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 dissolving. The probe is designed to find out what students think happens to sugar when it dissolves in water.

Justified List

dissolving, mixture, physical change

The best answers are D, E, and G. A grain of sugar is actually a large collection of sugar molecules. When these grains of sugar are added to the water, they dissolve—forming a mixture called a solution. This solution is formed because water is a polar molecule, in which one part of the molecule has a slight positive charge and the other part has a slight negative charge. Opposite charges attract. When grains of sugar (tiny sugar crystals) are added to water, the positive part of the polar water molecules attracts (through dipole forces) the specific groupings on the sugar molecules (called hydroxyl groups) that have a slight negative charge. This dipole force does not break the molecular bonds in the individual sugar molecules, resulting in individual atoms that make up the sugar. Instead, the force overcomes the intermolecular attraction that holds the large number of individual sugar molecules together in the form of a “sugar grain,” or crystal. The sugar molecules become surrounded by the attracted water molecules so that the individual sugar molecules are no longer part of the crystal. This process is repeated until either (a) all the sugar (the solute) is dissolved in the water (the solvent) to form a solution or (b) there are no longer any “unattached” water molecules and no more sugar can dissolve.

When sugar dissolves, it is physically, not chemically, combined with water, and therefore it does not form a new compound. Instead, it forms a mixture—sugar water. The sugar can be separated from the water in solution through evaporation or boiling off of the water. Sugar crystals form again through intermolecular forces among the sugar molecules that are no longer surrounded by the polar water molecules. During the dissolving process, the sugar still exists as a molecular compound. It has not melted because melting involves a change of state and does not require the interaction between two substances as dissolving does. According to the conservation of mass principle, the weight or mass of the sugar remains the same even though it cannot be seen in the solution. It is still there. No additional sugar molecules have been added or taken away after dissolving.

Elementary Students

In the elementary grades, students explore a variety of observable physical changes, including dissolving and melting. Mixing sugar in water and evaporating water to recover sugar crystals is a common experience for elementary students. They see the sugar “disappear” but may not understand where it goes. By evaporating the water, they see that it is does “come out” of the solution. This probe is useful in eliciting early ideas about what happens to substances that dissolve. However, a molecular explanation of what happens during dissolving exceeds this grade level.

Middle School Students

In the middle grades, students begin to use particulate ideas to explain phenomena such as sugar dissolving or substances melting. They should be able to differentiate between chemical changes of the same substance (e.g., burning sugar) versus a physical change (e.g., dissolving sugar). Mixtures and solutions are commonly investigated by students at this grade level. However, students at this level may still confuse dissolving with melting, particularly when a liquid is involved as part of the system. At this level the difference between dissolving and melting can be explained by interactions. Middle school students may begin to use the idea of molecules or simple particulate models to explain what happens when the sugar dissolves, although an explanation of the nature of attraction between sugar and water molecules should wait until high school. At this level, students should also be able to use the idea of atoms and molecules to explain how the mass (or weight) is conserved during the dissolving process.

High School Students

Students at the high school level develop a more sophisticated understanding of a particulate model of matter that can be used to explain dissolving. They encounter formal concepts and ideas in chemistry dealing with the attraction among and between particles and their arrangements and begin to develop an understanding of the hydrogen bonds and the attraction between molecules of a solute and solvent. This more sophisticated particulate model can be developed to help overcome ideas about dissolving and melting being the same process. This probe is useful in finding out if students have changed previously held ideas or if they still hold on to their preconceptions about dissolving, even after formal instruction.

Be sure students are familiar with the phenomenon described. It may be useful to demonstrate by dissolving a teaspoon of sugar into a glass of warm water so that they can see that the sugar is no longer visible in the glass of water. For younger students, consider eliminating choices C, G, and H if they have not yet had an opportunity to develop ideas about the structure of matter at the molecular level.

• From an early age through to adulthood, conceptions about dissolving include the following: the solute “disappears,” “melts away,” “dissolves away,” or “turns into water.” Older students imagine that as sugar dissolves, it “goes into tiny little bits,” “sugar molecules fill spaces between water molecules,” or sugar “mixes with water molecules” (Driver et al. 1994).
• Students’ ideas about what happens to sugar as it dissolves frequently fail to include the conservation of mass. The gap between the proportion of students who conserved substance but not mass widened between ages 9 and 11 but narrowed in later school years. After age 12, many, but not all, students begin to develop a conception of weight and mass and begin to conserve mass of the solute (Driver et al. 1994).
• Students’ ideas about solutions include thinking that sugar solutions are not a single phase but rather that invisible gross particles of sugar are suspended in the solution. They may suggest that the particles can be filtered out or will settle out from the solution. Others see the solute and solvent as a single substance rather than as a homogeneous mixture (Driver et al. 1994).
• Cosgrove and Osborne (1980) sampled children ages 8–17 and found they regarded melting and dissolving as similar processes because they were both gradual.

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

Ashbrook, P. 2006. Mixing and making changes. Science & Children (Feb.): 29–31.

Kessler, J., and P. Galvan. 2006. Dynamics of dissolving. Science & Children (Feb.): 45–46.

National Science Teachers Association (NSTA). 2005. Properties of objects and materials. NSTA SciGuide. Online at http://learningcenter.nsta.org/ product_detail.aspy?id=10.2505/5/SG-01.

• This probe can be used to launch an inquiry-based investigation related to what happens to the mass of sugar during the physical process of dissolving in water. Ask students to predict and explain what will happen to the combined mass of the sugar and water before and after dissolving, then test it.
• Challenge students to come up with a way to separate sugar once it is dissolved in water.
• Ask older students to draw “particulate pictures” to show and explain what happens to the sugar and where it goes when it dissolves in water.
• If students experience only colorless solutions, such as sugar in water, the idea that the solute “disappears” may be reinforced. Provide colored solutes such as drink crystals and use the presence of uniform color throughout the solution to help students understand that this is additional evidence for the matter still existing, even though its form has changed. However, it may still be necessary to provide additional experiences to confirm that the total mass (or weight) does not change.
• When teaching about physical changes, have students come up with a “rule” that can be used to identify a type of physical change. For example, help students recognize that dissolving involves two materials whereas a phase change such as melting involves one. This can explicitly be used to help students develop a rule to decide if something dissolves or melts.
• With middle school students, use the “Scientists’ Idea” strategy to compare their initial ideas about the probe and dissolving with how a chemist would answer the probe. You can use the explanation on pages 12–13 as the chemist’s ideas or provide a reading in their text or other resources that describe dissolving. Several other formative assessment strategies described in Science Formative Assessment: 75 Practical Strategies for Linking Assessment, Instruction, and Learning (Keeley 2008) can be used with this probe.