Activity 5: pH


  • Learn about pH, a water quality characteristic.
  • Learn about ocean acidification, a water quality issue caused by climate change.
  • Learn about how seashells are formed.
  • Model building shells, including how it can be harder in more acidic water.
  • Use pH paper to measure the pH of some household substances.

Learning Targets

  • Understand what pH is.
  • Understand how species are affected by pH.
  • Model changes in pH.


60-80 minutes.


Included in this kit:

  • 10 Shells (real) (10 per kit)
  • Water Beads (500 beads per group of 3-4 youth)
  • 7 sets of beads for the acidic ocean (bags with 5 white and 10 orange beads)
  • 7 sets of beads for normal/healthy ocean (bags with 10 white and 10 orange beads)
  • 14 aluminum pans
  • 14 Chenille Stems
  • 10 Plastic Beakers
  • 7 packs of pH Paper
  • 10 Pipettes
  • 1 Water Pitcher
  • 1 Aquarium net
  • 25 safety glasses

Not included, but needed for the activities:

  • Tap Water (not provided)
  • pH samples (not provided):
    • Bismuth subsalicylate (1 bottle)
    • Lemon Lime Soda (1 can)
    • Cola (1 can)
    • Vinegar (1 bottle)
    • Hand Soap
    • Milk of Magnesia (1 bottle per kit)
    • Calcium-Carbonate Chewable Tablets (1 bottle per kit)
  • Evaluations (choose one or more of the following):
    • Exit Ticket (1 for each youth)
    • KLEWS Chart (1 for each youth)
    • Journals (1 for each youth)


  • pH: a unitless measure of the concentration of hydrogen ions in a solution.
  • pH Scale: a logarithmic scale, meaning that when you go from 7 to 6, the amount of hydrogen ions has been multiplied by 10. Acids are below 7, bases are above 7, and 7 is neutral. As the concentration of hydrogen ions increases, the concentration of hydroxide ions decreases.

Background Information for Facilitators

Note:  All links are provided purely for educational purposes. No responsibility is assumed for any content on the linked site(s).

pH videos:

Ocean acidification videos:

  • Demystifying ocean acidification and biodiversity impacts – California Academy of Science (YouTube) (length: 12:12)
    • Summary:
      • Beginning (up until about 4:35) has a more in-depth explanation of the pH scale, followed by an excellent explanation of the chemical reaction, or how carbon dioxide in the atmosphere is making the oceans more acidic
      • Ending (after about 4:35) describes how ocean acidification affects the earth, the oceans, and the plants and animals living in water; chemical reaction for calcium carbonate (shells), coccolithophores, larval stages of animals are particularly vulnerable


No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

  • Gather all the necessary materials.
  • Put water beads to soak in water in bowls the night before. It takes time for them to expand. (Minimum soak time of 4 hours. Overnight is recommended.)  Divide the plastic water-beads evenly into the 14 bowls (just under 1 level tablespoon/bowl).  Don’t worry if they’re not exactly equal, you can move them around after they have expanded. Add water to each bowl. Fill the bowl until the water is just under an inch from the top.  Let soak at least overnight, but no more than a few days. Once water-beads reach the appropriate size (~1-1.5 cm), drain any excess water so that you only have hydrated water-beads in the bowl, and no extra liquid. Make sure each bowl contains approximately the same amount of water-beads.
  • Prepare a regular and an acidic “ocean” for each group of 3-4 youth. DO NOT LABEL THESE OCEANS as acidic/normal. Youth should not know ahead of time that there is a difference:
    • For a regular ocean, add 10 white (carbonate) and 10 orange (calcium) beads to seven of the prepared pans of water beads and mix
    • For an acidic ocean, add 5 white (carbonate) and 10 orange (calcium) beads to seven of the prepared pans of water beads and mix
  • Label beakers and put samples in beakers. (For testing pH of samples.)
  • Calcium-Carbonate Chewable Tablets: Put a tablet or two in water about 2 hours before the activity. This should give enough time for the tablets to dissolve. (Measuring pH does not work unless tablets are dissolved in water.)
  • Decide which evaluation method you will use:


Before you begin, review the discussion norms that youth agreed to during the Introduction to Aquaculture activity. These norms are intended to help youth have productive, respectful conversations. Reminding youth of their norms helps youth to remember to follow their norms.

