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November 2022 | Resource Feature
Safely Introducing Students to the Chemistry Lab by Modifying a Classic Investigation
By W. Patrick Cunningham
Instructional Strategies, Demos & Labs
My chemistry teaching career began in 1969, the same year as the first manned lunar landing — which means that my students’ birth years have ranged between 1954 and 2006! One advantage of this career longevity is that I have experienced the first days of a chemistry education with more than three generations of students.
What nearly all those students have had in common is they’ve come into the chemistry laboratory with limited, sometimes nonexistent, manual skills. What’s more, the later in my career that my students arrived, the emptier were their skill sets. My sense is that this is typical of many students new to learning chemistry — which means easing them into safe and responsible lab behavior must be one of the principal tasks of any teacher of first-year chemistry.
Learning about laboratory equipment and its safe handling, however, has not always been engaging for the students. Moreover, the overarching objective of safety-first seems to compete with the students’ desire to have “WOW” experiences, such as occur in the oxidation-reduction between hot chlorate salts and Gummi Bears under a fume hood. In fact, I barely trust myself with presenting that reaction during the first week of school.
So how does a teacher strike a balance, giving students a safe, hands-on experience with a real chemical reaction, using tools that they might already have used? And would it be possible to offer such an investigation at a low net cost, with a minimal carbon footprint as well?
Initial chemistry lab experience
In my own first-year chemistry teaching practice, like that of many colleagues, one of the first goals of demonstrations is to help students to distinguish between chemical and physical changes. The Chemical Education Materials Study1 taught many of us in the 1970s to use candle flame as the first chemical reaction studied, because it is relatively safe, inexpensive, and permits some useful measurements. These include linear distance, mass, volume (with density calculations), and time duration of the reaction.
Corollary studies answering the question, “What is burning?” are possible with activities such as collecting the molten wax to show that the liquid around the burning wick is not water. And the vapor can be collected using a cool ceramic cup with a cobalt (II) chloride test strip to show that it is water.
However, the changes in the candle flame are both chemical and physical, and there is no way to collect all the products to measure them quantitatively. Since almost all students have seen burning candles, there is no real “WOW” aspect to the experience, so this commonly used option seems to fall a little short on delivery.
Over the years, I have developed a “WOW week” unit to use about halfway through the first semester, with activities investigating the gas laws, to help students transition into a way of learning based on laboratory work. I position this week of activities right after we study the properties of solids and liquids, including their density. It is an ideal follow-up to these studies, and even leads into an acceptance of Avogadro’s important hypothesis. However, the chemical reactions involved are either teacher demonstrations (relative density of gases produced by reactions) or videos supporting the Law of Combining Volumes. Since this unit has been successful with my students, I’ve also reviewed content typically presented later in first-year chemistry to look for experiences that would lend themselves to being introduced earlier in the year.
Finding the right fit
For example, very early in each first-year chemistry course I teach, I include measurement and lab safety modules. I’m now planning to add in a very simple chemical reaction that will help students master some unfamiliar, but safe, skills. The reaction will include a “WOW” experience, and will also lead to more advanced learning later in the year. After consideration of the curriculum and potential lab experiences for students, I’m envisioning a simple decomposition with no oxidation-reduction component, which I’ll position before our density investigations into solids, liquids, and gases. It should require only basic lab skills that students would already have developed in middle school.
One inexpensive and fairly safe chemical reaction frequently encountered in first-year high school curricula is the dehydration of certain inorganic salts. Salts such as magnesium sulfate, calcium chloride, cobalt (II) chloride, or copper (II) sulfate are often acquired in hydrated form, and can be decomposed with heat into the anhydrous salt and water vapor. In most cases, the reaction can be reversed afterwards by preparing a concentrated salt solution using the anhydrous salt and water, and then carefully evaporating the liquid water. This classic investigation is very frequently employed as part of a mid-year stoichiometry unit, because with the proper calculations, a mole ratio of water-to-anhydrous salt can be determined, and an empirical formula such as MgSO4●7H2O reported. My suggestion for an early positioning does not envision analysis beyond:
- Gathering evidence that gaseous water is produced, and
- Performing a simple calculation of percent water in the inorganic hydrate.
The teacher then can reconstitute the hydrate with water to validate the original reaction and demonstrate its reversibility by a hydration synthesis.
Of all the hydrated salts readily available for this investigation, I always recommend copper (II) sulfate pentahydrate, for several reasons. First, the crystalline hydrate comes as an intense blue regular crystal which, on heating for dehydration, changes to a white powder (see Figures 1 and 2). That’s the attribute change behind the “WOW.”
