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.

Access this probe as a Google form: English

Download this probe as an editable PDF: English 

The purpose of this assessment probe is to elicit students’ ideas about the mass of a gas. The probe is designed to reveal whether students recognize that an uninflated balloon will increase in mass when inflated with a gas, even though the balloon intuitively seems lighter when it floats in the air.

Familiar Phenomenon

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

The best response is A: The floating balloon has a mass greater than the mass of the uninflated balloon. Gases are a form of matter and thus have mass (or weight) and take up space. Putting a gas (such as helium in this case) in a balloon adds mass, even though some students intuitively think the balloon may now be lighter because it floats. Balloons filled with helium rise because they are less dense (the total massto- volume ratio) than the surrounding air and not because they have a lesser mass than an uninflated balloon. Students tend to confuse density with mass. In this case, the mass of the gas-filled balloon has increased while its density has decreased with the increase in volume. A balloon filled with helium has a lesser mass (and weighs less) than a balloon blown up to the same size with air because it is less dense. However, this problem is not about comparing density, but rather comparing mass. The balloon filled with helium has a greater mass than the uninflated balloon because additional mass (matter in the form of a gas) was added to the “empty” balloon.

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 as they have not yet developed a particulate notion of matter. However, before students proceed to middle school, it is important for elementary school students to understand that gases are matter and that they have weight (and mass).

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 students in elementary school grades, middle school students have more experience investigating gases. At this level, they should understand the idea that gases are made of molecules that have mass (and weight). As they investigate density, they discover that some gasses, like helium, are less dense than the surrounding air. However, they need to be able to distinguish between density and mass in order to reason why an object’s mass (not a substance’s mass) can increase while its density decreases. At this level, students begin to use the number of atoms and molecules to explain the conservation of matter or change in mass. If the number of atoms or molecules in an object remains the same before and after a change, then the mass stays the same. Conversely, if additional molecules or atoms are added to an object, such as putting gas into an “empty” balloon, then the mass increases.

High School Students

At the high school level, students deepen their understanding of gases by learning about the gas laws and behavior of fluids. However, they tend to hold on to their earlier ideas about the mass (and weight) of a gas if they are not confronted with their preconceptions. They may confuse density-related ideas with the comparison of masses (empty vs. gas-filled balloon) by not accounting for the change in volume in this phenomenon.

Students should be familiar with the rising of a helium-filled balloon. If not, consider bringing in a helium-filled balloon and an empty balloon of the same size and shape and ask students to think about the mass of each. If using this probe with younger elementary students, consider using the words empty instead of uninflated and weight instead of mass so that students’ unfamiliarity with the concept of mass does not interfere with their ability to answer this question and explain their reasoning.

• Students may believe that matter does not include gases or that gases are weightless materials (AAAS 1993).
• Researchers have suggested that one of the reasons students fail to recognize gases as having weight or mass is because one of their most common experiences with gases is with those that tend to rise or float (such as the helium). This view is supported by studies that show that children ages 9–13 tend to predict that gases have negative weight, such that when a gas like helium is added to a balloon, it will weigh less. Students believe that the more gas that is added to a container, the lighter the container becomes (Driver et al. 1994).
• Students at the end of elementary school and beginning of middle school may be at different points in their conceptualization of a “theory” of matter. Although some third graders may start seeing weight as a fundamental property of all matter, many students in sixth and seventh grade still appear to think of weight simply as “felt weight”—something whose weight they cannot feel is considered to have no weight at all (AAAS 1993, p. 336).
• 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).
• 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).
• Gases pose special difficulties for children since the ones 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

Adams, B. 2006. Science shorts: All that matters. Science and Children (Sept.): 53–55.

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.

Ontario Science Center. 1995. Solids, liquids, and gases: Starting with science series. Toronto: Kids Can 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.

• Before students can describe the properties of a gas, they must first accept a gas as being a substance or matter.
• Provide opportunities for students to use fundamental physical dimensions such as quantity, mass, volume, pressure, and temperature to describe the state of a gas (Sere 1985).
• Provide students with opportunities to enclose a gas (such as blowing up a balloon with air) and find the mass. Compare the mass of the empty balloon with the mass of the balloon filled with air. This can also be demonstrated by balancing two empty balloons on the end of a balanced meterstick or other lever. Blow up one balloon and reattach it to show the gain in mass.
• Use particle drawings to help students reason what happens when the uninflated balloon is filled with air. Have students draw particles of air in the “empty” balloon and then draw particles of air put into the balloon and tied off so the air does not escape. Have them compare the number of particles and then ask if an increase in the number of air particles increases the mass. Once students agree that the mass increases because air particles were added and each particle adds mass, then ask what would happen if other gases were used. Establishing the idea that all gases add particles, which add mass, may lead to helping students understand why a balloon’s total mass does not decrease when helium is added.
• For older students who have developed a concept of density, challenge them to show how the mass of an object containing a gas can increase while its density decreases. Have them connect this idea to the phenomenon described in the probe.