Engage (10-20 minutes)

Feel a real shell

Not everyone has had the chance to hold a real shell. We will start the pH lesson by becoming familiar with something that is affected by pH: shells.

  • Give youth the real shells.
  • Ask youth to handle shells.
  • Journal: have youth record observations on page 24.
  • Ask the youth to share observations.

Ocean Acidification in a Cup

  • Briefly introduce ocean acidification to the youth.
    • Ocean acidification occurs when carbon dioxide is absorbed by the ocean, increasing the acidity of the water.
  • Tell the youth that they will perform an experiment to demonstrate ocean acidification.
  • Give each youth two clear plastic cups and a paper cup.
  • Remind youth to always put on safety glasses when they do a science experiment.
  • Walk the group through the following instructions:
    • Pour 50 mL of acid-base indicator solution into each of the two clear plastic cups.
    • Add 2 g of baking soda to the paper cup.
    • Tape the paper cup inside one of the clear plastic cups so that the top of the paper cup is about 1 cm below the top of the plastic cup. The bottom of the paper cup shouldn’t touch the liquid in the plastic cup — you don’t want it to get wet!
    • The plastic cup without the paper cup in it is the control model in this experiment.
    • Place the plastic cups onto a sheet of white paper and hold up another sheet of white paper behind the cups (this is for observation purposes).
    • Add about 5 mL of vinegar to the paper cup. Be careful not to spill any vinegar into the indicator solution.
    • Observe the changes in the paper cup model. After a few minutes, you should notice a color change at the surface of the liquid. As time passes, the rest of the liquid will also change color.

Shell Building Activity

Youth will model building a shell in two different ocean conditions.

items set up to perform the first part of the pH activity


  • Tell youth to imagine they are a juvenile mollusk (e.g. clam, mussel, scallop), and need to build a shell to keep themselves safe from predators.
  • Divide youth into groups of 3-4 youth. One youth will “live in” the regular (neutral) ocean, one will live in the acidic ocean, and one will be the timekeeper.
  • Each group needs a set of pans with water beads/pony beads. One bowl is a regular ocean, the other is an acidic ocean. Youth should not know that there is a difference.
  • Youth will have 90 seconds to build the largest shell they can. In each ocean, there are white and orange pony beads.
  • White represents calcium ions and orange represents carbonate ions. To build a shell, youth will string beads onto a chenille stem, being careful to alternate the colors (white, orange, white, orange, …)
  • After one round is complete, count the number of pairs that each mollusk-youth has.
    • Journal: record results on page 24.
    • After counting, remove the beads and mix them back into the ocean they came from.
  • Next, have the mollusk-youth switch and build a new shell in the other ocean. Repeat the 90 second round and counting completed pairs.  Repeat again until all youth have the opportunity to build from each ocean.
  • Discuss how there is a difference between the acidic ocean and normal ocean — the number of carbonate ions
  • Discuss how ocean acidification happens (covered in the first video below)

Seashells are essentially an exoskeleton for the animal they protect. Seashells are made primarily of calcium carbonate. When CO2 dissolves in water, it forms carbonic acid (the same substance found in carbonated drinks like lemon-lime sodas or colas). The calcium carbonate in the shell reacts with carbonic acid in the ocean and can cause existing shells to weaken.  As the ocean becomes more acidic, animals with shells may also be impacted by a decrease in carbonate available for building shells.

Explain: What is pH? (10-15 minutes)

Ask youth what they know about pH. If youth have not had high school chemistry and are a few years removed from middle school physical sciences, they may not remember very much about pH. (This is fine.)

  • KLEWS: use the K column.
  • Journals: brainstorm on page 24.

