November 2023 | Advice Column
Dear Labby: Frustrated with Flames
The "Flame Test" is a popular lab for exploring the relationship between electron configuration and photon wavelength emission. I have done many different versions with my students over the years but always seems to have problems with barium salts. On-line and textbook references always show a light green or yellow green flame. No matter what I try or how pure my source, I only every get a bright yellow flame. I have had many other chemistry teachers express the same frustration. How do the experts get the nice green flame in the photographs? For that matter, it would be fantastic to get some ideas for household products students can flame test. I routinely use baking soda (Na), cream of tartar (K), and Deicer (Ca).
Frustrated with Flames
Dear Frustrated with Flames,
Though there are many ways to do the flame test, my favorite is dipping water-soaked wood splints (wood coffee stirrers work well) into small amounts of solid salts. Once in the flame, the salt typically melts into a blob on the end of the wood, allowing the student to put it in the flame many times before losing the color. It is important, however, to continue wetting the wood splint so the burning of the wood does not overpower the flame color. The student can use a wash bottle of distilled water to re-wet the wood when needed. An easy way to set up the lab is for students to transfer a pea-sized amount of each solid into separate wells of a spot-plate (well-plate), using a labeled grid. Wood splints can be soaked in distilled water for a few minutes before the lab begins, but they will better resist drying out if they are soaked overnight. You will still occasionally have some orange-yellow flame overpowering the very light yellow-green of the barium ion flame. If that is the case, you can view it through cobalt blue glass to filter out the orange color and allow you to view the green that arises from the barium. One last hint about flame tests is to do them in the dark (using appropriate safety procedures) so the colors stand out more.
Regarding household items, aside from the ones you mentioned, I have used banana peels for potassium ions and copper-based algaecide for copper ions. I've also had students test different road salts to determine which ions are present. Educational Innovations sells "all natural" ColorFlame candles that have different colored flames. These are a nice way to have students identify unknown ions using materials that were not among what they already tested. The candles come with a list of the chemicals used to create the different colored flames (including ions of lithium, potassium, and sodium, along with boron from boric acid).
Yours in Chemistry,