Teacher Background for Unit “How Does Garbage Impact Our Environment?”

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What Important Scientific Understandings Do Students Develop in This Unit?

Particle model of matter. In 5th grade, students learn that matter is made of particles that are too small to see. Particle models are developed for matter in each of the three phases: solids, liquids and gases. A key new understanding in this unit is that gas, while not visible to us, is just as much matter and just as real as solids and liquids.

In solids the particles are stuck together to form a particular shape. The particles do not move around relative to one another except that they can vibrate in place, as they do when a sound wave travels through the solid or because thermal (heat) energy has been accumulated in the solid. In fact the particles are always vibrating a little but vibrate more at higher temperatures.

In liquids the particles are touching one-another but can slide around relative to one another. They are moving enough all the time that they do not get stuck together, so any liquid can flow. Liquids do not have an intrinsic shape, and take the shape of their container.

In gases the particles are widely separated and are moving about in space. Each particle moves in a straight line path until it collides with another gas particle or with the particles in a solid or liquid (e.g. with the surface of the container that the gas is in). A gas does not “fill” a container in the sense that there is no space left, but the gas particles move throughout any container they are in. A given quantity of gas has a weight, but no fixed volume or shape.

Like all science models, particulate models are not just extra things to learn and remember, they are things to use to help explain and interpret a wide variety of phenomena. Without such models most phenomena that occur in matter cannot be explained. What is going on at the substructure level when a substance melts? Why does condensation form on the outside of a glass of iced water? How do smells get from the oven to your nose? Only as students see particulate models as useful in this way do they have any reason to remember and call on these models.

Conservation of matter. The other key idea is that matter is conserved and cannot be created or destroyed. In 5th grade, this is examined by weighing matter before and after different types of changes: tearing, crushing, dissolving, and chemical processes where one type of matter disappears and something that looks very different is produced. In all these instances, students see that the weight is conserved, meaning that the amount of matter stays the same. At the 5th grade level, we talk about conservation of weight, but more correctly what is conserved is mass. However, we do not bring up the distinction between weight and mass at this grade level. (Mass is an intrinsic property of the matter; weight depends not only on its mass but on the gravitational pull of the Earth on that mass.)

Once the particulate nature of matter is established it becomes more natural to see all of the processes that involve matter as being rearrangements of the particles. The idea of conservation of weight suggests that through all the various matter changes observed there are, at some level, the same set of particles present before and after the process. These core science ideas, conservation of matter and all matter made from particles too small to see, are critical underpinnings for middle and high school science.

What are Crosscutting Concepts?

A crosscutting concept is a science concept that is applicable across all domains of science. They are stressed in NGSS because they are useful thinking tools, each provides a lens for looking at problems and asking questions. In this unit several are being developed, and once they are used a couple of times it is useful to give the students time to reflect on their usefulness.

• Matter and energy: conservation, flows and cycles. If we know that matter is conserved then we can infer whenever the weight of a system changes that matter has left or entered the system, and we can look for where it came from or where it went. Thus, no matter what type of system we are looking at, we now have a tool (a lens) for asking questions about what is going on “what is happening in terms of matter flows?
• Systems and system models. Notice that the key to tracking matter is looking at a well-defined system. Defining a system leads us to think carefully about and answer the questions: What is in the system, what is not? What are its components? How do they interact? What is flowing in or out?; As we consider these questions, we develop a model for the system we want to think about. Whatever area of science we are trying to understand, thinking about what system is involved and attempting to construct a (usually diagrammatic) system model will help us get started to build a picture of what is going on.
• Scale, proportion and quantity. In this unit students work with models at two very different scales, the scale of visible objects, which we have called the macro-scale, and the much smaller scale where we think about the behavior of and effects due to the tiny particles within matter. We discovered that certain macroscale phenomena can be best understood when we understand what is happening to the particles in matter.

How Do Students Develop These Scientific Understandings Through the Course of the Unit?

The central phenomenon of this unit is the amount of waste that is generated in modern society, how that waste is disposed and what happens to various types of waste when placed in a landfill. The unit begins by investigating one day’s lunch garbage. The students sort and record the contents of this garbage pile, and this sorting introduces them to ideas about how different types of matter have different properties, in this case relevant to what will happen to them if they are placed in a garbage dump. We then model the garbage-handling system of the town to further develop student’s capacity to think in terms of system models and also to lead students to further questions about garbage.

