The Chemistry of Ocean Acidification: Exploring the Basics of pH

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Students will be able to discuss the concepts of acids, bases, and neutralization reactions using their own data.

Big Idea

Have students explore the pH of common substances and relate their work to our discussion about climate change.

Notes for the Teacher

Every biology teacher does some version of a pH lab during the beginning of the school year.  What makes this one more meaningful for me is that we started this unit discussing climate change and in particular, we worked on a virtual lab that demonstrated to students the impact of climate change basics on the ocean environment and marine organisms.  This hook leading into the pH lab gave my students the opportunity to relate to pH in a deeper context and the questions they asked me were more conceptual in nature than in previous years.  

Check out this short video I made connecting our pH work with our climate change discussions and activities to date.

In the past I have used pH probes and if you have them available, they can be a nice substitution/addition to traditional pH paper.  However, overall, the difference for me in the learning experience that students had with this lab has been the climate change context mores than the equipment they used to gather their basic pH data.   

I have set it up to use anywhere from 12-16 substances, however, you can use however many work with your classroom/time constraints.  In the past, I have used pH paper, pH probes, or both.  This is also a good lesson to use as a reminder to students about the differences between qualitative and quantitative data.

The Classroom Flow: Reviewing pH

10 minutes

1.  Using a popcorn protocol or any other group discussion norm that works with your class, ask the students to share out what they know about pH.  Document their answers on the board visible for everyone to see.

2.  After the brief share out, review basic concepts of pH, found on the powerpoint slides:

  • the definition of an acid and a base  related to H+ ions and OH- ions 
  • the pH scale, relating numbers on the scale to acids and bases
  • the basic neutralization reaction (H+ + OH- --> H20)

 Note: pH is a concept they will be familiar with and I use this lab as a starting point for our future discussion rather than a culminating event in their learning cycle about pH and water.  Therefore, I ask students to take brief notes but I keep this very short.  Students know our presentations are online for them to view and our classroom culture emphasizes the discussion and the experience as our primary entry point into exploring new topics. 

3.  Take any questions that come up for the class.

The Classroom Flow: Testing for pH

35 minutes

1.  Tell students that now it is time to investigate pH!  Pass out the lab document and ask students to fill in the names of the substances they will be testing from the list you have written out on the board.  You may use whatever materials you have available to you for this activity.  I have included a pH sample list of substances I typically use with my classes. I intentionally put any strong acids and bases on separate tables and make sure those materials are located at the tables closest to the front of the room so that I can closely monitor for lab safety during as well as in between classes.

2.  Show the students the pH lab set up they will find at each lab table. At each lab station, there are two samples to test for pH.  Because there are two set ups at each lab table, students can experience the lab as pairs.  I find that because the pairs are working independently rather than as a group of four, I have not needed to assign specific roles at each table.  The students tend to automatically take turns so that each pair partner gets to handle one sample at each table and then record information on their lab document as their partner handles the other sample.  If you prefer, you can delineate these roles or set up a system where they switch roles halfway through the lab session.

  • Note: By keeping the kids moving around the room as they test and discuss, it minimizes off task behaviors and helps students stay on focused at each station because they know their time there is limited.  Keeping reactive samples at different tables also prevents overly curious students who may or may not be focused on the lab activity from mixing things on their own without direct supervision or oversight.

3.  Tell students that they will have five minutes at each lab table to test the materials at the table and answer questions on the lab document.

4.  Ask for a student volunteer to be the time keeper who will announce when it is time for each group to shift to the next table.  Alternatively, you can be the time keeper, but i find students like to take turns doing that job for the class and it frees me up to interact about the content and monitor for lab safety without distraction.

5.  Remind students that at the end of the class period, their table group is responsible for ensuring that their lab set up is ready for the next class--clean, organized, and with all of the necessary materials.  They can get any replacement materials from you or from the labeled area you designate in the room.

Connecting to Ocean Acidification: A Review

10 minutes

1.  Now is a great time to spiral back to one of our original climate change lab activities utilizing the Stanford Virtual Urchin website resource for the acidifying oceans powerpoint slides connecting acid reactions to aquatic conditions and marine life.  

2.  Get onto the Virtual Urchin acidifying ocean website and replay the slides related to pH and the chemistry of ocean pH in relation to climate change greenhouse gases.  Students have viewed this site before in preparation for a virtual lab about acid effects on ocean life and it is a great resource. 

3. Use the spokesperson protocol for students to discuss and share out answers to the following prompts:

What do you know about pH now that you didn't know about before?

What can you say about the relationship between pH, climate change greenhouse gases, and the impact on ocean life?

You can see from this student sample the types of comments I hear all the time in class and in student reflection papers about how clear the connection between climate change and pH chemistry is for them.  This entire unit seems to be an a-ha moment for kids as they see how integrated science is to many things that they care about in their day to day lives.  In terms of the lab document, most students will get perfect scores on the activity written assignment.  If there are issues, they center on the definitions of acids and bases as they relate to H+ and OH- ion concentrations.  This is something to revisit when you review work with the class after you have graded and handed them back to students.