In this opening lesson in a unit about acids and bases students will participate in two hands-on activities that are designed to engage their interest about acids and bases. The first activity simply involves drinking lemon juice and tea. One is considered to taste sour, while the other is considered taste bitter. Interestingly, both are acidic. The second activity is performing a neutralization reaction between hydrochloric acid (the acid used in our stomach for digestion) and Milk of Magnesia.
This lesson aligns to the NGSS Practices of the Scientist of Obtaining, evaluating, and communicating information because students are getting some hands-on experience about acids and bases and learning how acids and bases play a significant role in the human body. Many students only think of acids as something dangerous, and they have even less understanding about what a base is. By giving them some time to manipulate acids and bases I hope that they think of these classes of chemicals in broader terms.
This lesson does not align to any of the NGSS Disciplinary Core Ideas, which do not discuss acids and bases, but it does correspond with many state standards that do, including Massachusetts Content Standard 8.2: Relate hydrogen ion concentrations to the pH scale and to acidic, basic, and neutral solutions. Compare and contrast the strengths of various common acids and bases (e.g., vinegar, baking soda, soap, citrus juice). The lesson does this because by getting students to think about acids and bases in terms of everyday materials they encounter.
There is no prior knowledge needed for this lesson as it is an introductory lesson.
The materials needed for this lesson include the following:
Do Now: Students enter my room and are asked to read a part of an article and answer Acids and Bases in the Human Body reading questions that relate to the corresponding portion of the article. This is a good way to start class because it gets students thinking about their own body and how acids and bases are a part of it. It is also an easy task that requires little in the way of clarification or teaching, allowing me to focus on tasks such as attendance and helping students who were absent last class.
Activator: After I have taken attendance and students have gotten some traction with the assignment, I begin class by greeting the students and I ask if they have any questions. Students do not have questions, and so I ask them to return to the assignment, and I explain that they have begun a jigsaw, in which each student reads a part of an article, and then students share what they learned with students who did not read their section. Students then return to their reading. After they are done, I give them several minutes to share what they learned with their classmates. I walk around to listen for correct interpretations of what was read.
Mini-lesson: After students have shared information about acids and bases in the human body, I review the directions for the two mini-labs for this lesson using these introduction to acids and bases mini-lab slides.
The first mini-lab is designed to give students the chance to differentiate between the words sour and bitter. I explain that this is important because acids generally taste sour while bases generally taste bitter. I emphasize that this is the ONLY time that we will be tasting things in this unit, and that the safety rule is that students do not taste anything unless specifically instructed to do so.
I then review the second slide. I remind students that HCl can cause significant eye injury, and so safety goggles must be warn during the second mini-lab. I also remind students that it is important to patiently and slowly add HCl to the Milk of Magnesia. If it is added too quickly, then we will have to dispose of the solution in the waste bucket so as not do harm the drain pipes.
This instructional choice reflects my desire to instill a level of safety and respect for the environment and to give students the chance to experience some of the characteristics of acids and bases at the start of the unit.
Student Activity: The majority of the lesson is spent by students experiencing acids and bases. First, they take turns tasting the lemon juice and the cooled brewed tea. In the past I have served saltines in between each of the beverages so that students have a chance to cleanse their palate. On this occasion I not have saltines, and so students drink water and wait several minutes between beverages. The theatrics that ensue when students drink each of the beverages is entertaining, as shown in this video of students tasting lemon juice.
The second mini-lab, in which students slowly add acid to milk of magnesia, takes more time. I encourage students to add the acid slowly so that they do not add too much. I explain that the goal of mixing the acid (HCl) and the base (milk of magnesia) is to neutralize the two so that we end up with a test tube that is neither basic nor too acidic to dump down the drain. In this acid base neutralization video, I am working with a student group engaged in this process. I keep an eye on students, and if they rush, then I make them use pH paper which shows that their sample is acidic. I do not explain that in fact even a little bit of acidity would make the litmus paper orange, as I am trying to emphasize the importance of patience so that when we go to do titrations they are not hearing this for the first time.
I have chosen these activities because I want students to have some experiences with acids and bases that I can refer back to later. We have used HCl a lot over the course of the year, and by now students have the misconception that acids are dangerous. I want them to recognize that acids are a part of many everyday items, and that not all acids and bases are dangerous. I will especially return to this theme when we discuss pH. I have also chosen these activities because I want students to have a day in the lab that is fun and low stakes, as they have just finished turning in a major lab report and test on reaction rates.
To wrap this lesson up I ask students to answer the following questions in a ticket to leave:
Ending class this way, with a debrief, allows me to preserve a structure and expectation that I have worked hard to create all year. The debrief time is a time to think back on the class and what it meant. These tickets to leave show that students did get something basic out of the class, but it is clear that we will need to spend a lot more time on what a neutralization reaction really is. I am not worried about this, as neutralization reactions using titration are the end point for this unit and all the lessons will lead to that point. What I am pleased about today is that students had some fun while diving into a new topic.