Water Quality: What is pH?

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SWBAT explain what pH measures and how it applies to water quality.

Big Idea

One component of water quality is pH, which tells us how acidic or basic a solution is; acids and bases have different properties; living organisms typically prefer neutral solutions.

Why This Lesson?

Students have already gained a background in how water quality is measured and will have participated in testing real samples from the community in the two prior lessons: Water Quality: How is it Measured? and Water Quality: Testing Samples.  One of those water quality tests was to measure pH, but students have not had any formal learning about pH at this point.  Water quality is a great framework for introducing and teaching acid base chemistry.

Treating water samples and exploring sources of poor quality water will help students meet Performance Expectation HS-ESS3-6: Use a computational representation to illustrate the relationships among Earth systems and how those relationships are being modified due to human activity.  Part of being able to treat water quality is to understand pH balance and be able to neutralize acidic or basic pH.  Before students are presented with the challenge of cleaning and neutralizing a water sample in ASSESSMENT: Removing Contaminants and Balancing pH of a Water Sample, they need to understand acid base chemistry (this lesson's focus) and be able to neutralize pH (the next lesson, Water Quality: Neutralizing pH).

Today, students are using acid base reaction equations to determine the location and movement of the hydrogen ion.  Students will be labeling acids, bases, conjugates acids, and conjugate bases.  Students are engaging in SEP 6: Constructing Explanations.


5 minutes

While I take attendance, students do a warm-up activity in their composition Warm-Up/Reflection books.  I use warm-ups to either probe for students' prior knowledge about the day's upcoming lesson or to have them bring to mind and review what they should have learned previously.  (To read more about Warm Up and Reflection Books please see the attached resource.)

Today’s Warm-Up: “What is an acid?  What is a base?"

In this case, the warm-up is asking students to draw upon their prior knowledge to give definitions of acids and bases.  It is also preparing students for today's activity during which they will learn that acids are proton donors and bases are proton acceptors.

If time permits, I walk around with a self-inking stamp to stamp the completed warm-ups indicating participation, but not necessarily accuracy.  In order to speed things up, my students have been trained to pass their books into the center of the table rows and stack them so that I can quickly pass by and stamp.  On days when there is too much business keeping, I do not stamp.  Students have been told that warm-ups are occasionally immediately checked and other times not.  At the end of each unit, Warm-Up/Reflection Books are collected and spot-checked.  Today, I do collect and stamp.  I need to know what the general prior knowledge is in my classroom about acid base chemistry before beginning today's lesson.

Introduction to Activity

15 minutes

Because I just checked Warm-Up books, I have a general sense of what my students know about acids and bases.  I expect that vWery few, if any, students will articulate that acids donate protons and bases accept them.  

I begin class discussion by asking the questions in bold, and probing for responses in italics:

  • How can I tell if something is an acid or a base?  Measure pH.
  • What kind of pH would tell me I have an acid?  Below 7
  • What pH tells me I have a base?  Over 7
  • What if my pH is 7 exactly?  The solution is neutral
  • What do acids do that make them acids?
  • What do bases do that make them bases?

For the last two questions, I am not expecting students to answer in ways other than giving me properties of acids/bases that they may have learned before.  They may also say that both are dangerous and/or corrosive.  This will lead to my explanation of what acids and bases do on a molecular level.  I use a football (hydrogen ion is the football) analogy to describe an acid base reaction (see the reflection video here for more explanation).

Partner Work

20 minutes

I begin by handing out the worksheet found here by John Erickson: Conjugate Acid Base Pairs, Chem Worksheet 19-2.  I allow students to pair up and work with a partner to complete the sheet.  Students gain practice identifying acids, bases, conjugate acids, and conjugate bases.  They also have practice predicting products in acid base reactions.  As students work, I check-in with partner groups to identify anyone struggling.  This gives me a chance to pull students to a table in the back of the room where we can go over the content again if necessary.  In one class period, I had two students who could not figure it out together that I sat and worked with.  In the other class period, I had three different groups who needed serious re-teaching.  I pulled those six students to a lab station and was able to work with them until they were able to work on their own.

As much as I hate just giving students a worksheet, from time to time they do need to practice.  Giving students time for independent (pair) practice allowed me the freedom to work with students who needed the extra attention.

Whole Class Debrief

10 minutes

After students have had plenty of practice time, we go over the correct answers as an entire class.  I am not planning to collect the student work--the point of today's lesson was to build understanding in identifying acids and bases so that students can later apply that knowledge to neutralizing pH in water samples.

Student sample work:

I love how this student drew the arrows to show proton transfer.  

This student did not draw the arrows, but clearly understands how to identify acids and bases.

This student also could identify acids and bases correctly.  In fact, all of my students did really well.  Tomorrow's lesson will reinforce the learning from today.

Student Reflection

5 minutes

In student's Warm-Up/Reflection Books, students should spend about 3-5 minutes writing a response to the day's reflection prompt.  Prompts are designed to either help students focus on key learning goals from the day's lesson or to prompt deeper thinking.  The responses also allow me to see if there are any students who are missing the mark in terms of understanding.  The collection of responses in the composition books can also show a progression (or lack thereof) for individual students. 

Today, I ask students to instead of recording their answers in their books, to record their answers on their activity handouts, simply because we were pressed for time.

Today's Reflection Prompt:  "How do we know if something is an acid or a base?  What is it that transfers from acids to bases?"

Desired student responses should indicate that acids are proton donors and bases are proton acceptors.  Students should indicate that the hydrogen ion (or proton) is what is transferred from acids to bases.

My students had no trouble identifying that acids are proton donors and bases are proton receivers after today's lesson.