Elementary School Science: Introduction to pH

This is a short unit that I’ve used to introduce upper elementary school students to pH and, more broadly, the concept of chemical testing.  It is strongly influenced by the charming publication Of Cabbages and Chemistry by Jacqueline Barber.  My teaching periods are quite long, so although I’ve organized this unit into three lessons most teachers would need to break it up further.

Introduction to pH:
Cabbage Juice and Color Change

Photos downloaded June232015am 079

Getting Started

• Do Now: Make a prediction: if you add a clear liquid to a pink liquid, what color will you get?
• Share and discuss predictions as a class.

Cabbage Juice Combinations
• Demonstrate how to use a pipette. Caution students not to overfill the wells in their mixing trays. Emphasize the importance of keeping liquids separate from each other and using each pipette in only one liquid.
• Let students look at the mixing trays and challenge them to develop a method to keep track of which chemicals are mixed in each well. Conduct class discussion and come to a conclusion about one or two record keeping methods that will be effective. Give students time to create charts for their observations.
• Allow students to begin testing the color changes that occur when they add a wide variety of colorless or almost colorless liquids to purple cabbage juice. (Liquids include white vinegar, rubbing alcohol, salt water, tap water, baking soda mixed with water, an antacid tablet mixed with water, aspirin dissolved in water, a calcium pill dissolved in water, lemon juice, and kitchen countertop cleaner dissolved in water.)
• When students have had time to test each liquid on their own, begin a class discussion and testing procedure. For each liquid, ask students what color the cabbage juice turned when that liquid was added. Combine a clear plastic cup of the liquid and cabbage juice in front of the students as a way of clearing up disputes and reinforcing the idea that multiple tests are an important part of science. Create a class chart listing the color of each mixture.
• Randomly group the class into pairs.
• Working with their partner, have students put the liquids into categories based on the color they turned when mixed with cabbage juice. Because there will be many shades of pink, blue, purple, and green there is a good deal of opportunity for discussion.
• Give groups an opportunity to share their categories. Discuss the differences and similarities between the different grouping systems.

Photos downloaded June232015am 076

Read “pH – Water Properties”

• Do a shared reading with “pH – Water Properties” (abbreviated from an article by the USGS).
• Lead a class discussion to elicit prior knowledge about pH, acids, and bases.
• Let students know that cabbage juice does not change color when mixed with a neutral substance, turns pink when mixed with an acid, and turns blue or green when mixed with a base. Ask them to re-categorize the substances they tested earlier as neutrals, acids, or bases.
• Lead a class discussion in which each substance tested is definitively identified as an acid, base, or neutral.

Cabbage Juice Challenges
• Let students work individually (not with a lab partner) for the cabbage juice challenges.
• Challenge half of the students to turn the liquid in every well of their mixing trays green. Challenge the other half of the students to turn all of the liquid in every well of their mixing trays pink.
• After students have had time to attempt the challenges, conduct a class discussion. What turns the mixtures green? What turns the mixtures pink? Can the pH of a mixture change when more is added to it?
• As a class, write notes on what we learned today.

Homework
• Pretend you are writing to a 3rd grader. Explain what pH is. Be sure to use the words “acid”, “base”, and “neutral.”

Materials
Chemical mixing trays, pipettes, purple cabbage juice, test liquids (white vinegar, rubbing alcohol, salt water, tap water, baking soda mixed with water, an antacid tablet mixed with water, aspirin dissolved in water, a calcium pill dissolved in water, lemon juice, and kitchen countertop cleaner dissolved in water), plastic cups, handout – “pH – Water Properties”

Continuing to Study pH:
Acids and Aliens from Outer Space

Getting Started
• Do Now: Write down 3 liquids that you guess are acids and three liquids that you guess are bases.
• Discuss the Do Now. Write a list of the substances and predictions on the board. To the extent that it is possible, gather these substances to test them in the next class.
• Check homework by randomly selecting students to read their explanation of pH.
• Randomly assign lab partners.

Acids and Aliens from Outer Space
• Tiny aliens have been discovered who are very sensitive to pH – they are unable to live in acidic or basic environments. Neutral liquids are the only liquids that are safe for them. Your goal is to safely escort an alien through a spaceship. Some rooms of the spaceship are filled with acid fluid, some are filled with basic fluid, and others are filled with neutral fluid. Your job is to test each room and adjust the pH so that aliens can safely live in it.
o Setup: The “spaceship” is actually a mixing tray filled with randomly placed liquids of varying pHs. The “spaceships” should be prepared before class.
• Class discussion of the spaceship activity. What was hard about it? What was easy about it? Are humans more or less sensitive to pH than the aliens? How did you neutralize acidic solutions? Basic solutions?

Reading –The Magic School Bus Inside the Human Body
• Read The Magic School Bus Inside the Human Body to the class.
• Pay particular attention to the role that stomach acid plays in digestion.

Review Game
• Allow students to play the “GEMS Alien Juice Bar” computer game. http://www.lawrencehallofscience.org/kidsite/activities/arcade/arcade.html?activity=alienjuicebar

Extension (if class finishes early)
• Draw a picture of one of the aliens you helped today. If you like, you can also draw its spaceship.

Homework
• Write a blog entry for the blog Alien Health and Safety. This blog entry should give practical, scientifically accurate information to aliens to help them survive in space ships that may be contaminated with acidic or basic liquids.

Materials
Chemical mixing trays, pipettes, purple cabbage juice, test liquids, The Magic School Bus Inside the Human Body, computers with internet access, drawing materials

Wrapping Up pH:
Going Beyond Cabbages

Getting Started
• Do Now: What are some advantages of using cabbage juice as a pH indicator? What are some weaknesses of cabbage juice as a pH indicator?
• Check homework – randomly select students to read their Alien Health and Safety blog entry.

Introducing Other Indicators
• Show students universal pH indicator strips, litmus strips (red and blue), universal pH indicator liquid, and an electronic pH meter. Demonstrate each one using a known acid, a known base, and a known neutral (i.e., liquids tested and discussed over the previous two days).
• Show students the testing stations set up around the room. Explain the data sheets that are at each station. Remind students about the importance of keeping accurate records.
• Randomly assign lab partners.
• Allow pairs of students to rotate through stations in which they test a variety of liquids with each of the new pH indicators. Each station has one new pH indicator and about 3 liquids to test.
• Discuss the results of the testing as a class. What did we learn about the pH of different liquids? If there is any disagreement about whether a particular liquid is acidic, basic, or neutral test it in front of the class using each of the indicators. What did we learn about different pH indicators? What are some advantages and disadvantages of each one? Does having more than one way to test pH make us more or less confident that we really know the pH of a substance?
• As a class, write notes on what we learned today.
Write – Which pH indicator do I prefer?
• Which pH indicator do you prefer? Write your preference and be sure to explain your reasoning.
• Allow volunteer students to share their preferences.

Homework
• Conduct a thought experiment. Can colorful fruits and vegetables other than purple cabbage be used to indicate pH? Describe an experiment you could do to find out the answer to this question.

Materials
Universal pH indicator strips, red and blue litmus paper strips, universal pH indicator liquid, electronic pH meter, liquids to be tested, data sheets

Bibliography
Barber, Jacqueline. Of Cabbages and Chemistry: Teacher’s Guide. Berkeley, CA: Lawrence Hall of Science, U of California, 2006. Print.
Cole, Joanna, and Bruce Degen. Magic School Bus inside the Human Body. New York: Scholastic, 1993. Print.
“PH – Water Properties.” PH: Water Properties. USGS Water-Science School, 05 May 2015. Web. 09 July 2015.

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