How Can You Measure pH?

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SWBAT use pH paper to identify common acids and bases. SWBAT categorize data collected about acids and bases to create a chart illustrating the data.

Big Idea

Why is it important to measure pH? Why should you measure pH of household solutions? Students investigate by testing common household acids and bases like cola, baking powder, vinegar, and lemon juice.

The Scientific Method: First

10 minutes

This is an 80 minute lesson. Day #1 students will carry out an investigation (SP#3) using pH paper and testing household solutions. Day# 2 students will categorize, analyze and interpret the data (SP#4) collected about acids and bases to create a chart illustrating the data.

At this point in the school year, students are able to work more independently so I chunk the learning. Students use the worksheet How can you measure pH? to record their writing and data. I ask students to take 10 minutes to complete steps #1 - 3 of the scientific method with their partner(s). That means that students will discuss the question "How can you measure pH?", read and discuss the background information, and then write an hypothesis. An appropriate student hypothesis could be: I think that you can measure pH by using pH paper because the pH paper will turn different colors when it comes in contact with an acid or a base. My students are more successful when I provide a sentence frame (I think that . . .because . . .) for the writing. It helps guide student thinking and encourages students to use evidence. 

The Scientific Method: Next

20 minutes

The next chunk of learning is the investigation (SP#3) where students conduct an investigation to produce data to serve as the basis for evidence that meet the goals of the investigation. The goal of the investigation are the questions: How does matter behave? Why is it important to measure pH? and Why should you measure pH of household solutions?

I direct students to read the materials and procedure with their partner(s), test each solution, collect (SP#8) and record the data, and analyze (SP#4) the results. As students perform the experiment, they will: test their hypothesis, analyze and interpret data to provide evidence for phenomena (acids & bases), and analyze and interpret data to determine similarities and differences in findings about acids & bases.

Lab Safety: Make sure that students wear goggles at all times when testing solutions. Do not eat or drink anything in the lab. Use common household solutions when testing (ie: lemon juice, vinegar, cola).

The Scientific Method: Last

10 minutes

The last chunk of learning is for students to (SP#8) evaluate and communicate information. This step is very important for students to "come full circle." I ask students to engage in a scientific discussion with their peers to derive meaning from the investigation. Students collaborate with peers searching for the best explanation by engaging in argument from evidence (SP#7). Students obtain evidence during the experiment.

For students to successfully write a conclusion, I have learned that you need to take students back to the questions so they can think about the process. How does matter behave? Why is it important to measure pH? Why should you measure pH of household solutions? Take 2 minutes for students to process the questions and write a conclusion which includes evidence from the investigation. I give them a sentence starter to help with the process, for example:  I learned that . . .because. . . Take 2 minutes to share answers with the class so students can hear other student thoughts.

Some students wrote:

  • I learned that oven cleaner is a strong base because it can harm you and it has warnings on the container.
  • I learned that you can measure pH because you can dip the pH paper into substances and match the color to see if it's an acid or a base and see how strong or weak it is.

pH Scale

40 minutes

Day# 2 students categorize the data collected about acids and bases to create a chart illustrating the data.

To ensure student achievement, I develop a Rubric specifically designed for this project which will assess the final product. A well designed Rubric provides a clear expectation for student work and clear expectations for grading.

I provide time in class for students to design, create, and complete their pH Scale where they will (SP#8) communicate their findings clearly and persuasively. This time allows me to circulate the classroom, address any misconceptions, and providing extra assistance to those struggling students. I provide supplies and materials for students to work such as: 8.5" x 14" paper, colored pencils, rulers, pencil sharpeners, and a pH Scale as a visual reference. For Special Education and ELL students, I provide pre-printed images of the solutions tested during the investigation. Students can cut and glue these images into the appropriate place on the pH Scale.