Figure 1. A sample of copper (II) sulfate pentahydrate.
Figure 2. A sample of anhydrous copper (II) sulfate.
Second, the crystals can be purchased at many hardware or big-box stores as “root killer.” Third, it is available online from Flinn Scientific and other science companies. Fourth, the only time it has decomposed for my students beyond the simple dehydration and produced sulfur oxides (mostly SO3), my students had been using an intense Bunsen burner flame. Consequently, my recommendation for this first decomposition is that students use either a standard microwave oven or, preferably, an electric heating element. Use of either heat source reduces the “carbon footprint” of the investigation when compared with the Bunsen burner, particularly if the electricity is generated by solar, wind, or nuclear sources, rather than hydrocarbon oxidation.
Comparing results from different heat sources
In my classroom, groups of one or two first-year lab students used all three sources of heat (Bunsen burner, microwave oven2, and heating element3), and produced acceptable results. The Bunsen burner, which is not recommended for use early in the lab year, was employed as a comparative heat source known to give excellent water-to-anhydrate ratios. Burner heating produced visible sulfur oxide byproducts during a five-minute heating period. Many hot plate options are available from scientific supply companies such as Flinn Scientific.
|Sample Data for Various Heating Sources*|
Mole Ratio Range, water to anhydrous CuSO4
(accepted value 5.0)
Average Mole Ratio, water to anhydrous CuSO4
(accepted value 5.0)4
|* Based on results from the author’s own lab students.|
Magnesium sulfate and calcium chloride hydrates deliver reproducible results, but both reactant and product are white, meaning that the only macroscopic change observable would be a small change in the crystalline form. Cobalt chloride gives a color change, (but it is much more expensive than the copper (II) sulfate) and, in my experience, can easily be overheated, producing chlorine gas. For these reasons, I prefer to use cobalt chloride test paper as part of the investigation, because students can make a further “WOW” discovery when they see that the paper changes visible color when exposed to water vapor or liquid water.
This investigation may be performed in the first weeks of the school year, as long as students are familiar with safe laboratory practices (which they probably encountered in middle school or junior high school). Teacher preparation of chemicals, hot plates, crucibles, and lab balances should take 30-45 minutes; the lab itself should last 25-30 minutes, plus time for cool-down and clean-up.
I suggest that preparation for the investigation begin in a class meeting immediately prior to the lab, and should include a review of safety considerations, lab procedures, and the objective. The investigation should be presented as one of discovery, allowing students to offer questions as they read the lab procedure. Students should perform this experiment wearing chemical-resistant lab goggles, laboratory aprons, and gloves.
Heating hydrated inorganic salts can decrepitate and spatter contents from the crucible. The chemicals must be handled with care, as they all are skin irritants and must not be ingested. The heating equipment should always be considered hot — and importantly, the crucible will be extremely hot. Teachers should demonstrate to students how to correctly use tongs to transfer the crucible, and remind students that neither the crucible nor its lid should be directly touched after heating. More details about the implementation of this lab activity can be found in the lab activity document.
This investigation, with its reduced student expectations (no use of the mole concept, only percent ratio calculations) is to be used in several schools during the 2022-23 school year. The complete student guidelines for the lab can be found as a separate document. A longer-term research project, measuring student performance outcomes by the quality of lab report produced, is planned. Teachers who have an interest in participating in this research by sharing student data and/or reports are invited to contact the principal investigator.
The author acknowledges the invaluable assistance of Matthew Eberly, Maxwell Iliff, and Madeline Seebeck (a student research team from St. John Paul II Catholic High School in New Braunfels, Texas for their contribution to the development of this investigation and article.
- Campbell, J. A. The Chemical Education Materials Study. J. Chem. Educ. 1961. 38, No. 1, 2-5. https://pubs.acs.org/doi/10.1021/ed038p2 (accessed Oct 26, 2022).
- Magic Chef Model MCD 1611 ST, 120 volt, 1.5 KW.
- Elite Gourmet ESB-301BF, 120 volt, 1000 watt on maximum heat.
- Experienced teachers who have used this lab as part of the curriculum may be skeptical about the almost perfect stoichiometric ratios shown here. In my practice, students report mole ratios between 4 and 5 using heating element warming. I recommend heating between 10 and 15 minutes total, in two successive heatings with the same crucible, cover, and salt.
(article cover) bigstockphoto.com/PCH.Vector