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 density and buoyancy. The probe is designed to find out how students think an object can be changed to make it float differently.

P-E-O

Properties of matter, buoyancy, density, floating and sinking

The best responses are C and G. To make the solid ball float so that most of it is under the water, you can either use a ball of the same size made out of a denser material or attach a weight to the ball. The degree to which a solid object will float when placed in water depends on the density of the material. To be further submerged, the density of the object must be increased. Density is defined as the ratio of the mass to the volume of an object. By using a ball of the same size made out of a denser material, the ratio of the mass to volume is greater, which causes the object to be further submerged. By attaching a weight to the ball, the proportion of the total mass relative to volume is increased, so the overall density is increased. This, too, will result in the object being further submerged. As more matter is attached to the ball, the buoyant force increases, which is indicated by the displacement of more water.

Adding more water to the tank has no effect on how an object floats. An object floats the same way regardless of how deep or shallow the water is. Adding salt to the water actually makes the object more buoyant because salt increases the density of the water. For example, when you swim in the ocean, you float better than when you swim in fresh water because salt water is denser than fresh water.

Elementary Students

At the elementary level, students typically have experiences with floating and sinking objects of different sizes and shapes. They are able to describe observable properties of objects that affect how they float. Although it is too early to expect them to quantitatively explain results and use the concept of density, they can make changes to objects and observe cause and effect. Upper elementary students may be more systematic in their investigation of floating and sinking objects and may make quantitative measurements of weight and volume. At this level, students can plan and carry out simple experiments that involve a fair test, which is a precursor to understanding independent and dependent variables.

Middle School Students

In middle school, students transition from having observational experiences that involve floating and sinking to developing a conceptual understanding of density and how it affects the buoyancy of an object. They use mathematics to understand how density is a proportional relationship between the total mass and volume of an object. Students also use ideas about pairs of interacting forces in fluids, such as the buoyant force that pushes an object upward and the gravitational force that pulls an object downward.

High School Students

Students build on their understanding of pairs of interacting forces (buoyant force and gravitational force) and use mathematics to predict the effect of changing the mass or volume of an object on its density and thus on how it floats in a liquid.

This probe is best used with grades 3–8. Consider using visual props as you introduce the scenario. Place a sphere that floats in a container of water. Then display objects and materials that represent each of the things that could be changed, and have students respond and explain their thinking. Remove answer choices that include terminology that is unfamiliar to younger students or simplify the terminology; for example, replace “made out of a denser material” with “made out of material that is heavier for its size.”

• Students will often mistake buoyancy-related phenomena for characteristics of density (Libarkin, Crockett, and Sadler 2003).
• Ideas that interfere with students’ conception of density include the belief that when you change the shape of something, you change its mass and the belief that heaviness is the most important factor in determining whether an object will sink or float (Stepans 2003).
• When students investigate and explain sinking and floating, they typically focus on only the object they are testing and ignore the liquid that the object is in (Houghton et al. 2000).
• Some students use an intuitive rule of “more A-more B.” They reason that if you have more material, density increases or makes an object sink more (Stavy and Tirosh 2000).
• Notions of weight and density develop as children begin to take account of viewpoints other than their own. At ages 9–10, students begin to relate density of one material to another. For example, they may say a material floats because it is “lighter than water.” Many students have misconceptions about volume that present difficulties for understanding density. Also, students’ ways of looking at floating and sinking include the roles played by material, weight, shape, cavities, holes, air, and water (Driver et al. 1994).
• In a study of 60 Australian 11-year-old students, more than 80% had misconceptions about volume, which led to difficulty in understanding density (Rowell, Dawson, and Lyndon 1990).
• Biddulph and Osborne (1984) conducted a study during which some students ages 7–14 suggested that things float because they are light. When asked why objects float, the students offered different reasons for different objects. The same study asked students ages 8–12 how a longer candle would float compared with a shorter piece; many students thought the longer candle would sink or float lower.

Bell, R., and H. Banchi. 2008. The many levels of inquiry. Science and Children 46 (2): 26–29.

Gomez-Zwiep, S., and D. Harris. 2007. Sinking and floating: Bringing math to the surface. Science Scope. 31 (4): 53–56.

Mayer, K., and J. Krajcik. 2017. Core idea PS1: Matter and its interactions. In Disciplinary core ideas: Reshaping teaching and learning, ed. R. G. Duncan, J. Krajcik, and A. E. Rivet, 13–32. Arlington, VA: NSTA Press.

NGSS Archived Webinar: NGSS Core Ideas—Matter and Its Interactions, http://learningcenter. nsta.org/products/symposia_seminars/NGSS/ webseminar27.aspx.

Peterson-Chin, L., and D. Sterling. 2004. Looking at density from different perspectives. Science Scope 27 (7): 16–20.

Shaw, M. 1998. Diving into density. Science Scope 22 (3): 24–26.

Yin, Yew, M. Tomita, and R. Shavelson. 2008. Diagnosing and dealing with student misconceptions: Floating and sinking. Science Scope 31 (8): 34–39.

• This probe can be used with the P-E-O (predict-explain-observe) strategy and the science practice of planning and carrying out an investigation. Have students predict, explain, and systematically test and observe how solid objects float in water when changes are made to the object or the liquid. If observations do not match students’ initial predictions, have students further explore the phenomenon and revisit and revise their initial explanations.
• This probe can be used with a station approach to investigate floating and sinking phenomena. Refer to the article Using Formative Assessment Probes to Develop Elementary Learning Stations (Keeley 2018) for more information on how to set up learning stations using formative assessment probes.
• When dealing with density-related phenomena, use the terminology mass, volume, and density with middle and high school students. With elementary school students, use the more familiar terms size, weight, and heavy for its size. Research indicates that young students mistake the word mass for massive and confuse the term with the size of objects. In other words, a large foam ball is more “massive” to them, and thus they may think it has more mass than a small wooden ball. The Next Generation Science Standards also avoid using the term mass until middle school.
• Change the object. Instead of using a ball, try other objects of different shapes and materials. Have students investigate whether cutting a banana into a variety of shapes and sizes will change how it floats in water. Put an orange in water and observe how it floats after the peel is removed, and then encourage explanations of the phenomena.