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 particles during a phase change. The probe is designed to find out if students recognize that the bubbles formed when water boils are the result of liquid water changing into water vapor.

Friendly talk

Phase change, boiling, water vapor, gas, properties of matter

The best answer is Grandma’s: The bubbles are an invisible form of water. This invisible water is called water vapor, a gaseous form of water that is not visible. Sometimes it is called steam. However, steam can contain very small droplets of liquid water, which allows it to sometimes be visible. When water is heated, the energy supplied to the system results in an increase in molecular motion. If heated enough, the molecules have so much energy that they can no longer remain loosely connected and thus start to slide past one another. The increased energy now allows the attractive forces between water molecules to be overcome, and they form an “invisible” gas in the form of water vapor. Because the molecules in the gas phase are so much farther apart than in the liquid phase, they have a much lower density, are more buoyant (causing them to “bubble up”), and escape into the air. The bubble is the invisible water vapor that rises to the surface and escapes from the liquid.

Elementary Students

At the elementary level, students have experiences observing changes in state. The idea of change is connected to physical properties of materials by subjecting materials to heating and cooling. Water is often used as a familiar substance for observing phase changes. In the primary grades, the focus is primarily on solids and liquids. Elementary students describe change in states of water from the solid to liquid to gas phase and vice versa, although the change from liquid to gas phase is an abstract idea developed more fully in upper elementary grades. Children develop ideas about bubbles early on through their everyday experiences, so it is not too early to ask students their ideas about particles and bubbles. However, it is best to hold off on expecting a scientific explanation until middle school when students learn about kinetic molecular theory.

Middle School Students

In middle school, students use the kinetic molecular theory to explain what happens at the particle level during phase changes. They compare evaporation of a liquid under ordinary ambient conditions as well as in situations where increased application of heat is involved, such as boiling water. They are encouraged to use models to explain everyday phenomena such as the water boiling in a kettle.

High School Students

At the high school level, students connect ideas about energy to phase change. They develop the idea that the continuous addition of energy during the heating of a liquid overcomes the attractive forces between molecules of a liquid during the liquid-to-gas transition. They understand that a chemical change in which the water molecules break down into simpler substances does not take place.

This probe can be used with students in grades 3–12. If used with elementary students, remove the last answer choice. You may wish to use visual props for this probe. Bring a beaker (or some other clear, boiling-safe container) of water or to a full boil so that students can see the bubbles forming and rising to the surface. Be sure students are wearing safety goggles and are not too close to the heat source if they are observing the boiling up close. Continue to heat the boiling water as students respond to the probe and explain their thinking.

• Because students cannot observe gases, they often do not think of a gas as being the same type of matter that makes up solids and liquids (Mayer and Krajcik 2017).
• It has been well documented that secondary students think the gas produced from boiling water is a mixture of hydrogen and oxygen gas (Mayer 2011).
• Students’ understanding of boiling precedes their understanding of evaporation from surfaces such as dishes and roads. In a sample of students ages 6–8, 70% understood that when water boils, vapor comes from it and that the vapor is made of water. However, the same students did not recognize that when a wet surface dries, the water turns to water vapor (Driver et al. 1994).
• An analysis of middle school students’ test results showed that explaining changes of state in molecular terms was among the most difficult tasks for many students. Before instruction, almost none of the students in the study could give molecular explanations of changes of state. After instruction, significantly more students demonstrated understanding, although many students still had difficulties understanding changes of state in molecular terms. For example, one student thought that when molecules are in the water, they move farther apart, they move faster, and then they turn into air (evaporate) (Lee et al. 1993).
• Osborne and Cosgrove (1983) studied New Zealand students ages 8–17. An electric kettle was boiled in front of the students so they could see the bubbles in the boiling water. They were asked what the bubbles were made of. Heat, air, oxygen, hydrogen, and steam were common replies, with the percentage of students answering steam increasing between ages 12 and 17. However, most 17-year-old students thought that water can be split into its component elements by heating, or that heat is a substance in its own right, or that air is contained in water. Osborne and Cosgrove attribute the idea that water molecules break up to knowing the formula of water is H2O, so naturally it comes apart.

