Ocean Acidification: pH and the Ocean's Balance

Big Question

What happens to ocean animals when the ocean's pH changes?

What's This About?

About one-quarter of all the carbon dioxide that people release into the air (by burning fossil fuels) ends up in the ocean. Carbon dioxide is called an "acid gas." When it gets into ocean water, it makes the water more acidic. Scientists measure this using something called pH, which tells us how acidic or basic (the opposite of acidic) a liquid is.

The ocean isn't actually acidic—but even a small shift toward being more acidic can cause big problems for ocean animals. This works the same way in the human body!

What You'll Learn

By the end of this activity, you should be able to:

• Understand that pH is one way scientists track ocean acidification
• Understand the pH scale, and know what acids and bases are
• Understand that a small change in pH number actually means a much bigger change in chemistry

Key Ideas to Remember

• The ocean is a vital part of Earth. It's home to sea creatures that help keep all life on our planet healthy.
• Human activities are changing the chemical balance of the ocean.
• Small shifts in pH can make a big difference in the health of sea creatures. (In humans, dropping blood pH by just 0.05 causes a sickness called acidosis.)

Important Connections

• Everything is connected: Both sea creatures and humans need to keep a very careful pH balance to stay healthy.
• Small numbers, big changes: A tiny move up or down on the pH scale means a much bigger shift in chemistry than you'd expect.
• How it all connects: Burning fossil fuels releases CO2 → the ocean soaks up the CO2 → CO2 is an "acid gas" → this makes the ocean more acidic → even a small drop in pH (just 0.1) can tip the balance from healthy to unhealthy for sea creatures, just like it does in humans.

What Can We Do?

The less acid gas (CO2) that humans release into the air, the less ends up in the ocean. There are many new inventions in clean energy and transportation that can help—not just for one family, but for whole neighborhoods and cities.

Ideas to explore in your own community:

• Organize a group project to make a home more energy-efficient with better appliances, windows, or insulation.
• Take the bus, ride a bike, or walk instead of driving when you can.
• If your area doesn't have good public transportation or bike lanes, talk to local leaders and start that conversation.

Activity Instructions

Getting Ready: Before you start, write the numbers 1 through 14 on separate sheets of paper. Lay them out in order from left to right (on a table or floor). Write "8.1" on an index card and place it in the top corner of the "8" paper.

Steps:

1. Ask: Do you know what an acid is? What about a base? Want to find out?
2. Introduce the pH scale, which runs from 0 to 14.
3. Anything above 7 is a base. Anything below 7 is an acid. The number 7 is neutral—neither acid nor base.
4. 0 = strongest acid. Acids usually taste sour, can sting your skin, and can wear away metal.
5. 14 = strongest base. Bases taste bitter, feel slippery, and can dissolve fats and oils.

1. Ask: Did you know that many foods and household items we use every day are acids or bases?
2. Have your student pick (or hand them) a household item and guess where it belongs on the pH scale. Use the pH list below to check their guess. You can give hints like "getting closer" or "getting farther" or ask questions like "Does it taste sour?"
3. If they guess correctly, celebrate! Try as many items as you have time for, saving "the ocean" for last.
4. When it's the ocean's turn, explain how special it is. You could even offer a small prize for guessing its pH correctly.
5. Talk about how the ocean's pH has changed over the last 100 years—from 8.1 down to 8.0 today. That might not sound like much, but here's why it matters:
6. If human blood pH drops by just half that amount (0.05), it can cause headaches, confusion, tiredness, shaking, sleepiness, and trouble thinking clearly. (Healthy blood pH is 7.45; a drop to 7.4 is called "acidosis.") Scientists have seen similar effects in ocean fish—like clownfish (think of Nemo!)—who can become confused and even swim toward predators when ocean water becomes more acidic.

1. Ask: Do you know what's causing the ocean's pH to change?
2. Carbon dioxide is an "acid gas." When it enters the ocean, it makes the water more acidic (which shows up as a lower pH number).
3. Carbon dioxide pollution comes from things like:
4. Driving cars, boats, planes, and trains
5. Using electricity at home
6. Cutting down trees (deforestation)—trees store carbon dioxide in their leaves, which keeps it out of the ocean

Let's Talk About It

• Can you think of ways to help lower the amount of carbon dioxide we release into the air (which eventually ends up in the ocean)?
• Talk about solutions that fit your community (see ideas above)—these work well for families and older students.
• For younger kids, simple personal actions might work best:
• Turn off lights when you leave a room
• Unplug electronics you're not using, like chargers and computers

Materials Needed

• Fifteen sheets of 8x11" paper
• One 5x7" index card
• Everyday household items and foods with different pH levels
• Something to represent the ocean (like a snow globe, a container of water, or a toy sea creature)
• The pH chart below
• A pH scale

Approximate pH of Everyday Items

Item	pH
Large battery	2.2
Lemon	2.3
Coca-Cola (classic)	2.5 (up to 4.2 for other brands)
Vinegar	2.8
Grapefruit	3.0–3.3
Orange	3.0–4.0
Apple	3.5
Banana	5.0
Milk	6.5
Bottled water	7 (6.5–8 depending on brand)
Human blood	7.4
Ocean (today)	8.0 (was 8.1)
Baking soda	8.4
Tums (antacid)	10.5
Bleach	12.6
Drain cleaner	14

Learn More

• http://www.cisanctuary.org/ocean-acidification/
• http://oceanacidification.noaa.gov/

This lesson was created by NOAA's Ocean Acidification Program. Questions? Email noaa.oceanacidification@noaa.gov