The Flame Test
Lesson 5 of 7
Objective: Students will use a flame test to identify the presence of various metals in solutions.
In this lesson students heat a number of metallic salts in a Bunsen burner and compare the colors that emerge with known colors of different elements. The goals of this lab are to give students a way to interact with atoms and electrons, to introduce the idea that electrons have wave-like properties, and to engage students.
This lesson aligns with the NGSS Disciplinary Core Idea of HS-PS 1-1, Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms because it emphasizes that "Each atom has a charged substructure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons."
This lesson aligns with the Crosscutting concept Energy and matter because it shows how electrons absorb energy and then give it off in the form of visible light.
This lesson aligns with two NGSS Practices of the Scientist-- SP3, Planning and carrying out investigations and SP4, Analyzing and interpreting data because students conduct an investigation and then wrestle with the data they collect.
While students do not need prior knowledge to conduct the lab, this lesson asks students to calculate the frequency of the light emitted from the electrons as part of the post-lab questions. Advance teaching of this optional material such as happens in this lesson would ensure that there was enough time on lab day for students to focus on collecting and interpreting the data.
Students should know how to use a Bunsen burner. If they have no experience then they should receive explicit instruction before the flame tests as to how to light , adjust , and work safely with them.
various chloride salts on a Petri dish
wooden splints (the latter should be soaked over night in distilled water)
Do Now: During this time students read a procedure for the flame test. I use this website as it gives a good overview of the procedure as well as the cause of the colors that are emitted and what the colors may indicate. Students take what they have learned about the procedure and create a cartoon of the procedure, which causes them to not only read the procedure, but also to begin to process the meaning behind each step in the procedure. Students use the flame test cartoon graphic organizer so that they are using a common format, which will make discussing the procedure easier. Here is one student's flame test cartoon that was used by the student to discuss the procedure for the lab.
Activator: As students are finishing their cartoons, I show a video of a pop song whose theme is fireworks. This seems like a good choice because today's lesson focuses on how colors are produced in fireworks. A more complete rationale for this choice can be found in this Classroom Video: Using a Pop Culture Reference.
Mini-lesson: Part 1.
I begin class by acknowledging that last class’s learning objectives were not met, and I note that today we will spend some time reviewing and learning what I hoped we had mastered yesterday. A video that shows how I discussed the speed of light formula and what my rationales were for the lesson can be found in this Classroom Video: Responding to Data with Reteaching.
Guided Practice: Once I have reviewed how to use the speed of light formula, I know that students will need to learn how to use their calculators with scientific notation. I give them some time top practice and get help, as outlined in this Classroom Video: One-on-One Help.
Mini-lesson, Part 2: Once I feel confident that students understand how to process the data they will gather today, i continue with the next part of the lesson showing students a model of the electron shell for a typical atom. I ask students to describe where the electrons are typically found. They respond (hopefully!) that they are found close to the nucleus. I ask them to describe what happens when electrons absorb energy (they move farther away from the nucleus), and help them to see in the text where it discusses this. Lastly, I want them to understand that when electrons return to the ground state they give off electromagnetic radiation in the form of visible light, and this is why we see colors during the flame test.
I then discuss the procedure. I note that students will have 5 minutes per station, and that we will rotate from one station to the next clockwise. I model what this looks like in honor of the digital age. I ask students about flame safety--they should know and explain that loose clothing and/or hair needs to be secure and that safety goggles must be worn at all times. I am very concerned about student safety; I model how I address this fact in this Classroom Video: Safety Procedure. Review
I remind students that the goal of the lab is to record the colors from each of the stations, and to try and decide which element exists at each of the stations. I explain that this requires that they observe the colors and try to match the colors to elements for that color. I note that in the past I have noticed students arguing about which color they are seeing, and I ask that they are respectful of one another's opinions, and we can try to settle on what the correct answer is during the debrief. During the lab they can use the Flame Test Lab handout to record their color observations.
Student Activity: During this time students are conducting the flame tests at the different stations. While they are doing this my main concern is their safety. I carefully monitor the lab to make sure that safety procedures are being followed. I also interact with students to discuss what they are seeing and to make sure that they are recording data. I am also a time-keeper. I ask students to rotate every five minutes, and give them a one-minute warning so that they have time to return their work station to the condition that they found it. This section of the class, and how I feel about it, is documented in this Classroom Video: Individual Support as Needed.
In this flame test video students can be seen testing the various elements. They love this lab as evidenced by the random comments some of the students make. The colors are vibrant! However, there are some challenges. For example, this video shows limitations of the flame test--namely, that it can be difficult to see the subtle colors that are evidence for the presence of some elements. In this element I was trying to help students see the subtle presence of pink, because I knew that the element was lithium, but it was hard to see because it was masked by impurities such as sodium that are found in many lab grade chemicals.
During this time I project the Flame Test Known Values spreadsheet. I ask students to review each of the unknown metals and see what color they chose and what element they thought they had. Then I reveal the true identity of the element. In some cases the students guess the right color, but in some cases they do not. One inherent weakness of the flame test is that color is a subjective experience. Because I want students to work with the speed of light formula as part of their lab write-up I make sure that I name the element that each beaker contained, and I assign them (slightly arbitrarily) a wavelength so that students are able to to calculate frequency. Here is the flame test class data that they will use in their write-up of the lab.
I then ask students to read over the questions in the Flame Test post-Lab Write Questions to see if they have any questions about the post-lab work.
In this student's lab report it was clear that the student learned how to calculate frequency and understood that the colors were given off by electrons going from the excited state to the ground state. However, in another student's lab report it was clear that the student did not realize that it was the electrons that were giving off energy in the visible light spectrum, a point that I reemphasized in a subsequent class.