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 food, transformation of matter, growth and development, conservation of mass, and systems. The concepts underlying this probe are complex. It is not important that students know exactly what happens to the mass of an egg and why. Rather, this probe is used as an interesting context to draw out their ideas about several interrelated concepts in science.

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conservation of matter, embryo development, food, system, transformation of matter

The best answer is Group B’s: “We think an egg will lose mass. An egg’s mass is less just before hatching than when the egg was laid.” During normal incubation, chicken eggs lose approximately 16%–18% of their original mass (Snyder and Birchard 2005). On an average, bird eggs from small hummingbirds to large ostriches lose 15% of their original weight. This weight loss is primarily the result of water vapor passing through the permeable eggshell. Water vapor is a waste product of metabolism.

The egg yolk serves as food for the developing embryo inside. This food is used for the energy the embryo needs to carry out life processes, such as respiration. During respiration, carbon dioxide and water vapor are released as waste products. Molecules from the food are converted into the building material the embryo needs for growth and development.

Intuitively it would seem that the egg would weigh more after the chick has developed inside. The liquid matter (yolk and “white part”) inside the freshly laid egg is transformed into the body tissues of the embryo, which continues to grow and develop as cells divide. The yolk provides the energy and source of building material the chick needs for its development. Although oxygen does diffuse through the porous cell and is used during respiration, most of the material the embryo needs for development is packaged inside the cell at the time it is laid. Because the eggshell is permeable to gases, oxygen enters into the egg through the shell and some water vapor diffuses through the eggshell to the outside environment. If the eggshell were a perfect closed system, there would be no change in mass.

It is not important that students know that an egg can lose a significant percentage of its original mass. What is significant is that they recognize that matter is conserved during the transformation of the egg material and development of the chick but that some of this matter may escape through the egg because it is not a closed system.

Elementary Students

In the elementary grades, students study the life cycles of different organisms. Incubating chicken eggs is a common activity in some classrooms. Students at this level can examine eggs, learning that developing embryos, like all animals, need food and that the yolk is the source of the embryo’s food. However, the more complex notion of food being transformed into the body material of the embryo should wait until middle school.

Middle School Students

In the middle grades, students develop a scientific conception of what food is and how it provides energy as well as building material for organisms. At this level, students can begin to understand the transformation of the yolk into the body material of the developing chick as a result of chemical reactions and cell division. They also develop the idea of open versus closed systems and can use this idea to consider whether some materials can diffuse in and out of living and nonliving membranes or other porous materials.

High School Students

At this level, students investigate more complex ideas about embryology, chemical processes of metabolism, and passage of materials in and out of an open system. They should know about the breakdown and recombination of molecules during biochemical changes and how matter is conserved in each of these changes. They should recognize the semipermeable nature of membranes and porosity of other seemingly impermeable materials and explain how and why materials pass from an internal to external environment or vice versa.

This probe can be accompanied by a visual representation that shows the changes inside a chicken egg at different stages of development. However, be aware that the representation may reinforce the misconception that the egg would weigh more because it looks like there is more “stuff” inside the egg with the late stage embryo compared with the freshly laid egg that contains mostly liquid-like matter. When used as a discussion prompt, this probe can lead to a very lively discussion and argument among students that elicits ideas about food, conservation of matter, transformations, role of gases, and open and closed systems.

• Field tests of this probe with middle and high school students reveal that most students think the mass will be greater because the chick inside the egg is getting bigger. Some students use conservation reasoning to say the mass stays the same. Although their ideas about conservation of mass during the transformation of matter are correct, they fail to consider whether the egg is a closed system.
• Some students use an intuitive rule of “more A, more B” (Stavy and Tirosh 2000). They reason that if the chick gets bigger inside the egg, then the mass or weight of the egg increases.
• Children often do not recognize that food is the material basis for growth—that is, that the food becomes transformed and incorporated into the body, thus making the body bigger (Driver et al. 1994).
• In a study conducted by Russell and Watt (1989), elementary children assumed that the growth inside an egg is associated with an increase in mass within what they assumed was a closed system. They described the process of growth as creating new material rather than transforming material (yolk) that was in the egg. Only a very small minority considered some type of transformation of the contents inside the egg into a complete chick.
• Some ideas about embryonic development contribute to students’ notions about what is happening inside the egg. Some children believe that the chick had always been there inside the egg waiting until it was time to hatch. Others thought all the parts of the chick were there when the egg was laid and that they came together in the egg (Driver et al. 1994).

American Association for the Advancement of Science (AAAS). 2001. Atlas of science literacy. Vol. 1. (See “Flow of Matter in Ecosystems” map, pp. 76–78.) Washington, DC: AAAS.

• This probe can be used as a P-E-O probe (Predict, Explain, Observe) with activities that involve egg incubation. Have students predict what would happen to the mass of the eggs as the eggs develop over the course of their incubation. Have students explain the reasons for their predictions. Then have students test their ideas and observe the decrease in mass. Encourage students to come up with alternative explanations to account for the discrepancy between their predictions and their observations.
• Probe further to find out students’ conceptions of an open versus closed system in relation to the egg. One way to show that water can pass through an egg shell is to place a raw egg in corn syrup. The mass of the egg in corn syrup will decrease because water from inside the egg flows through the membrane and shell into the syrup. It moves from a higher concentration inside the egg to a lower concentration in the corn syrup. The corn syrup molecules are too large to pass into the egg. You can also try this with molasses. Careful observations will reveal a thin layer of water resting on top of the molasses.
• Find the mass of a raw egg and leave it in a warm, dry area for two to three weeks. Find the mass of the egg again, noticing a decrease in mass. Encourage students to propose ideas about where the loss in mass came from. Did anything leave the egg?
• Safety caution: Always have students wash their hands after handling eggs.
• Use a graphic of a chick embryological development such as the one at http://msucares.com/ poultry/reproductions/poultry_chicks_embryo. html (Mississippi State University Extension Service 2004). Have students propose ideas about (a) where the material is coming from that leads to the growth of the chick inside the egg and (b) the chemical life processes that are occurring.