A
Framework for K–12 Science
Education begins a description of the Crosscutting Concept of Systems and System Models as follows: “as noted in the National Science
Education Standards, the natural and designed world is complex; it is too large and complicated to investigate and comprehend all at once. Scientists and students learn to define small portions for the convenience of investigation. The units of investigations can be referred to as ‘systems.’ A system is an organized group of related objects or components that form a whole. Systems can consist, for example, of organisms, machine, fundamental particles, galaxies, ideas, and numbers. Systems have boundaries, components, resources, flow, and feedback” (NRC
2012, p. 91–92). In keeping with this idea of defining a system, the NGSS describe the grades 3–5 learning target as “A system can be described in terms of its components and their interactions” (NGSS Lead States 2013).
One of the first steps in formatively assessing how elementary students use ideas about systems to explain phenomena is to determine how they define or recognize the boundaries of a system, starting with a small observable system. The “Ice Cubes in a Bag” probe (Figure 1) has been used with elementary-, middle-, and high-school students to uncover their ideas not only about conservation of matter but also whether they also recognize a system as being open or closed, which helps them determine the boundaries of the system (Keeley, Eberle, and Farrin 2005).
“Ice Cubes in a Bag” is an example of how students can use the Crosscutting
Concept of Systems and System Models to define the boundaries of an interaction, such as the change in state happening inside the “bag system,” and use that idea to help them understand and explain a phenomenon.
The best response to this probe is
“C. Mass is a measure of the amount of matter in a substance.” When water changes from one state to another, the number of molecules of water does not change; therefore, the amount of matter or mass does not change. This principle applies to changes between liquids and gases as well as solids and liquids. However, one could also use the idea of a closed system to explain what happens to the mass. The sealed bag implies a closed system (even though at a microscopic level a plastic bag is semi-permeable). The components of the system are the bag, ice
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cubes in the bag, and the melting process that is happening inside the bag.
If a system is closed, such as the sealed bag containing ice, then no new matter can get into the bag, and nothing inside the bag can get out. Therefore, the total mass remains the same because nothing is added or taken away.
If the bag was not sealed, it would be an open system, and matter could enter or leave the bag and interact with components outside the bag.
Figure 2 shows responses from a fourth-grade class. The word weight was substituted for mass, which, according to the NGSS, is an appropriate stepping stone for elementary students. (Note: Some changes were made to students’ original spelling, grammar, and punctuation for the purpose of readability.)
Examining students’ responses reveals several misunderstandings about properties of matter, change in state, and conservation of matter in the context of a physical change.
The probe is useful in eliciting students’ initial ideas to inform instruction that will prepare students for the fifth-grade Performance Expectation
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 (NGSS Lead States
2013). While this performance expectation does not include the Crosscutting Concept of Systems and
System Models, teachers should continually look for ways to assess and provide opportunities for students to use Crosscutting Concepts in various contexts. In examining the student responses from this fourth-grade class, only one student, student #14, explained that the bag was closed so
Student #1: Because the ice will melt and it would get lighter. It did happen to me and the melted ice was lighter than the ice cubes.
Student #2: I think the in the bag with water will be less weight than the bag with ice because ice is heavier than water.
Student #3: I think the answer is A because as the ice melts the weight of the water gets smaller. I know this because when I got hurt in the summer and put ice on my leg and the longer the ice is on my leg it weighs not as much.
Student #4: Because ice cubes are solid and water is a liquid and usually water is heavier.
Student #5: The weight of the water will be more because I carried a bag of ice and a bag of water, the bag of ice was bigger and the water was still heavier. I think it will work the same way with this project.
Student #6: I picked that because the water can get heavier and it will take a lot of ice to get a pound and less water.
Student #7: It’s more spread out.
Student #8: The weight of the water in the bag will be more than the weight of the ice in the bag, because there is more water than there was ice. I know this because when ice changes it also changes in weight.
Student #9: Because when you have an ice cube in a cup it feels light and with water in a cup it feels heavy just like a cloud it gets heavy with water and flows out as rain.
Student #10: It is just that way.
Student #11: I picked that because if the ice melts, it could be, possibly the same weight. I know this because I’ve seen it happen, and tried it.
Student #12: The weight of the water in the bag will be the same as the weight of the ice cubes because the water is coming from the ice cubes and if the ice cubes are 1 pound then the water is coming from the ice cubes.
Student #13: Because when you melt the ice cubes and let them melt it should be the same as the ice cubes but it will be the same for pretty much only for a certain amount of time in the bag.
Student #14: It will be the same weight because the bag is zipped up so nothing can leak.
Student #15: Same because water and ice are the same.
January 2015 21

nothing could get in or out of the bag.
The student recognizes the boundaries of the “system” and uses this to explain why the weight does not change. This lack of using systems ideas is an indication to the teacher that this probe can then be used as a springboard into investigation to develop the idea of interactions that occur within closed systems, starting with small-scale systems such as the ice cubes in the bag. Students could also predict and test what happens to the weight over time when melted ice is left in an open container, which represents an open system. The idea of open and closed systems can be found in several of the Uncovering
Student Ideas probes. Taking the time to uncover and then develop this important key idea about simple open or closed systems will help students explain various phenomena and prepare them for later grades when they encounter more complex systems.
■
Page Keeley (pkeeley@mmsa.org)
is the author of the Uncovering
Student Ideas in Science series
(http://uncoveringstudentideas.
org) and a former NSTA President.
Arlington, VA: NSTA Press.
National Research Council (NRC).
2012. A framework for K–12 science education: Practices, crosscutting concepts, and core ideas.
Washington DC: National Academies
Press.
NGSS Lead States. 2013. Next
Generation Science Standards: For states by states. Washington DC:
National Academies Press.
References
Keeley, P., F. Eberle, and L. Farrin. 2005.
Uncovering student ideas in science:
25 formative assessment probes.
Download the “Ice Cubes in a
Bag” probe at www.nsta.org/
SC1501 .
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22 Science and Children