We then introduce a planned experiment, a set of closed and open jars with various bits of garbage in them. The bottles provides a visible system for which students can develop a model, and make multiple versions of it to represent what they observe and measure over time. The bottle simulates what occurs to some types of matter in a landfill. This classroom investigation can help us answer the question of what happens to garbage in a landfill. Students observe and weigh their landfill bottles initially and again after some of the the garbage has decomposed. In this unit the investigations are mostly planned and structured by the curriculum design. Later in the year students will be given responsibility for the full cycle of planning, carrying out and analyzing results from an investigation, but early in the year there is much for them to learn about how to carry out investigations, take careful records, and analyze their data, as well as how to develop and use models that represent the system they are studying.

While we wait to see changes in the landfill bottles, the unit adds activities and models to support the idea that all matter is made from particles that are too small to see. They use sand to model the particles in solids and liquids and crush cans and tear paper to learn that weight is conserved despite these changes to matter. Then the class turns its attention to the smell that is emerging from the landfill bottles. They observe how smell moves around the room and experiment with gas in balloons and syringes to discover that gas has weight and that the model of it as being particles moving in space and bouncing off surfaces can explain the phenomena that they are observing. They also conduct an experiment that allows them to observe that new substances can be created when materials are mixed, but that their measurements show that the total amount (weight) of matter stays the same when nothing is allowed to flow in or out.

By comparing the initial and final weights of the closed and open landfill bottles, students establish that the weight of the closed system does not change, thus reinforcing the concept of conservation of matter. They then discuss their comparison of the closed and open systems and the smell of the open systems to infer that as the garbage decomposed, it transformed into a smelly gas that flowed out of the open bottles. The matter did not disappear, the particles simply transformed into a different form of matter. The gas has left the open systems, thus explaining the difference in weight. Finally, students investigate and read about the microbes that are causing some of the trash to decompose.

How is Modeling Part of Learning Science?

Throughout the unit, modeling is central to students’ sense-making. The unit begins with macro-scale models, for example of the landfill jars, where the components are mostly visible. Then it moves to particulate models of matter. As students learn about particles and gas, their models of the landfill bottles become more sophisticated, adding the invisible gas. They create models that show what is happening at the particle level, as well as visible changes to the properties of materials. Many phenomena that occur in matter can be explained using the particulate models for what is going on inside solids, liquids and gases. As students progress in their science education, they will need to be able to call upon particle-scale models of matter to support their explanations of many phenomena.

Models vs. pictures. When students use a model to express their ideas about a system, they have to observe it carefully and be explicit about their ideas. Even when the model is at the visible scale, their model diagram must show not only the visible components of the system but also find a way to indicate invisible aspects such as particles, relationships, or interactions between components. So the model diagram is much more than a picture. It is a representation of ideas about the relationships of the components in the system, often at multiple scales.

Students need to be reminded that a model is also less than a picture, in that it does not have to look exactly like the thing it represents as long it is accompanied by a key to any coded symbols for objects or invisible elements. Students start out trying to make pictures and can waste a lot of time beautifying their picture without adding anything to its usefulness, or to their science learning. So it is important to introduce the distinction between a picture and a diagram early on.

Modeling and reasoning. Remember that models in science play a very important role as a way of expressing key understanding and as a tool for thinking through what is going on in a given system or process. A model can provide the tools for a student to reason about the invisible mechanisms and relationships that cause the phenomenon to occur and hence help the student answer how and why questions about a phenomenon they have observed, rather than simply describing what occurred. They are a tool for students to reason scientifically and to make the conceptual changes that are the key to deeper learning.

However, models are only valuable in this way if students are developing and refining models for themselves, not just learning about the models scientists have found to be useful. It is critical to take the time to let the students struggle to form their own models and then to revise and refine them. Early in the unit certain models are suggested to them as examples, but as the unit progresses more and more of the work is done by the students. The more the students take ownership of their models, the more they will internalize the ideas and concepts of the unit.

Students should be encouraged to see incomplete or inaccurate models not as “mistakes” but as central to the scientific process. Revising the model that expresses your own thinking is a key step in revising your thinking, and this is what we call learning—changing your understanding of something. So the cycle of testing and refining models is where the learning occurs, whether for the students or for scientists themselves.

It goes against prior experience for most teachers to let the students go home confused or with ideas that you know are not correct. However, the struggle of sense-making is important and it takes time, so often will not be “done” by the end of a class. Sometimes the students will return to the next class with new ideas, or counter-arguments for another student’s idea that convince the class to discard it. If they do not and a “wrong” idea is still on the table, rather than refuting it by your authority as a teacher, you need to raise questions and examples that will lead the students to discard it because they can see it is not working. It’s like the old saying—teach a man to fish rather than giving him a fish and you have given him a better future. In this case, teach the students to think things through rather than giving an answer that they must remember and not only will they remember what they figured out longer than if you told it to them, but they are now better prepared to figure out the next thing they want to know or understand.

