November 2017 | Classroom Commentary
Part I: Rethinking Common Practices in High School Chemistry
By Kaleb Underwood
Physical vs. Chemical Changes
The distinction between a physical change and a chemical change usually takes a prominent role at the beginning of a physical science or chemistry course. For many instructors, this seems intuitive: in a course dedicated to the study of matter and change, we should distinguish between types of change early to provide a foundation on which to construct our future studies. A brief survey of five common high school and college chemistry texts shows that all address the distinction in the early chapters, and a search of the resources on the website of the American Association of Chemistry Teachers reveals about 30 resources dedicated to the practice.
The problem with categorizing changes at the beginning of a course in chemistry is that the rationale used necessarily rests on macroscopic observation alone — students do not yet have knowledge of the particulate level of matter to justify in terms of atom rearrangement. These macroscopic observations are not sufficient for categorizing each change because they are rife with ambiguity. That ambiguity, when encountered, does not contribute to student understanding of chemistry and is often at odds with students’ premature notions of the arrangement of matter.
Rather, classroom time should be spent developing a more robust understanding of the particulate nature of matter and exploring the details of different types of changes over the course of the year. Only once the particulate nature of matter is thoroughly understood should we seek to define physical and chemical changes in terms of particle rearrangement. All of the different changes normally discussed at the beginning of the year can be dispersed and addressed in turn throughout the curriculum.
My argument is that the common exercise of classifying changes as physical or chemical should be removed from introductory chemistry courses, or at minimum saved until later in the course following a careful development of the particulate nature of matter. All the different types of change are complex and rich in teachable moments. Do not write them off as either/or exercises early in the year.
Definitions of Physical and Chemical Changes
A physical change is usually described as a change that does not alter the “composition” or “identity” of a “substance.” Textbooks usually refer to visual appearance or physical state as properties altered during a physical change. A chemical change is usually presented in opposition to a physical change in that it does alter the “composition” or “identity” of a “substance” in order to produce “a new substance” that was not there before. Additionally, “signs” or “evidence” of a chemical change usually follow the definition and the list commonly includes formation of a precipitate, formation of a gas, temperature change, and color change. In the author’s experience, reversibility and irreversibility of the process are sometimes referenced as criteria as well, though usually in grades K–8. Common practice is to have students classify changes as one or the other using worksheets or simple lab scenarios.
The definitions of the words in quotes above provide the source of the first problem with this practice. In the mind of a student, what do the terms composition, identity, and substance mean? The dependence of identity on atomic or molecular makeup is foreign to the novice and the term “substance” appears vague. Although the broader use of that term (as opposed to element or compound), is appropriate early in the yeari, we must be careful. Chemists use the term “substance” when referring to matter with constant composition, i.e. an element or compound, so when we say a “new substance is formed” we know that we have not created something new from nothing, but our students may still harbor naïve notions that such an event is possible.
To an instructor who understands the particle-level changes occurring in each of the examples they present to students, classifying changes into these categories seems a straightforward activity for the beginning of the year. However, the naïve ideas of matter that students bring with them to chemistry are well documentedii, and serious work is required to help them move past these preconceived notions and develop understanding. Asking them to make distinctions regarding change early in a course amounts to asking them to memorize a list of keywords. Additionally, a student cannot be expected to draw correct conclusions about the change in particle arrangement from qualitative observation alone when they have no prior knowledge of atoms, molecules, particle behavior, or even evidence to support their claims. Students need time to develop their conception of the particulate nature of matter before meaningful learning can take place.
A common example to illustrate physical change is the changes of state of water. We inform our students that changes of state are physical changes because steam, water, and ice are all “the same substance.” The student might well disagree due to the existence of three different names for the three different “things” combined with their unfamiliarity with the word “substance.”
In my classroom, I use the terms “gaseous water,” “liquid water,” and “solid water” to emphasize the unity between the three. We are, from the students’ perspective, asking them to take three “different” substances and consider them as one without any evidence whatsoever — because, from our perspective, the identical chemical composition of the three states is an obvious fact. Other common misconceptions include the belief that the boiling of water produces hydrogen and oxygen gases and that condensation is the combination of the two gases.iii For those of us familiar with the chemical properties of hydrogen and oxygen gases, this seems absurd — but students have more than likely never interacted with elemental gases, other than possibly helium, and have no understanding of their properties.
In my classroom, we discuss phase changes early in the year. We generate a heating curve of water, and draw particle diagrams of water at the various points on the curve, but we represent water with single particles because we have not yet differentiated elements and compounds. I continue my emphasis that water is still water after it boils. We also put a lid on a beaker of boiling water, showing the condensation of the vapor, continually emphasizing that it is still water.