Ask youth what they know about acids and bases. These are words that youth should have some exposure to and familiarity with.

  • KLEWS: use the K column.
  • Journals: brainstorm on page 24.

Watch this video: Demystifying Ocean Acidification and Biodiversity Impacts (YouTube) by California Academy of Sciences (length: 12:12).  Note: if time is limited, you may stop at about 5 minutes in.

As a whole group, decide on a definition for ‘pH’.

  • KLEWS: record pH definition in S column.
  • Journals: record pH definition on page 23.

Discuss how pH affects aquatic life. (This is covered in the second half of the longer video.) The shell-building activity modeled how pH affects mollusks. Have youth explain what was happening to the shells in water with different levels of acidity. Have youth explain how the differences in shells would affect the animals living inside the shells.

  • KLEWS: use L, E, and S columns
  • Journals: brainstorm on page 24. Following discussion, record effects on page 23.

Explore: Testing pH (10-15 minutes)

Use pH paper to test the pH of various samples and substances.

other items tested for pH level in pH activity
No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.


  • Journals: record data on page 24.

Please note: all samples we have provided or encouraged you to procure are over the counter substances and/or foods, and should not be harmful if youth or facilitators are exposed to them. While we have not included very strong acids or bases, we have found substances that should show a range of pH values.

Here are the steps:

  • Label beaker and put pH samples in them (if this has not been done already).
  • To measure pH, youth will dip a strip of pH paper into a sample. The color of the paper should change.
  • Compare the color of the paper to the key on the case for the pH paper.
    • Note: use the color that is farthest from the end dipped in the sample. (Sometimes multiple colors are seen on the paper, this one is most accurate.)
  • Record the measured pH for each sample.
  • Have youth compare their measured pH values to each other and to expected values.

pH Sample Answer Key

Substance Expected pH Notes
Cola 2-3 Do not drink the sample from the sample beaker.
Lemon-Lime Soda 3-4 Do not drink the sample from the sample beaker.
Bismuth subsalicylate 5-6 See label on bottle for warnings. Eating large amounts may result in constipation. Long term use may result in aspirin poisoning.
Vinegar ~2.5 Vinegar is a mixture of acetic acid and water. Acetic acid is an excellent nail polish remover. (It also strips the finish off of wood.) Food-grade vinegar (what you have) is safe.
Milk of Magnesia ~10.5 See label on bottle for warnings. Drinking small amounts of milk of magnesia will settle the stomach (it’s an antacid). Drinking large amounts of milk of magnesia can cause diarrhea (it’s a laxative).
Calcium-Carbonate Chewable Tablets > 7 (depends on the amount of tablets and amount of water) See label on bottle for warnings. Consuming small amounts settles the stomach (it’s an antacid). Consuming large amounts can cause constipation (too much calcium carbonate) and/or diarrhea (irritation of intestines).
Hand Soap 9-10 pH depends on the amount of lye in the soap. Soap with more lye has a pH 11-14, and will irritate your skin – do not use these soaps. (Lye reacts with the fats in your skin … pure lye will literally dissolve your skin.)
Water ~7 Water should be roughly neutral.

Extend: Aquaculture Project (10-15 minutes)

Use both supplemental resources to help youth with their research:

Test the pH of your water sample.

Find out the pH range of your target species.


Evaluate (Assessment) (5 minutes)

Approximately 5 minutes before the end of the session, have youth finish up what they are doing, help clean up the materials, and do one of the following evaluation methods:

Have youth complete and turn in an exit ticket.

Have youth finish and turn in the KLEWS chart.

After completing this activity youth should have the following in their journals:

  • Definitions of pH and ocean acidification (page 23)
  • Aquaculture project: (pages 25 and 26)
    • pH range of their species
    • pH range of their waterbody
    • Comparison of the species and water body


Adapted with permission.  Original activity brochure available online (Towson University) to download and print: Ocean Acidification and Oysters, How will increasing levels of CO2 affect oysters? (PDF)