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. The probe is designed to reveal whether students recognize that the mass of a warm gas in a closed system is the same after it has been cooled, even though the volume it occupies has decreased.

Familiar Phenomenon

conservation of matter, gas, kinetic molecular theory, mass, properties of matter, weight

The best response is C: The mass of the warm balloon is the same as the mass of the cold balloon. In the warm balloon, the gas molecules are free of each other and moving rapidly. They collide with each other and with the wall of the balloon exerting pressure, which gives the balloon its size (volume). As the balloon cools in the freezer, the air molecules transfer energy to the surrounding freezer. The molecules of air are still free of each other but they do not move as rapidly or as far apart and their collisions with the balloon wall are not as forceful. As a result, the volume of the balloon containing the cool air decreases. The conservation of matter principle explains why the masses are the same. Mass is the measure of the amount of matter in an object, material, or substance. Heat and cold only speed up or slow down the motion of molecules. Changing the temperature of the air inside a closed system does not change the mass because the number of molecules remains the same. The balloon is sealed so nothing can get in or out.

Elementary Students

At the elementary school level, students describe the properties of materials or objects and classify them as solids, liquids, or gases. Their experiences with solids and liquids are based on matter they can see. Gases are more difficult for them to understand, because they have not yet developed a particulate notion of matter. However, before proceeding to middle school, it is important for students at the elementary school level to understand that gases are matter and they have weight. At this age level, conservation of matter is taught using phenomena with pieces that are observable, such as parts of objects. The shrinking balloon phenomena used in this probe is appropriate at the observational level for elementary school students, but asking students to explain what happens in terms of the particles should wait until they are ready to use a particulate model.

Middle School Students

At the middle school level, students transition from focusing on the macroscopic properties of solids, liquids, and gases to explaining states of matter in terms of the position, arrangement, and motions of the atoms or molecules. Compared with elementary school grades, middle school students have more experiences investigating gases. At this level, they should understand the idea that gases are made of molecules that have mass (and weight). They develop the idea of a closed system and can use that idea to reason conservation of matter–related phenomena, although understanding conservation of matter in a gas context is more difficult.

High School Students

At the high school level, students deepen their understanding of gases by learning about the gas laws. They use Charles’s law to explain what happens to the volume of a gas when the temperature changes. At this grade level, students are expected to be able to use the conservation of matter principle to explain a variety of changes within a closed system. However, they tend to hold on to their earlier ideas about the mass (and weight) of a gas if not confronted with their preconceptions.

Consider demonstrating this phenomenon with a balloon. If a freezer is not available, put the balloon outside in the cold, in a refrigerator, or in an ice chest. Make sure students understand that for the purpose of this probe, the balloon is sealed, although in reality some air can escape.

• Students may believe that matter does not include gases or that gases are weightless materials (AAAS 1993).
• Many researchers have noted that students do not initially seem to be aware that air and other gases are a type of “material” and thus have properties, such as weight or mass, like other materials (Driver et al. 1994).
• Research shows that some students have a difficult time conserving matter in a closed container when a gas is involved and the volume of the container changes. They confuse volume with quantity (Sere 1985). This can also be explained by the intuitive rule, “more A, more B” noted by Stavy and Tirosch (1995). Since the volume of the room temperature balloon is larger, students reason that the balloon has more matter, thus more mass.
• The idea that air or gas has mass is not obvious to children. Yet, when it is taught, it is a concept children can acquire easily and remember (Sere 1985).
• Some students believe a warmed gas weighs less than the same gas that is cooler (Driver et al. 1994).

American Association for the Advancement of Science (AAAS). 1993. Benchmarks for science literacy. New York: Oxford University Press.

Driver, R., A. Squires, P. Rushworth, and V. Wood- Robinson. 1994. Making sense of secondary science: Research into children’s ideas. London and New York: RoutledgeFalmer.

Keeley, P. 2005. Science curriculum topic study: Bridging the gap between standards and practice. Thousand Oaks, CA: Corwin Press.

National Research Council (NRC). 1996. National science education standards. Washington, DC: National Academy Press.

Robertson, W. 2005. Air, water, and weather: Stop Faking It! Finally Understanding Science So You Can Teach It. Arlington, VA: NSTA Press.

Sadler, T., T. Eckart, J. Lewis, and K. Whitley. 2005. Tried and true: It’s a gas! An exploration of the physical nature of gases. Science Scope (Nov./ Dec.): 12–14.

• Have students carry out investigations to test their ideas. Use their findings to engage them in resolving the discrepancy between their prediction and ideas and their findings. However, be careful in humid climates that additional mass from condensation of water vapor in the air is not added to the balloon when it comes out of the freezer.
• Explicitly teach the concept of a closed and open system. Link conservation of matter to changes that happen in a closed system.
• Gases pose special difficulties for children because the gases they commonly experience, like air and helium, are invisible. It is suggested that this invisibility prevents students from developing a scientific conception of a gas. Explicit instruction is needed for children to understand the properties of a gas, including properties like mass and weight. This is in contrast to solids and liquids where students tend to learn about them intuitively (Kind 2004).
• Provide other opportunities to compare changes in volume of a gas with temperature, such as slipping a balloon over a flask and then heating it, observing the balloon as it expands. Ask students to explain what happens to the mass of the total system before and after heating.
• Encourage students to draw a “particle picture” of what is happening inside the balloon in both situations. Use their drawings to probe deeper into their understanding of the numbers and motion of the particles.
• Combine this probe with probes in Volume 1 (Keeley, Eberle, and Farrin 2005) to further examine students’ ideas about conservation of matter during a physical change.