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 the boiling point of a pure substance. The probe is designed to find out whether students recognize that the boiling point of a pure substance stays constant no matter how long it boils.

Familiar phenomenon, P-E-O

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

The best answer is A: The boiling temperature did not change. Temperature is a measure of the average kinetic energy of the molecules in a system. When a liquid is heated, the kinetic energy of the molecules increases. 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 gained by 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 temperature of the remaining liquid essentially stays constant as heating continues. In the case of pure water under standard conditions, this boiling point is 100°C. (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 is transformed into a gaseous state. If the heat continues to be applied to the gaseous state, the temperature of the gas will rise.

Elementary Students

At the elementary level, students’ experiences with objects and materials are primarily observational. Students subject objects and materials to temperature changes through heating and cooling and observe which changes can go back and forth. Upper elementary students are familiar with the changes 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 (or slightly more if there are impurities in the water), but understanding the difference between heat and temperature and the notion of boiling point as a characteristic property exceeds expectations for this grade level.

Middle School Students

In middle school, students shift from observable properties to explaining properties at the particle level, including transitions during phase changes. They learn that some properties, such as boiling point and density, can be used to identify a pure substance. They learn to distinguish between heat and temperature, although this distinction is still difficult to understand at this grade level. Opportunities to observe and measure characteristic properties such as boiling and melting points can be used to distinguish and separate one substance from another. Students have had experiences with boiling liquids, and this probe may be useful in determining students’ ideas about heat and temperature and whether they recognize that boiling temperature is a constant. Although students can identify the boiling point of pure water as 100°C under normal conditions, they may still intuitively believe that the temperature rises the longer heat is applied to a boiling liquid.

High School Students

During high school, instructional opportunities connect the macroscopic properties of substances to microscopic properties, including the attraction between molecules. Students relate the particulate nature of liquids and gases to the role of heat during phase changes. By high school, students should distinguish between heat and temperature, although it is still a very difficult concept for most students to understand.

This probe is best used with grades 6–12. Make sure students understand the difference between a pure substance and an impure one (e.g., distilled water versus tap water). Students may want to know what type of liquid is boiling. It may be helpful to have visual props for this probe. If you use water, make sure students realize that the probe is asking about any pure liquid. Bring a beaker of water to a full boil (while wearing safety glasses). Continue to heat the boiling water as students respond to the probe and explain their thinking.

• Students do not make a clear distinction between heat and temperature, and they often believe that temperature is the measure of heat (AAAS 2009).
• Students often will use a “more A, more B” type of intuitive rule for reasoning about what happens to the temperature the longer a liquid boils. Based on the everyday experience that the temperature of an object rises when heated, students may reason that the longer you heat a substance after the onset of boiling, the higher the temperature will be (Stavy and Tirosh 2000).
• Some students think that the boiling point of water increases the longer it is allowed to boil. Much of this confusion is related to a misconception that heat and temperature are the same thing. Thus, students are apt to argue that the longer you heat something, the hotter it gets (Driver et al. 1994).
• A standard laboratory exercise is to plot a time-temperature graph of water as it changes from melting ice to boiling water. Although students can readily see the steady temperature as they make their observations of the boiling water, the counterintuitiveness of the phenomenon often results in disbelief statements such as, “This thermometer is not working properly” (Erickson and Tiberghien 1985, p. 64).

Brockway, D., and M. Papaleo. 2009. Watching the pot to improve inquiry skills. Science Scope 33 (2): 37–43.

Cavallo, A. M., and P. Dunphey. Sticking together: A learning cycle investigation about water. The Science Teacher 69 (8): 24–28.

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.

Purvis, D. 2006. Fun with phase changes. Science and Children 32 (5): 23–25.

NGSS Archived Webinar: NGSS Core Ideas—Matter and Its Interactions, 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. However, be aware that tap water that contains impurities may change a little during 20 minutes of boiling. As water boils away, the remaining solution becomes more concentrated, and boiling temperature increases slightly.
• Conduct a similar investigation to examine the effect of continuous heating on the temperature of a substance existing in two different phases, such as a container filled with water containing ice cubes, snow, an ice “slush,” or another familiar substance as it melts. Measure temperature at different time intervals while the substance is still melting. Contrast the findings from melting (a liquid and solid phase being heated over time) with heating just a liquid phase to boiling (a liquid and gas phase being heated over time).
• When having students develop time-temperature graphs, be aware that they may be able to explain their findings as shown on the graph yet revert to their belief that the temperature does not remain constant. Be sure to provide sufficient time to discuss the graphs and what the data show.
• Be explicit about developing the generalization that a constant, specific boiling point applies to all pure liquid substances, not just water.