Later, after we differentiate elements and compounds, I demonstrate the properties of hydrogen gas, oxygen gas, and water vapor using splint tests and cobalt(II) chloride test paper. Following this, I use a Hoffmann Apparatus to perform water electrolysis and demonstrate that the products of splitting water are different from the product formed from boiling water using the same tests. We can now revisit our particle diagrams of water phase changes from the heating curve lab and update them, now representing water as a molecule of two hydrogen atoms and an oxygen atom. These can be compared to diagrams we draw of the electrolysis demonstration.
The Failure of the Four Signs of a Chemical Change
Students are usually presented with the task of categorizing various changes in matter as physical or chemical changes based on whether or not they observe one of the four signs of a chemical change. In my view, however, the four indicators students are asked to use are flawed and cannot be applied broadly. Let’s evaluate each of the so-called “four signs of a chemical change.”
The formation of a precipitate. The details of precipitation reactions will be taught later in the course, so at first glance this criterion does not seem so bad. However, consider how a student could view this. They have been taught that state changes are physical changes, so why then should a liquid (as far as they are concerned) turning into a solid not be a physical change? Remember that at this point they currently have no ability to distinguish a solution from a pure liquid. Macroscopically, in addition to solidification, this process may also appear to a student as the opposite of dissolving and their conclusion that it’s a physical change makes sense if they were taught that dissolving is a physical change.
Alternatively, I recommend holding off on teaching about precipitation reactions until your unit on chemical reactions. At this time, students should have knowledge of solutions and ion dissociation. The simple question, “Why does a precipitate form when the solutions are mixed, but not in the original solutions themselves?” can lead to a rich discussion of the relative attractions between the ions and water molecules that surround them. The precipitate forms because the ions of the precipitate are more attracted to each other than to the water molecules.
Gas Formation. This criterion fails when we consider the phase changes of vaporization or sublimation. Gases are formed in both phase changes and the teacher is forced to qualify the “formation of a gas” statement as “unexpected gas formation,” “gas formation that does not arise from heating,” “or gas formation that is not a phase change.” Notice that what constitutes an unexpected event is a direct consequence of the level of experience one has. Common classroom demonstrations of vaporization include the boiling of water or oil and demonstrations of sublimation include that of dry ice and iodine. Boiling water and oil fit within all three qualifiers mentioned above, but the sublimation of dry ice is spontaneous at room temperature, so the caveat of a gas formed that “does not arise from heating”fails. . Additionally, it is worth pointing out that the “gas” actually seen during dry ice sublimation is not actually gaseous carbon dioxide being produced, but microdroplets of liquid water that have condensed in the region of cold air surrounding the dry ice block. Gaseous carbon dioxide is not visible to the naked eye.
A temperature change. Temperature changes accompany all physical and chemical changes, even if they are not easily perceived or measured. A transfer of energy occurs whenever forces of attraction, of any kind, are overcome or formed. It cannot be used as a sign of a chemical change alone.
A color change. Color changes are not unique to chemical change. Color change can accompany dilution, the addition of food coloring to a substance, or a phase change. The teacher is then forced to qualify this statement in a number of ways, all which lack meaning to the novice.
It is important to acknowledge that the failure of these signs does not render the concepts of physical and chemical changes as invalid. Indeed, the distinction between the two types of change can be a useful tool for students to categorize their thoughts. But I argue that the dichotomy, if used, should be delayed until later in the year and with a focus on particulate-level explanation.
Classifying the dissolving of an ionic salt in water as a physical or a chemical change is often a topic of much debate among teachers. Usually there is no debate surrounding the dissolving of a molecular substance in water, such as table sugar. I offer the following discussion of the solution process to illustrate that the categorization as either a physical or a chemical change represents an unnecessary practice that lacks a useful end. What would be more useful are lessons designed to teach these differences, spread throughout the year, rather than lessons designed to categorize them. Of course, students are not prepared in the first weeks of a first-year chemistry course to make these distinctions, and as such, it is unwise of the educator to broach the topic with them until later in the year.
On one hand, some claim the dissolution of all compounds are purely physical changes. The argument goes that in ionic compounds, the ions are unchanged themselves (instead they are merely hydrated), and the salt can be recovered through evaporation of the water. For solid molecular compounds, the molecule does not dissociate into ions, and the solid can be recovered through the evaporation of water. Macroscopically, the two processes appear identical unless the conductivity of the solutions are tested. Students lack the requisite knowledge to differentiate between these two processes and it is unnecessary that they be led to equate the two, as that will create a misconception that will be broken later on.
Upon further inspection, the dissolution of all molecular substances cannot be categorized together. Certain molecular substances do ionize in solution: acids. Bubble HCl(g) through water and a solution of hydrochloric acid is formed as the HCl(g) molecules readily transfer hydrogen ions to water molecules forming H3O+(aq). This is a proton-transfer (Bronsted-Lowry acid-base) reaction. Dissolve solid oxalic acid in solution and the same occurs, though only a small fraction of these molecules ionize as it is a weak acid.
