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 a physical property of matter. The probe is designed to reveal whether students recognize that under the same conditions, boiling point remains the same.

Familiar phenomenon, P-E-O

Boiling point, characteristic properties, intensive properties, temperature, properties of matter

The best answer is C: The boiling temperature is the same at both settings. Temperature is a measure of the average kinetic energy of the molecules in a system. When heat is transferred to a liquid, the molecules gain energy. The motion of the liquid’s molecules, and hence the temperature of the liquid, increases until the temperature of the liquid reaches its boiling point (each pure substance has a specific temperature at which it will boil). Once a liquid is at its boiling point, the energy supplied to the system is used to overcome attractive forces between particles in the liquid. This results in the change from a liquid phase to a gas phase, allowing the molecules to escape into the air. The molecules that escape into the gas phase take away some energy from the liquid phase. Because the liquid continues to heat, molecules continue to escape and the temperature of the remaining liquid essentially stays constant as heating continues. In the case of pure water, the boiling point is 100°C (212°F). (Impurities in tap water may result in a slight temperature rise during an extended period of boiling as the remaining solution becomes more concentrated.) The temperature will not rise again until all the water changes to the gaseous state. If heat continues to be applied to the gaseous state, the temperature of the gas will rise. Whenever a pure substance undergoes a phase change, its temperature remains constant as long as the two phases are present.

Elementary Students

At the elementary level, students’ experiences with materials are primarily observational. They observe changes in temperature by heating or cooling materials. Upper elementary students are familiar with the change in states of water from the solid to liquid to gas phase and vice versa. They learn how to use thermometers to measure the temperature of water. They may observe that water boils at 100°C under standard conditions, but the concept of the difference between heat and temperature and an explanation of why boiling point remains constant is beyond this grade level.

Middle School Students

In middle school, students shift their focus from properties of materials to the properties of the substances from which the materials are made. Students learn about the characteristics of different states of matter and the properties associated with phase changes from solid to liquid to gas. Opportunities to observe and measure characteristic properties such as boiling and melting points can be used to separate materials. Students identify characteristic properties that can be used to identify substances, such as boiling point, melting point, and density. They observe that pure substances have a constant boiling point and that this boiling point does not change under standard conditions, no matter how vigorously or gently the liquid boils. Students are beginning to connect the ideas of heat, temperature, and constant boiling point, although the distinction between heat and temperature is still difficult to understand at this grade level.

High School Students

During high school, instructional opportunities connect the macroscopic properties of substances to microscopic properties, such as the attraction between water molecules. At this level, students can use particle ideas to explain the role of heat and temperature during phase changes. However, the distinction between heat and temperature is still a concept that eludes many high school students.

This probe is best used with grades 6–12. Make sure students know that water from a tap contains small amounts of other substances such as minerals. For the purpose of the probe, have students assume the water is pure. Make sure students are familiar with the dial used to adjust burner temperature on a stove since they may have experienced a variety of stoves and dials in their home settings. It may be helpful to have visual props for this probe. While wearing safety goggles, bring a beaker of water to a vigorous boil. Lower the amount of heat supplied to the boiling water, and keep the water at a gentle simmer as students respond to the probe and explain their thinking.

• Students may use the intuitive rule “less A, less B” to reason what happens when you turn down the dial of a stove and the water boils gently. Using this rule, students may think that when the temperature dial is turned down, the boiling temperature decreases (Stavy and Tirosh 2000).
• Many students think that the boiling point of water increases when the setting on a stove is “turned up.” Much of this confusion is related to a misconception that heat and temperature are the same thing. Students argue that if you increase the amount of heat, you will increase the boiling temperature (Driver et al. 1994).
• In a study of high school students’ understanding of basic chemical concepts, students were asked to explain a change from ice to water when two phases were present. Some students thought the reason for the constant temperature was because it takes a period of time for the thermometer to change, and others thought the heat was not hot enough (Abraham and Williamson 1994).

Cavallo, A. M., and P. Dunphey. 2002. Sticking together: A learning cycle investigation about water. The Science Teacher 69 (8): 24–28. Konicek-Moran, R. 2013. Pasta in a hurry. In Everyday physical science mysteries: Stories for inquiry-based science teaching, R.

Konicek- Moran, 185–193. Arlington, VA: NSTA Press.

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.” Available at http:// learningcenter.nsta.org/products/symposia_seminars/ NGSS/webseminar27.aspx.

• This probe can be followed up with the science practice of designing and carrying out an investigation. Ask the question, encourage students to commit to a prediction, and then test it. The dissonance involved in discovering that the boiling temperature did not change should be followed with opportunities for students to discuss their ideas and resolve the dissonance.
• Encourage students to use the crosscutting concept of cause and effect in their explanation: When water boils, lowering the dial setting on a stove will _____ the boiling temperature of the water because ______.
• Have students use phase change graphs to analyze patterns and notice that when two phases are present (e.g., boiling water includes water in the liquid form and vapor), the temperature remains the same until there is only one phase present.
• Connect the idea of a constant boiling point to other characteristic properties of a substance that remain constant under the same conditions, such as melting point, solubility, and density.
• Be sure to explicitly develop the generalization that boiling point is a constant property for all liquid substances, not just water.
• Use caution when introducing the difference between heat and temperature in the context of this probe until students are ready to understand this difference.