ice may be thicker or thinner depending on temperature. As the ice is warmed, the number of these slippery layers increases. As the number of layers increases, the players' skates need to "slosh" through more of these "water-like" layers; more friction occurs in these conditions, slowing the players down.)
- What did Professor Somorjai discover when he first examined the surface structure and composition of the atoms and molecules that make up ice? What did he discover upon further research to indicate what makes ice slippery? (At first, the structure indicated that every second water molecule on the surface was missing. After further study, Somorjai's team found that the "missing" water [or ice] molecule was indeed there, but it was vibrating so rapidly that it was invisible to the technique they were using. Then, he found that these molecules behave like a liquid, but they only move up and down; they do not move from side to side on the surface of the ice. This is an important distinction. If the atoms moved side to side, the "liquid-like" layer would literally become liquid [which is what happens when the temperature rises above 32° Fahrenheit]. This "liquid-like" layer is thought to be what makes the ice slippery.)
- Describe the steps involved in making ice for a hockey arena at the beginning of the season. (At the beginning of the hockey season, the arena uses an advanced refrigeration system that pumps freezing "brine water" [salt water] through a system of pipes that run through a large piece of concrete known as the "ice slab." When the "ice slab" gets cold enough, layers of water are applied to it. The first few layers are painted with the hockey markings and the advertisements that you see on [or more correctly "in"] the ice. These layers are then covered with 8 to 10 more thin layers of ice. When complete, the ice is only one inch thick.)
- What is a Zamboni? How does it work? (The Zamboni is a mechanical ice resurfacer. It works by scraping the ice surface and collecting the snow [ice that the been scraped loose.] Next, it "cleans" the ice, by putting down water which flushes the grooves deep in the ice, loosening any dirt or debris. The excess water and dirt is then collected. Finally, the Zamboni puts down a thin layer of heated water that freezes and creates a smooth surface.)
Assess students' understanding by asking them to describe the "old" theory about why ice is slippery. Then, in their own words, describe Professor Somorjai's more current findings that explain why ice is slippery. Ask students to draw a picture that illustrates what is happening at the molecular level on the surface of the ice according to Professor Somorjai.
(The "old" theory stated that either pressure or friction melted the ice, creating a water lubricant that allowed skates and pucks to slide. According to Professor Somorjai, skates and pucks do not generate enough pressure to instantly liquefy ice. He found that ice molecules vibrate extremely rapidly, but they only move up and down; they do not move from side to side on the surface of the ice. This creates a liquid-like layer on the surface of the ice. It is this layer that makes ice slippery. The students' illustration should show the molecules on the top layer of the ice moving very rapidly up and down.)
- The Physics of Ice Skating uses ice skating to explore several basic principles of physics, including Newton's second and third laws, kinetic and potential energy, friction, and torque. It also includes links to other websites related to physics and ice skating.
- Ask Mr. Science addresses the interesting question, "Why is ice slippery?" Students can read and discuss the response, which is listed below. (This is an interesting question because a few years ago I would probably have given the wrong answer. The old story was that the pressure of your skate would be enough to cause the ice under the blade to melt, or friction would melt the ice under your skate or ski. Wrong! Recent research (1999) has shown that there is a thin film of liquid on the surface of the ice, even at temperatures far, far below freezing. This is true for ice, but also for other materials. So ice is slippery because of this layer of water which is always there, regardless of pressure or friction.)