Substitute Lesson Plan: Ocean Acidification: pH and the Ocean's Balance

Objective

Students will be able to:

• Recognize that pH is one way scientists measure ocean acidification
• Understand the pH scale, acids and bases
• Understand that a small shift in the pH scale represents a much larger shift in the chemical balance

Materials

• This resource (printed copy of the activity outline and pH values list)
• 15 sheets of 8x11" paper, numbered "1" through "14" (one number per sheet), plus one extra sheet if needed
• One 5x7" index card with "8.1" written on it
• Household objects or edibles with different pH values (see list below) — use whatever is easily available; if none are on hand, just use the printed list and have students place the names of items instead of physical objects
• An object to represent "the ocean" (e.g., a snowglobe, a cup of water, a small toy sea creature)

Approximate pH of Everyday Objects (for reference):
Battery = 2.2, Vinegar = 2.8, Coca-Cola = 2.5, Lemon = 2.3, Grapefruit = 3.0–3.3, Orange = 3.0–4.0, Apple = 3.5, Banana = 5.0, Milk = 6.5, Water bottle = 7, Human blood = 7.4, Ocean (now) = 8.0 (was 8.1), Baking soda = 8.4, Tums/antacid = 10.5, Bleach = 12.6, Drain cleaner = 14

Warm-up (~5 min)

1. Before class (or as soon as students arrive), lay the numbered sheets of paper (1–14) in a line, in order, along the floor or a long table. This is the pH scale.
2. Ask the class: "Do you know what an acid is? A base? Would you like to learn about it?"
3. Briefly explain the scale as students look at it:
4. Numbers 0–14, with 7 in the middle
5. Anything above 7 is a base (alkaline); anything below 7 is an acid; 7 is neutral
6. 0 = strongest acid (sour, stings if touched, reacts with/degrades metal)
7. 14 = strongest base (bitter, slippery if touched, dissolves fats and oils)

Main Activity (~25 min)

1. Tell students: "Did you know there are acidic and basic substances we use or eat every day?"
2. One at a time, name a household item from the pH list (or hand out an actual item if available). Ask a student to guess where it belongs on the number line and physically stand on/point to that sheet of paper.
3. Give hints if they're stuck: tell them they're getting "hotter" or "cooler" (closer/farther from the right answer), or ask guiding questions like "Does it taste sour?"
4. When a student places an item correctly, congratulate them. Reveal the correct pH value from the list.
5. Go through as many items as time allows, saving "the ocean" for last.
6. For the ocean: hand the student your ocean object (snowglobe, cup of water, or toy creature). Explain how special the ocean is. If you have a small prize, offer it for placing it correctly.
7. Once placed near "8," tape/place the index card labeled "8.1" on the upper right corner of the "8" sheet. Explain:
8. 100 years ago, the ocean's pH was 8.1. Today it is 8.0.
9. This may seem like a tiny change, but it isn't.
10. Make the comparison to human blood: healthy blood pH is 7.4. A drop of just 0.05 (half of the ocean's 0.1 change) causes "acidosis" — headaches, confusion, tiredness, tremors, sleepiness, and trouble with brain function.
11. Explain that scientists have observed similar effects in ocean fish — for example, clownfish (like "Nemo") can become confused in more acidic water and may even swim toward predators.
12. Ask: "Do you know what is causing this change in the ocean's pH?" Explain:
13. Carbon dioxide is an "acid gas." When it enters the ocean, it makes the water more acidic (lowers the pH).
14. CO2 pollution comes from things like driving cars/boats/planes/trains, using electricity at home, and deforestation (trees normally hold CO2 in their leaves, keeping it out of the ocean).

Wrap-up / Exit Ticket (~10 min)

Ask students to answer the following on a sheet of paper (individually or as a group discussion):

1. What is the difference between an acid and a base on the pH scale?
2. What was the ocean's pH 100 years ago, and what is it today?
3. Why does such a small change in pH matter for ocean life (and for humans)?
4. Name one thing that causes carbon dioxide to enter the atmosphere (and eventually the ocean).
5. Name one thing you personally could do to help lower the amount of carbon dioxide released into the atmosphere.

Collect the papers or have a few volunteers share their answers aloud.

If Time Remains

Lead a short group discussion using these prompts from the resource:

• "Can you think of anything you can do to help lower the amount of carbon dioxide we release into the atmosphere?"
• Share simple individual actions: turning off lights when leaving a room, and unplugging "vampire electronics" like chargers and computers when not in use.
• If students are older (middle/high school), you can also mention larger-scale/community solutions: using public transportation, biking or walking, or talking to local government about improving energy efficiency and transportation options in the community.