Chemistry Experiments at Home - Acid-Base Reactions
By Jodye Selco
Learning chemistry should include hands-on experimentation, but can this be done safely while under stay-at-home orders? There are a variety of chemical processes that used to be done at home, but are now done industrially; one example is making soap. Historically, soap was made by collecting lye from ash and adding that to fat .
Although most urban dwellings no longer have fireplaces or wood stoves, there are two readily available sources of chemical base – household ammonia and Milk of Magnesia. Household ammonia is typically 2 to 10% by weight NH3 in water. While the concentration of ammonia is not necessarily consistent, it can be used as a basic sample or as a source of hydroxide ions in precipitation tests.
Milk of Magnesia is a suspension of Mg(OH)2 in water. Milk of magnesia contains 595 mg Mg/15 mL or 1.63 M in the milky, white, opaque suspension . This can be used as both a source of base for an acid-base chemical reaction and its own indicator because when sufficient acid is combined with the Milk of Magnesia, the magnesium hydroxide dissolves and the solution becomes transparent.
Using a dropper (as opposed to a burette) allows students to titrate acidic samples (e.g. fruit juice or types of vinegar) to determine relative acidities. If students have access to a graduated pipette or syringe (without the needle), the exact amounts of base added to the acidic solutions can be determined. (If you want to provide measuring devices to students, be sure to check with the district first! Many will not allow this due to liability reasons.) Even without being able to determine the volume of base (or acid) added precisely or accurately, the ability to observe these experiments first-hand is still valuable.
This experience can be coupled with a virtual titration from the Royal Society of Chemistry. The scenario of this virtual experiment is that there was a spill into a river, and the user has to determine the downstream concentrations of acid following an acid spill at a conservation site. This simulation uses HCl and NaOH, but following the structured calculations included, the students could then return to their own experiments and determine the concentration of acid in their samples if the volumes are known.
Done this way, the students not only examine portions of NGSS HS-PS1-1 and HS-PS1-2 by comparing the number of hydroxide ions coupled with magnesium as opposed to sodium, but also HS-PS1-7 that has students using mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
Many chemistry laboratory acid-base investigations involve either HCl or NaOH – presumably to simplify calculations. However, these solutions are colorless and require indicator compounds to indicate endpoints of titrations by changing color. Although not a strong acid like HCl, vinegar is approximately 5% by mass acetic acid. If the experiment you want your students to do involves colorless, transparent solutions, there are some widely available acid-base indicators. One is red cabbage extract which contains multiple indicator compounds and displays a wide range of colors from pH = 1 to pH = 14. Other anthocyanin-containing materials are flower petals. In Figure 1, a purple pansy turns blue in base (sodium carbonate) and red in acid (vinegar). Plants are the producers of most of the known acid-base indicators; many colored plant parts (except those that are green) often have anthocyanins which are used as acid-base indicators9,10 (e.g. lichen is the source of litmus11). Although not a high school standard, knowing the natural source of synthetic materials is a performance expectation in middle school.
There are multiple types of assessments that your students could submit to document their experiments. Besides the “standard” laboratory report, consider having students create a video, slide deck, news bulletin, or… Creating videos of what students did and their results can be done easily on a phone or from a computer using Zoom or Screencast-O-Matic, depending on what is available. If need be, they can demonstrate their learning for you during a synchronous class meeting. These videos in MP4 format can be edited on YouTube . Although video files are large, students can upload them to YouTube, make them “unlisted” and provide you with a link (without a link, the video cannot be found by searching YouTube). Students can also upload video files to your storage space (e.g. Google Drive or Dropbox).
The advantage of grading videos is that they are much quicker to grade than written documents whether the video is live-action or slides with audio. Just like any other type of assessment, you as the instructor need to specify what the required elements are, and you can specify how long the video can be.
This Fall (and possibly longer) the academic learning environment will be different from anything we have experienced before as educators. It is a great time to “think outside the box” at new ways of teaching, new experiments that can be done with what is available in-home (or readily available), and with new types of assessments.
Professor Jodye Selco is a Chemist and Science Educator at Cal Poly Pomona. She has developed a variety of simulations and hands-on chemistry experiments. Professor Selco has worked with Rialto Unified School District to design and implement district-wide hands-on science assessments. Visit Joyde's website for more information.
- See for example, David Swart: https://davidswart.org/2019/05/22/acid-and-base-lab-with-report/