When ionic compounds dissolve in water, the ions dissociate because strong electrostatic attractions between the ions are overcome by the stronger attractions between the ions and the water molecules. In some cases, a proton-transfer reaction occurs, known as salt hydrolysis, such as the reaction of NH4Cl(s) in water or NaCH3COO(s) in water, producing acidic and basic solutions respectively:
NH4+(aq) + H2O(l) ⇌ NH3(aq) + H3O+(aq)
Figure 2. In a solution of NH4Cl, the NH4+ ion donates an H+ ion to water, making H3O+ and therefore an acidic solution.
CH3COO−(aq) + H2O(l) ⇌ CH3COOH(aq) + OH−(aq)
Figure 3. In a solution of NaCH3COO, water donates a proton to CH3COO−, making OH− and, therefore, a basic solution.
More broadly, the dissolving of any salt could be viewed as a Lewis acid-base reaction in which the water and metal cation form a complex, with the water acting as the Lewis base (electron-pair donor) and the cation acting as the Lewis acid (electron-pair acceptor). An example is a solution of AlCl3, a strong Lewis acid. In solution, the metal ions are complexed by water molecules:
Al3+(aq) + H2O(l) → [Al(H2O)6]3+(aq)
And the acidic solution is created by the equilibrium established:
[Al(H2O)6]3+(aq) + H2O(l) ⇌ [Al(OH)(H2O)5]2+(aq) + H3O+(aq)
The strengths of these acids and bases rest on the relative strengths of attraction for electrons between the various species, and we will not go into details here.
The solution process is complex, and discussion of its intricacies extends in to second-year courses. In a first year course, we all must draw the line somewhere. For me, I include the differences between ionic and molecular dissolving, the distinction between strong and weak acids, and the proton-transfer reactions they undergo. Salt hydrolysis, complexes, and Lewis acid/base theory I leave until my second-year course.
Conclusion and Implications for Instruction
I suggest that the distinction between physical and chemical change be saved until, at least, the unit in which you cover chemical reactions. Up until that unit, time should be spent on ensuring that students build a working understanding of the particulate nature of matter and are comfortable creating and interpreting particle diagrams.
If high school teachers dispensed with the common classification exercises (except for those involving the interpretation of particle diagrams), I think chemical education would be improved, because students would be connecting macroscopic change directly to particulate-level change. Framing their understanding in particulate-level terms is going to best help the students understand the difference between the two changes and help overcome the vocabulary hurdle I mentioned at the beginning. Bridle and Yezierskiiv suggest that a possible follow-up activity is to use particle diagrams of different changes to evaluate the usefulness of various macroscopic means of classifying change. I am considering trying a similar activity this year.
Should I classify a particular change as a chemical or a physical change? It really does not matter if you understand the particle-level change that is occurring. If by the end of the year, my students can discuss the differences between water phase changes and water synthesis or decomposition, then I can be confident they have learned some good chemistry.
This article was inspired by Eric Scerri’s series of opinion pieces for the Royal Society of Chemistry entitled “Five Ideas in Chemical Education That Must Die.” I recommend that all chemistry educators read this series. My goal is to expand on Dr. Scerri’s original list of five with direct attention paid to additional concepts usually discussed in K-12 chemistry education.
Special thanks to Ben Meacham, Roxie Allen, Erica Posthuma-Adams, Dr. Russell Kohnken, Scott Miliam, and Denise Sanders for their contributions to this article. In addition, I am much indebted to the thoughtfulness of the three reviewers whose thorough reading and numerous suggestions greatly improved the original draft.
i Kind, V. Beyond Appearances: Students’ Misconceptions about Basic Chemical Ideas. Royal Society of Chemistry 2004, http://www.rsc.org/learn-chemistry/resource/download/res00002202/cmp00007478/pdf.
ii Ibid.; Bridle, C. and Yezierski, E. Evidence for the Effectiveness of Inquiry-Based, Particulate-Level Instruction on Conceptions of the Particulate Nature of Matter. Journal of Chemical Education 2012, 89 (2): 192-198. DOI: 10.1021/ed100735u; Nakleh, M. Why Some Students Don’t Learn Chemistry: Chemical Misconceptions. Journal of Chemical Education 1992, 69 (3), 191. DOI: 10.1021/ed069p191.
iii Johnson, P. Children’s Understanding of Changes of State Involving the Gas State, Part 1: Boiling Water and the Particle Theory. The International Journal of Science Education 1998 20 (5), 567-583.
iv Bridle and Yezierski, 2012.