Cavallo, A. M., and P. Dunphey. 2002. Sticking together: A learning cycle investigation about water. The Science Teacher 69 (8): 24–28.

Dasgupta, A. 2008. Water in disguise: Demonstrations exploring water as a solid, liquid, or gas. Science and Children 46 (4): 28–31.

Duncan, R., J. Krajcik, and A. E. Rivet. 2017. Disciplinary core ideas: Reshaping teaching and learning. Arlington, VA: NSTA Press.

NGSS Archived Webinar: NGSS Core Ideas—Matter and Its Interactions, http://learningcenter. nsta.org/products/symposia_seminars/NGSS/ webseminar27.aspx.

Pentecost, T., S. Weber, and D. Herrington. 2016. Connecting the visible world with the invisible: Particulate diagrams deepen students’ understanding of chemistry. The Science Teacher 83 (5): 53–58.

Peters, E. 2006. Building student mental constructs of particle theory. Science Scope 30 (2): 53–55.

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

Smith, P., C. Plumley, and M. Hayes. 2017. Much ado about nothing: How children think about the small particle model of matter. Science and Children 54 (8): 74–80.

• Use the phenomenon of bubbles to explain what happens to water molecules during a change in state from a boiling liquid to a gas. Extend the probe by having students use the science practice of developing and using models to explain what is happening at the particle level as the water forms bubbles and bubbles rise to the surface and burst.
• Use the formative assessment classroom technique (FACT) called BDA drawing to explain what is happening as water boils (Keeley 2015). This type of model involves three drawings: B-before, D-during, and A-after. For B, have students draw their particle model of matter to show molecules in the liquid state before the water is heated. For D, they should show what is happening to the molecules during heating and as bubbles begin to form. For A, they should show what is happening when the bubbles rise to the surface and burst. Have students share and explain their representations. As they share, carefully note how students get to the bubble stage—do the bubbles appear spontaneously in their drawings, or does the act of drawing help them make sense of what is happening to the water to form bubbles?
• Extend the probe by asking students to use the crosscutting concept of cause and effect to explain the phenomenon.
• Students may have trouble accepting that water vapor is in the bubbles if they do not understand the idea that water vapor is invisible. Help students contrast the concept of invisible water vapor with visible water in the air, such as clouds and fog, which are made of tiny suspended droplets rather than water molecules in the form of a gas spread far apart.
• Ask students to observe and describe what happens to the water level as the water boils. Encourage them to explain where the water went. How was it able to leave the glass container? Probe students to consider how the bubbles were involved in decreasing the water level. Challenge students who had the idea that the bubbles were air or nothing to explain how their model could account for the decreased water level.
• Consider how to present phase changes as reversible. Allow students to see heating and cooling cycles for themselves, so they can realize that phase changes do not result in a new substance being formed. This cycle may help them see that the water escapes as a gas in the bubbles and can be recovered again through cooling.
• By fifth grade, students should be using terminology such as water vapor. Using the correct terminology combined with an understanding that “invisible” water is in the air may help them overcome the idea that water changes into air rather than remaining the same substance but in a form that you cannot see.
• Be cautious when using the term steam with students to describe the gas or vapor form of water. What students are actually seeing over the boiling water when they refer to steam is a wispy mist—it is visible because it is water in a gaseous state that also contains tiny water droplets. Those tiny droplets scatter light at their surfaces, allowing us to “see” the “steam” in much the same way that we can see fog or clouds. The common use of the word steam is different from the way scientists or engineers use the word steam. To scientists and engineers, steam and vapor are both invisible forms of water in the gaseous state. However, when students (and often teachers) use the word steam in the context of this probe, they are usually referring to the visible substance that forms above the boiling water—a gas. Technically, this common use of the word steam is incorrect because a gas is invisible. Use the term water vapor (not steam) to describe the invisible, gaseous form of water. Explicitly point out that clouds and fog are made up of tiny droplets of water in order to distinguish forms of water in the air that we can see from forms we cannot see, such as water vapor. After they have grasped the idea that substances in the gaseous state are not visible, older students may be introduced to the scientific use of the word steam and compare it with how it is used in our everyday language.