This is the new updated edition of the first book in the bestselling Uncovering Student Ideas in Science series. Like the first edition of volume 1, this book helps pinpoint what your students know (or think they know) so you can monitor their learning and adjust your teaching accordingly. Loaded with classroom-friendly features you can use immediately, the book includes 25 “probes”—brief, easily administered formative assessments designed to understand your students’ thinking about 60 core science concepts.

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The purpose of this assessment probe is to elicit students’ ideas about conservation of matter during a chemical change (oxidation). The probe is designed to find out if students think the mass changes as rust forms inside a closed system.

Familiar Phenomenon

chemical change, closed system, conservation of matter, oxidation

The best answer is C: The mass of the jar and its contents stayed the same. Rusting is an example of oxidation—a chemical change in which electrons from the iron atoms are transferred to the oxygen atoms, resulting in the formation of a new compound. The chemical equation for this change is 4Fe + 3O2 --> 2Fe2O3. Oxygen chemically combines with iron atoms on the surface of the nail. As a result, additional mass is added to the nail as a new compound (iron oxide) is formed. Although the appearance of rust on the surface of the nail makes it look as if it is “breaking down,” it is actually gaining mass as it changes from iron to iron oxide. However, because the source of the oxygen is from the air that was inside the jar, any added mass to the nail surface can be attributed to an equal loss of mass from the air in this closed system. No mass is added or lost in the total system.

Elementary Students

In the elementary grades, students begin developing ideas about changes in objects and materials. They can recognize rust as a change in the appearance of the nail. Upper-elementary students begin distinguishing between physical and chemical changes on the basis of observation of changes in properties. Rust is often used as an example of a chemical change. This probe may be useful in determining students’ early notions of rusting, particularly whether they view rusting as a “decomposing” process. Conservation of matter in the elementary grades focuses on parts and wholes of objects and changes of state. Although the chemical change context of this probe is rather sophisticated for elementary students, it can be used to find out elementary students’ intuitive ideas about the change from a shiny nail to a rusted nail.

Middle School Students

In the middle grades, students link ideas about chemical change with formation of compounds, including basic ideas about oxidation. Rust is commonly used as an example of a chemical change that results in a new compound with properties that differ from the original substance. These basic ideas about chemical change are included as grade-level expectations in the national standards. However, the mechanism of that change, explained by the interaction between iron and oxygen atoms, is a more sophisticated idea developed in high school. The probe is useful in determining students’ initial ideas about what rust and the rusting process are.

By the end of middle school, all students should know that matter or mass is conserved in a closed system as well as in chemical reactions. Conservation ideas about objects begin in elementary grades and increase in cognitive sophistication as the ideas of atoms, interactions, transformations, and closed systems are considered. Transformation of matter is addressed in middle school, although it remains a difficult concept and one to which students may have difficulty applying conservation reasoning. The notion that gases are involved in the transformation may be missing, and the appearance of the rusty nail may influence students’ thinking that the nail is breaking down and losing mass. Knowing the ideas students hold prior to learning that oxygen combines with iron during the rusting process is useful in designing learning experiences that challenge their intuitive notions that have been influenced by observation.

High School Students

Students at the high school level make a transition from a basic understanding of types of chemical changes, including oxidation, to understanding the mechanism for oxidation. Conservation of matter at the high school level is implicit in other matter-related ideas in biological, physical, and geological contexts. The probe is useful in determining whether students recognize a closed system as justification for matter being conserved during a chemical change. The probe is also useful in determining whether students still hold on to preconceived ideas about rusting, even after they have received middle school instruction targeted toward the idea that oxygen combines with the iron to form rust.

Be sure students understand that the nails are contained in a sealed jar and nothing can enter or escape from the jar. It may help to have visual props for this probe. Seal five clean, wet, iron nails (not galvanized) in a jar. Have students observe the nails. After a few weeks, observe the jar again and consider what happened to the mass.

The probe “Burning Paper” (p. 23) can be used to further probe students’ ideas about conservation of matter during a chemical change involving oxygen. “Nails in a Jar” can be combined with “The Rusty Nails” in Volume 1 of this series to further probe ideas about rusting in an open system in which the mass increases (Keeley, Eberle, and Farrin 2005).

• Middle and high school students’ ideas about chemical change tend to be dominated by the obvious features of the change (AAAS 1993). For example, students are likely to think that when something rusts it weighs less because it looks as if parts of the metal are being “eaten away” or the powdery rust is less substantive than the iron (Driver et al. 1994).
• Some students describe rust as a type of mold (Driver et al. 1994).
• Some students think that rust comes from the nail itself. Some students explain that it is already under the surface of the nail and is exposed during the rusting process (Driver et al. 1994).
• Students’ everyday experiences with rusting often involve iron getting wet. Consequently students are likely to think that rusting happens as a result of the water eating away at the metal, rather than rusting being an interaction with oxygen in the air (Driver et al. 1994).
• In a survey conducted of English 15-year-old students, one-third said the rusty nail would lose mass, one-third said it would gain mass, and one-third said its mass would stay the same. Of these students, just over 10% of those studying chemistry said the mass would increase because the mass of the rust is added to the mass of the nail. There was no indication from their response that the iron in the nail was involved in the formation of rust. Others who understood the reaction explained that the mass would not change because oxygen doesn’t weigh anything (Driver et al. 1994).
• Some students use the “taught” ideas about oxidation but adapt them to their intuitive notions of rusting using reasoning such as “the oxygen dissolves some of the iron” (Driver et al. 1994).
• Studies show that the way students perceive a change may influence their ideas about conservation during that change. For example, if their view is dominated by the appearance of a new material, they may think mass has been added (AAAS 1993).

American Association for the Advancement of Science (AAAS). 2001. Atlas of science literacy. Vol. 1. (See “Conservation of Matter,” pp. 56–57.) Washington, DC: AAAS.

Cobb, C., and M. L. Fetterolf. 2005. The joy of chemistry: The amazing science of familiar things. Amherst, NY: Prometheus Books.

Drigel, G. S., A. M. Sarquis, and M. D’Agostino. 2008. Corrosion in the classroom. The Science Teacher (Apr./May): 50–56.

National Science Teachers Association (NSTA). 2005. Properties of objects and materials. NSTA SciGuide. Online at http://learningcenter. nsta.org/product_detail.aspx?id=10.2505/5/ SG-01.

Robertson, W. 2007. Chemistry basics: Stop faking it! Finally understanding science so you can teach it. Arlington VA: NSTA Press.

• Using nongalvanized iron nails, have students carry out an investigation to test their ideas. Use their observations to resolve the differences between their prediction and results. Note: Rusty nails by themselves do not present a safety hazard; however, if students handle the nails, have them wash their hands afterward.
• Develop an understanding of open versus closed systems and explicitly link conservation of matter during a chemical change to changes within a closed system. Carry out an investigation to compare the change in mass as iron nails rust in a closed versus an open system.
• Help students understand the idea that there is oxygen in the air that is also being transformed in the jar. Explicitly develop the idea that air is a substance and has mass; revisit this idea throughout the grade levels.
• Help students draw parallels between other types of chemical change that involve combination with oxygen, such as combustion reactions, to rusting.
• Extend the idea of conservation of matter in a closed system by changing the context to a physical change (see the probe “Ice Cubes in a Bag” in Volume 1 of this series; Keeley, Eberle, and Farrin 2005) or a chemical change in a living system (see the probe “Seedlings in a Jar” in Volume 1 of this series; Keeley, Eberle, and Farrin 2005).