There is a place for introducing and taking time to teach new science ideas and to give students texts and videos that introduce scientific concepts, and that happens in this unit. For example, the idea of particles too small to see most likely must come from the teacher. The right time to do this is when students need this science concept to explain something that they have observed, so it does not come at the beginning, nor at the end of the unit, but rather is threaded into the middle of it at intentional moments.

Background Teacher Knowledge That Goes Beyond Grade 5 Level

It is always good to know a little more than you expect the students to learn. Some of them will surprise you with the level of their questions. However, remember that “Let’s find out” is a valid and useful answer, even sometimes when you do know the answer but want them to explore or think further. Answering a question with a question to lead them in the right direction is also a very useful strategy to help them learn how to think things through.

The emphasis at this grade level is on how these models help describe and explain observed phenomena (such as air pressure or sound travelling through a solid) not on the nature of the particles themselves. The difference between atoms and molecules is not stressed. However, it is helpful if you as a teacher have a concept of atoms as the basic building blocks of all matter and molecules as many and varied structures made from two to hundreds of atoms. In chemical processes the same number of each type of atom is present before and after the reaction but they have been rearranged into a different set of molecules.

There are a great variety of solid types and structures. Metals, for example, do not have a defined molecular substructure, while crystals have repeating patterns of basic blocks containing a specific set of atoms, but these are not actually separated out as molecules. In a liquid the particles are generally molecules, though as the liquid gets hotter some of the molecules in it may be broken into fragments.

The particles in a gas are typically simple molecules or atoms of inert elements (those in the last row of the periodic table that do not form molecules) such as helium. Three molecules in air that tend to crop up in other grade 5 standards are Water (H₂O), Carbon Dioxide (CO₂) and Oxygen (O₂ ). Students do not need to be able to decode these names as structures made from hydrogen, carbon and oxygen atoms and understand photosynthesis until the middle school grades. In grade 5 this is not emphasized, simply that plants get matter to grow from air and water. However since these terms are in common usage students may introduce them and want to understand them. It is quite acceptable to talk about these molecules as some of the particle types found in the mixture that is our air, and the first as the molecules that are particles of water.

Conservation of mass is actually conservation of atoms—the number of each type of atom does not change in all the processes we have discussed. Since, to the level of accuracy measured with any scale you might use, the mass of any structure formed from atoms is the same as the sum of the masses of the atoms it contains, if atoms are conserved then obviously mass is conserved. This rule breaks down in nuclear processes. Because some student may know enough to raise it, you should be aware that the conservation of the number of atoms of each type is not absolute. While the number of each type of atom is unchanged in any physical change such as melting or dissolving and in any chemical process, nuclear processes break this rule. These processes include fission (a big atomic nucleus falling apart into two or more smaller ones), fusion (a pair of small atomic nuclei fuse to form a somewhat bigger one) and radioactive alpha and beta decays (processes in which neutrons turn into protons or vice versa (always with emission of electrons or positrons and neutrinos) and so change one type of atom into another. Gamma decays are nuclear processes too, but they do not change the type of the nucleus involved (they are processes where a nucleus that has been formed in a high energy configuration emits a high energy photon, known as a gamma ray, and relaxes into a lower energy configuration). In nuclear processes the sum of the number of protons plus the number of neutrons does not change, so there is a matter quantity (this sum) that is still conserved. However since the mass of a proton is a little bit less than the mass of a neutron, and the mass of any atomic nucleus is also a little bit less than the sum of the masses of the protons and neutrons in it, nuclear processes result in small changes in the total mass of matter present. This is not something you should raise at this grade level, but it is something you need to be aware of in case your students raise it.

The conservation of matter is a cross cutting concept (along with conservation of energy) because it applies no matter what kind of system we are looking at (as long as no nuclear processes occur, which they rarely do under everyday conditions on Earth). The availability of each type of matter limits what can occur. Thus tracking each type of atom (where it comes from, where it goes), helps us understand what happens and what cannot happen. Processes which need matter, for example as fuel, must get that matter from somewhere, and must find a way to dispose of waste products.

Energy is also conserved. As for matter, that it makes it useful to track energy flows into, out of, and within a system. Unlike matter, energy is only conserved overall, not each type. It changes from one manifestation to another freely, so tracking each separately makes no sense. In NGSS the stress is on energy as a single quantity and on transfers of energy between systems or subsystems, rather than on “forms of energy.” What science teachers have traditionally referred to as different forms of energy are just the phenomena that we perceive in different ways that show us that energy is present. Sound, for example, is not “a form of energy” it is a physical phenomenon, a particular pattern of motion of matter (a sound wave) that carries energy with it as it moves from place to place. We perceive that energy transport by hearing sound.