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 floating and sinking. The probe is designed to find out if students think changing the size of an object affects how it floats.

P-E-O

Properties of matter, intensive properties, density, floating and sinking

The best response is B: Half of the larger log floats above the water surface. The degree to which a solid object will float when placed in water depends on the density of the material. Density is an intensive property of matter, which means that it is independent of the amount of material. If one sample of material is very large and another sample of the same material is very small, the proportion (ratio) of the mass to volume of each sample is still the same, so the density remains the same. The first and second logs were both cut from the same tree, so they are made of the same material and have close to the same density. (There may be a slight difference because the logs are a mixture and are not made of a homogeneous substance.) Because the densities are essentially the same, the two different-sized logs will float at equal levels. Half of the first (smaller) log floated above the water’s surface, so half of the second (larger) log will also float above the water’s surface.

Elementary Students

At the elementary level, students have observational experiences with floating and sinking objects of different sizes and shapes. They are able to describe observable properties of objects, such as how much of an object floats above the water’s surface. They begin to connect the crosscutting concept of cause and effect to properties. Although some things may change (such as size), other things may stay the same (how an object floats). This probe may be useful in determining students’ ideas about floating objects and whether things made of the same material have the same properties. However, the concept of intensive properties, such as density, should wait until middle school.

Middle School Students

In middle school, instructional experiences with density progress from observational (floating or sinking and heavy for its size) to a conceptual understanding of density as a characteristic property of matter that describes the proportional relationship between mass and volume. Students begin to use mathematics and the crosscutting concept of scale, proportion, and quantity to describe density of different amounts of matter. Middle school is a good time to make the distinction between not only properties such as volume, mass, or weight that change with amount but also properties such as density that are not affected by the amount of matter. By the end of middle school, students should understand that two objects composed of the same substance and in the same state (solid, liquid, gas) under the same conditions of temperature and pressure will generally have the same characteristic properties, which can be used to identify them or predict their behavior. Students can now use technical vocabulary such as mass, volume, and density. However, it is important to determine if they have a conceptual understanding of density before introducing the D = M/V mathematical relationship (density equals mass divided by volume).

High School Students

Instruction at the high school level builds on the concept of characteristic properties of substances, such as density, which was developed in middle school and integrates the details of atomic structure with how atomic architecture plays a role in determining the properties of materials. The terms intensive and extensive properties of matter are introduced in high school. By high school, students should be able to explain the distinction between intensive and extensive properties at both a substance and particle level.

This probe is best used with grades 3–12. You may wish to use props to help younger students visualize the manner in which the first log is floating with respect to the water’s surface and to show students what it means when logs float on their sides, rather than upright like a buoy. Place an object that floats in a clear container of water so that students can see what is meant by “above and below the water’s surface” and “floating on its side,” or draw a picture to explain it. Show students a second object composed of the same material that is longer and wider than the first object (such as a dowel of a different width), but don’t place this object in the water. The probe can be extended for middle and high school students by asking them to use mathematical reasoning in their explanation.

• Some students age 15 and older still use sensory reasoning about matter, despite being able to think logically and use mathematics. They may recite a definition of density as mass over volume and perform density calculations yet hold a common belief that the more massive or heavy an object, the denser it is (Kind 2004).
• 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).
• 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).
• Students’ ways of looking at floating and sinking include the roles played by material, weight, shape, cavities, holes, air, and water. Also, researchers have found that some students have misconceptions about volume that present difficulties for understanding density (Driver et al. 1994).
• In a study of 60 Australian 11-year old students, over 80% had misconceptions about volume that 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.

German, S. 2017. Creating conceptual storylines. Science Scope 41 (5): 26–28.

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

Grooms, J., P. Enderle, T. Hutner, A. Murphy, and V. Sampson. 2016. Argument-driven inquiry in physical science: Lab investigations for grades 6–8. Arlington, VA: NSTA Press.

Keeley, P. 2010. “More A-more B” rule. Science and Children 48 (2): 24–26.

Keeley, P. 2014. More A-more B rule. In What are they thinking? Promoting elementary learning through formative assessment, P. Keeley, 9–16. Arlington, VA: NSTA Press.

Libarkin, J., C. Crockett, and P. Sadler. 2003. Density on dry land: Demonstrations without buoyancy challenge student misconceptions. The Science Teacher 70 (6): 46–50.

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 followed up using the science practice of planning and carrying out investigations. Give students wooden dowels of different lengths and thicknesses or different-sized wooden blocks of the same type of wood to determine if they float differently. Have students use their results to support their answer choice to the probe and explain the phenomenon.
• The probe “Mass, Volume, and Density” in Uncovering Student Ideas in Physical Science, Volume 3 can be used to reveal further student ideas about these concepts (Keeley and Cooper 2019).
• Try a different material to explain the phenomenon. Cut a very small piece of Ivory soap (a soap that floats) versus the rest of the bar of Ivory soap. Or cut off a tiny piece of a soap that sinks, and ask students if they think that piece of soap will float, sink, or float differently depending on its size.
• Investigate the floating and sinking of the same kind of material—for example, Styrofoam balls—with different sizes and the same shape. Similar investigations can be conducted with strawberries, blocks of wood, or rubber objects. Then have students investigate how different shapes of the same material float.
• When middle school or high school students have developed the conceptual understanding of density, have them use the science practice of using mathematics and computational thinking to support their explanations with proportional reasoning and connect it to the crosscutting concepts of scale, proportion, and quantity. It is counterproductive to start by using D = M/V if students have not developed a conceptual understanding of density first.
• Connect the observation of how the wood floats in water to the concept of density. Have students discuss whether they can identify the type of wood based on its density.
• Make the probe three dimensional by having students use the crosscutting concept of cause and effect in their explanation. When you change the size of an object made from the same material, what effect does it have on how the object floats?
• Probe further to determine if students use the same reasoning to explain what happens to different properties when