Water Quality: Neutralizing pH

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SWBAT determine how to neutralize acidic and basic solutions, then perform those neutralization reactions.

Big Idea

Acids can be neutralized by adding base; bases can be neutralized by adding acid.

Why This Lesson?

Water quality is a great framework for introducing and teaching acid base chemistry.  Students have already gained a background in how water quality is measured and will have participated in testing real samples from the community in two previous lessons: Water Quality: How is it Measured? and Water Quality: Testing Samples.  Then, students spent the lesson right before this one (Water Quality: What is pH?) learning how to identify acids, bases, conjugate acids, and conjugate bases.   

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 have practice neutralizing pH, which we do in this lesson.

Today, students will be experimenting by combining different acids and bases to note the resulting pH and learn that bases neutralize acids and vice versa.  This engages students in SEP 3: Planning and Carrying Out Investigations and SEP 4: Analyzing Data.


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 does pH measure?"

In this case, the warm-up is asking students to draw upon their prior knowledge about pH, which they should have from biology and as they experienced measuring during our water testing lab (Water Quality: Testing Samples).  It is also preparing students for today's activity during which they will be measuring pH and investigating how to neutralize acids and bases.

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 pH before beginning today's lesson.

Pre-Lab Discussion

10 minutes


  • pH paper that measures 1-14 (with color reading scale)
  • various acids and bases:
  • 1M HCl
  • 1M NaOH
  • ammonia
  • vinegar
  • citric acid
  • baking soda
  • beakers
  • 10 mL graduated cylinders
  • 1/4 teaspoons

I explain that today students will be investigating how to shift pH levels with the goal of reaching 7, which is neutral.  I ask why might we want to make pH levels neutral.  I am expecting students to suggest that neutral means it will not hurt us or that living organisms often need neutral water.

Today is purely investigational.  I tell students to first test and record the pH levels for each liquid that they have (which is everything except baking soda).  Then, I tell students that after they know the initial pH levels, they can add varying substances, a little at a time, until they reach neutral.  I suggest that students decide which substances raise pH and which lower pH.

Student Investigation

30 minutes

I remind students to wear goggles during this entire class session due to the nature of the chemicals.  I also tell students to notify me immediately of any spills.

Students work first by getting a baseline pH check on their starting chemicals.  Then, students work in small groups to decide which substances move pH levels a lot and which move pH levels a little.  This allows students to investigate relative strengths of their acids and bases.  There are no right or wrong answers really, there is just discovery.

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:  "What should be added to an acid to neutralize it?  What would neutralize a base?"

Desired student responses should indicate that adding base to an acid would raise the pH and adding acid to a base would lower it.  The pH change will depend on how much acid/base is added as well as the initial pH levels.

My students were able to identify acids as being able to neutralize bases and vice versa.  They had difficulty articulating that the amounts and strengths were also a consideration and that neutralization was not guaranteed by adding any acid to any base.