The ABC's of Biology: Acids, Bases, and Compounds!

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SWBAT demonstrate their understanding of acids and bases through this inquiry-based activity by creating their own laboratory model of the pH scale.

Big Idea

What does it mean to have a pH of 2 versus a pH of 12? Students will discover first hand the pH values of unknown substances and construct their own model of the pH scale.

Introductory Video: Why is the pH scale important in Biology?

1 minutes

This video clip explains the importance of this lesson to scaffold the students' understand of acids, bases, and the pH scale.  Students are able to create an inquiry lesson geared toward discovering the pH of common household substances and creating a model of the pH scale based on their scientific findings.


Hook - pH In Action

5 minutes

As students walk into the classroom, they will notice the random assortment of household supplies on the front demonstration counter.  Once students are settled in their seats, they will respond to the following prompt, "Identify two similarities and two differences that would describe all of the substances you see here."

After a minute of reflection and writing time, students will pair-share their responses with their neighbor.  Pairs of students will  volunteer their responses in a whole-class discussion.  After a bit of a discussion, students will be able to conclude that the substances presented are all household items that they have used before and none of them have a pH of 7.  

The video clip below will reinforce the concept of connecting household substances with the students' comprehension and application of the pH scale.  Students will write down two facts from the video that supports our class discussion.

The concept of the pH scale is abstract and difficult for students to visualize without making the connections to their real lives.  By allowing students to identify everyday substances and determine the substance's pH, students will begin to conceptualize the concept of pH and formulate a model based on their experiences throughout this lesson.

Direct Instruction - Making Sense Of The pH Scale

20 minutes

This activity is building on the prior knowledge from the Properties of Water Lecture Notes that discusses acids, bases, and the pH scale that were shared in a prior lesson.

Student Inquiry Lab Development: In the past (pre-NGSS), students were usually provided a teacher-developed lab that outlined a step-by-step procedure that all students must follow with little to no creative thought.  This activity follows the previous concepts as a guide, but enables the students to test the pH of the household substances through their own inquiry.  Rather than provide a rigid set of procedural directions, students are instructed to collaboratively create their own procedure that will lead them to construct their own model of the pH scale based on the results of their inquiry process.

To ensure all students know how to properly use the supplies, the teacher will demonstrate how to use the ph indicator paper and remind students how to read the color-coded pH scale that was provided to each lab group/student pair.

Student pairs have 15 minutes to develop their procedure for their inquiry lab which will result in the development of their own pH scale using common household substances.  The students will need to list the following in their procedure:

  • Supplies needed
  • A detailed set of directions - VERY specific!
  • Safety precautions other than wearing goggles since that is non-negotiable.

The teacher will rotate around the room to offer guidance and support for student groups who are struggling with the the creative process of developing their own lab investigation.  Most students have participated in a teacher-developed lab and need to activate their prior knowledge and incorporate a similar format for this activity.  

Sample of Student Work: pH Lab Procedure- This student group worked well together to develop a detailed procedure that would test the pH of common household substances.  All safety precautions were easily followed since this group decided to test only common household substances and avoid harsh chemicals (either acids or bases).



Guided Practice - Creating Your Own pH Scale Model

30 minutes

As student pairs complete their proposed inquiry procedures, they will raise their hands and have the teacher proofread their ideas.  As the teacher reviews the student work a special emphasis will be placed on the following areas:

  • Are all safety precautions being followed at all times? For example, wear eye goggles, report spills, take only the volume of solution that is necessary to avoid spills/overflow
  • Do all steps of the procedure support the student identification of the pH of the substances?
  • Are students specific in their procedural details?  Can another lab group replicate the inquiry process?
  • Does the proposal describe how the laboratory data will be used create the pH scale?

If all of these areas are achieved, the student lab group is excused to begin work at the supply counters and bring their laboratory supplies back to their lab station.

The staggered start to begin student work will help to minimize students standing in line waiting to gather their supplies.  An extra effort is always placed on using instruction time effectively and reducing the amount of time wasted by students waiting to set-up their lab supplies.  Some groups will have to revise their inquiry proposals multiple times to ensure that all four areas of emphasis are sufficiently implemented into their work,, while others  will be approved on their first attempt. 

Once all student lab groups have been approved, the teacher will rotate round the room conferencing with groups to assess comprehension of the objective and execution of their inquiry lab. 

Data Analysis: After students have tested and recorded the pH for each of the substances, they will arrange the substances from strongest acid to strongest base to reflect the pH scale values 1-14. Students will need to arrange the pH indicator papers in order based on the color of the pH scale.  Students will use a class set of color copies of the pH scale that have been laminated as a reference.  By arranging the pH papers by color students will have a visual image of their very own pH scale that they have created through their inquiry lab.  Thus, they will be able to identify common household substances by the pH value they measured as part of the inquiry lab. This lab allows students to connect Biology curriculum with their everyday life experiences!

Creating A New pH Scale: Students will create a paper copy of the pH Scale Continuum that represents substances for each of the 14 values on the pH scale.  These paper models will be turned in at the beginning of the next class period.

Sample pH Scale Linked With Common Substances

pH scale and substances


Images of Student Lab Experiences: Testing the pH of Common Household Substances

Testing the pH of an Egg

Testing the pH of Whipping Cream

Testing the pH of Tomato Juice


Sample of Student Work: pH Lab Data and Conclusion - This group created a detailed procedure that allowed them to efficiently test each substance and record their pH value.  Good record keeping, attention to detail, and proper lab techniques set the foundation for success of this student lab group. 

Sample of Student Work:  My pH Scale Samples - Students created a custom pH scale to represent the data that was collected in their inquiry pH Labs.  It was my intention that students would provide more details in their pH scale and will emphasize these supporting facts to describe the student pH scales next year!




Close - Student Show And Tell: pH Models

5 minutes

Group Share: Now that the students have created their own inquiry-inspired pH scales, it is time to share their efforts with the class.  Student groups are encouraged to share their inquiry process and pH scale based on the common household substances.  After three groups have shared their results, the class will have a discussion to identify the similarities and differences that were observed between the three groups.  

Class Reflection and Discussion: If the student groups all used the same household substances and the same type of pH paper, should their results have come to the same conclusion?  Students suggested the following reasons to account for the varying results that were shared in class:

  • Different inquiry processes
  • Laboratory error
  • Inaccurate data analysis
  • Faulty household substances (did the solutions settle?)

Making Real World Connections: At the conclusion of the class discussion an emphasis was made to encourage the students to continue to make connections with the curriculum that is studied in Biology and the experiences they have in their everyday lives.  By making the Biology content part of the students' lives they will become more invested in understanding the lesson!