Acid and Base Scientific Explanation Lab

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Objective

Students will be able to explain which of four unknown solutions are acids through performing a lab activity.

Big Idea

There are multiple ways to test a solution to determine whether that solution is considered an acid or base.

Introduction

In this lesson students are planning and performing a lab to determine which of four unknown solutions are acids.

  • This lesson does not align with any specific performance expectations with the Next Generation Science Standards; however, it is important that students understand how to identify unknown solutions.  This is an important skill for students who are going onto college level science where they will be expected to be able to characterize solutions.  Additionally, students will be practicing argumentation skills which are an important aspect of both NGSS and the common core curriculum.
  • This lesson aligns with the Next Generation Science and Engineering Practice 3: Planning and Carrying out Investigations.  It does so because students are performing a lab activity where they are measuring the pH of various solutions.
  • This lesson also aligns with the Next Generation Science and Engineering Practice 6: Constructing Explanations and Designing Solutions.  It does so because students have to write a paragraph explaining which of the four solutions are acids.

For the lab activity I provide each group with the following materials:

  • four solutions labeled A, B, C, and D (A is .1M sodium hydroxide, B is .1M ammonia, C is .1M HCl, and D is vinegar).
  • plastic well plates (preferably with 16 squares)
  • blue and red litmus paper
  • pH paper
  • zinc metal
  • magnesium metal
  • phenolphthalein
  • bromothymol blue
  • pH meter and/or pH probe

Explain

10 minutes

To begin this lesson I pass out the acid base scientific explanation lab paper and have students spend 3 minutes reading it over to themselves.  I then go over the lab with students through the following steps:

  1. I tell students to make sure they have a blank piece of paper on which they will be recording their lab.
  2. I then ask students, "What is the goal of this lab?"  I call on a student for an answer and they will usually say something like, "to figure out which are the acids."  I show students that on their separate sheet of paper that they should write down the objective of the lab.
  3. I then hold up the four solutions (A, B, C, and D) and ask students if they can tell they are acids just by looking at them.  They will respond, No.  I then ask them if we have learned any tests to determine if substances are acids or bases.  I help lead them to the idea that they should go back to their Introduction to Acids and Bases lab as well as their lecture 1 notes (from Introduction to Acids and Bases lesson) for ideas.
  4. Next, I show students the list of materials on their paper and have them add magnesium ribbon, pH probes, and have them take off conductivity tester (I decided to add these two things and then not give students conductivity testers as that it was a distractor anyway which would not tell students if the substance was an acid or base as that they both conduct electricity).
    • This movie shows how I explain this lab to my students. 
  5. I then tell students to not write down the materials yet, as that they are only going to write down the materials that they are going to use with their groups.
  6. After materials, I tell students that they will be writing out procedures for the tests that they choose to do, and to make sure that they have at least 3 tests.
  7. I also tell students that they will need to make a data table to record their data.  I show them an example of a data table on the board to demonstrate how they will record data for each solution for each test.
    • This movie shows how I review how to setup the table with my students. 
  8. Finally, I tell students that they will write a conclusion which explains which of the solutions are acids, the evidence that demonstrates this, and the scientific rationale. 
  9. For this lab I have students choose their own groups, but remind them, "you can choose your own groups for this lab, but make sure that you are choosing to work with people who will work well with and keep you productive as that you are coming up with your own procedures."

 

Elaborate

70 minutes

In this section of the lesson students are actually performing the lab.  

  • They are in groups of 4 students (several groups of 5 or 3 depending on the class size).
  • They are expected to write out their materials, procedures, and data tables BEFORE they start the lab.
  • Students are expected to perform at least 3 tests on the four solutions.  Here is a picture showing one group's results of 3 tests. 
  • When they are done with gathering their data I have them clean up their stations (metal and papers go in the waste beaker while everything else gets washed down the sink) and write out their explanation conclusion.

As students are working I walk around and do several things..

  • I check that groups are first recording procedures, materials, and having a data table before starting the lab.  This movie shows how I am helping my students with how to write their procedures as instructions of what they will be doing in the lab.
  • I make sure that they are aware of why they are performing the tests (what results they would expect if they are looking for the acids).  This movie shows me doing this with students.
  • I instruct students how to clean up their stations.
  • I ask students if they figured out which of the four solutions are acids and what evidence they collected to determine this.  This movie shows how I did this with one group. 

Evaluate

When students are done with the lab I have them turn it in.  I then grade the lab using the rubric found at the bottom of the acid and base scientific explanation lab.

Overall, students tend to earn most of their points.  

The place where students tend to miss points is in their conclusion with their reasoning where they do not use scientific reasoning to show why there evidence supports their claim.  

  • For example, I was expecting answers such as... The first piece of evidence that supports this claim is that the blue litmus paper turned red for solutions C and D.  This evidence is appropriate because acids turn blue litmus paper to red.
  • This is an example of a lab where a student missed their reasoning.

Another place where students tended to miss their points was if they were not specific with their materials used in the lab, but rather just listed all of the materials given to them at their tables. 

  • Here is an example of one such lab.

A final area where students miss points is by not completing their procedures to where someone else can reproduce the lab.

  • Here is an example of a lab like this.