🙋Need Support?: Look for green and red "Hint" and "Walkthrough" boxes!
❓ Driving Question: Why is the ocean becoming more acidic?
❓ Driving Question: Why is the ocean
dissolving shells now when it didn't 50 years ago? What has changed in
ocean chemistry?
🐚 The Phenomenon: The Dissolving Shells
Mystery
Anchoring Context & Focus Question
🔗 Before We Begin: Activate Your Prior
Knowledge
Think back to Cycle 3: You learned that CO₂ is a
greenhouse gas that traps heat. You also learned that carbon cycles
through Earth's systems. This week: CO₂ → dissolves in
ocean → carbonic acid → pH drop → shells dissolve.
The same CO₂ causing climate change is also changing ocean
chemistry!
A scale from 0-14 measuring how acidic or basic something is
carbon dioxide (CO₂)
A gas that dissolves in water and makes it more acidic
carbonic acid
Weak acid (H₂CO₃) formed when CO₂ dissolves in water
calcium carbonate
Chemical (CaCO₃) that makes shells and skeletons
marine organism
Any living thing that lives in the ocean
concentration
How much of something is in a certain amount of space
Diagram unavailable.
📊Diagram: Carbon moves between the atmosphere, ocean, and living things in a continuous cycle — the same process causing ocean acidification.
🏙️ St. Louis Connection
St. Louis sits where the Mississippi and Missouri Rivers meet. Our
industrial CO₂ emissions contribute to global ocean acidification. Local
freshwater mussels face similar threats as ocean pH changes affect river
chemistry. Scientists at UMSL and Washington University are monitoring
these changes now.
🧠 Step-by-Step Solution (All 5 Steps
Shown)
Step 1: Identify what you know
💭 "Let me write down the givens:
Starting pH = 8.25 (1800), Current pH = 8.10 (2020), Projected pH
= 7.95 (2100). The pH scale is logarithmic."
Step 2: Recall the pH-acidity relationship
💭 "Key fact: pH is logarithmic.
Every 0.1 pH decrease = 26% increase in acidity. The total change
from 1800 to 2020 is 8.25 - 8.10 = 0.15 units."
Step 3: Calculate the acidity change
💭 "0.15 pH change ≈ 0.1 + 0.05 ≈
26% + 13% ≈ 40% more acidic since 1800. That's
significant! By 2100 (pH 7.95), it could be 50%+ more acidic."
Step 4: Compare to organism thresholds
💭 "Thresholds: Coral (8.0) →
Oysters (7.9) → Pteropods (7.8). At current pH 8.10, all organisms
are OK but stressed. At pH 7.95 (2100), CORAL fails. At 7.85,
OYSTERS fail. At 7.75, PTEROPODS fail."
Step 5: State the answer and check
💭 "Answer: Ocean is ~40% more
acidic now than in 1800. By 2100, corals will struggle first
(below pH 8.0). ✓ This makes sense
because lower pH = harder to build shells/skeletons."
💡 Fading Support:
This week shows all 5 steps. By Week 2, steps 4-5 will be YOUR turn.
By Week 3, you'll do most steps independently! This builds your
problem-solving stamina.
Connect Cycle 3 CO₂ concepts to ocean chemistry changes.
🐚 The Challenge
What You'll Do (~10 minutes)
Observe the phenomenon: shells dissolving in modern oceans (2 min)
Connect to Cycle 3: What does CO₂ do in water? (3 min)
Make predictions about ocean chemistry changes (3 min)
Answer diagnostic questions (2 min)
🏠 COMPLETING THIS AT
HOME?
Think about: What gas is increasing in the atmosphere? (Cycle 3)
What happens when gases dissolve in water? What do acids do to
calcium carbonate (shells)?
▼ 📗 Need extra support? Click for
hints ▼
Key Concept Reminder:
CO₂ + H₂O → H₂CO₃ (carbonic acid)
More acid = lower pH = shells dissolve faster
This is the same CO₂ from Cycle 3 greenhouse effect!
▼ 🆘 Stuck? Click here for step-by-step CER help ▼
Try these steps in order:
Review Cycle 3: CO₂ is a greenhouse gas that traps heat
CO₂ also dissolves in ocean water
When CO₂ dissolves, it forms carbonic acid (H₂CO₃)
Acid attacks shells made of calcium carbonate (CaCO₃)
Result: Shells dissolve while animals are still alive!
📝 COMPLETE THE HOOK FORM
BELOW
Submit your predictions before moving to Station 1.
📝 Complete Your Worksheet — Click to
Expand
Complete the "AFTER HOOK FORM" section on your worksheet:
Write what you learned in the "I learned that..." box
Review your initial thinking about dissolving shells
📊Diagram: Carbon moves between the atmosphere, ocean, and living things in a continuous cycle.
👇📖 Worked Example 👇
👇⚠️Common Mistake — Read Before Solving
👇
✗ WRONG:
"WRONG:
"WRONG:
"Ocean acidification means the ocean is becoming acidic (pH below
7)."""
✓ RIGHT:"RIGHT:
"RIGHT:
"Ocean acidification means the ocean is becoming LESS basic. The
pH is dropping from 8.2 toward 8.1, but it's still above 7.
'Acidification' means moving TOWARD acidity, not becoming
acidic."""
Step-by-Step Problem Solving
📋 Problem Scenario
A coastal monitoring station records an average ocean pH of 8.2
in 1980 and 8.1 today. A classmate says, “The ocean is now acidic
because its pH decreased.” Decide whether that claim is accurate.
Use the pH scale to explain what changed, what did not change, and
why a decrease of 0.1 pH unit can still matter to shell-building
organisms.
"Why do you think a small pH change (like 0.1) can have such a big
effect on marine life?" Share your thinking with evidence from the
simulation or data tables.
🧭 Interactive Simulation: PhET pH Scale
Use this simulation to compare the pH of different liquids, including
carbonated drinks!
Instructions: Select "Macro" tab → Click "Molecules"
to see H₃O⁺ ions → Compare "Soda Pop" (pH ~2.5, carbonated with CO₂)
to "Milk" (pH ~6.5) → Notice how many more H₃O⁺ molecules appear in
the acidic soda!
Key Insight: Soda Pop is acidic (pH 2.5) because CO₂
dissolved in water forms carbonic acid (H₂CO₃). This is the same
chemistry that makes oceans more acidic when they absorb atmospheric
CO₂!
Can't access the simulation? Use the pH data tables above to answer all
questions.
📖 Worked Example: How to Analyze pH
Data
Full Scaffolding
Example Question: If ocean pH was 8.20 in 1950 and is
8.10 in 2020, how many times more acidic is the ocean now?
Expert Thinking Process (5 Steps):
Step 1: Identify what you know
💭 "Let me write down the givens:
Starting pH = 8.20, Ending pH = 8.10, Change = 8.20 - 8.10 = 0.10
units"
Step 2: Recall the pH formula/relationship
💭 "I remember that pH is
logarithmic. The key fact: 0.1 pH change = 26% change in acidity.
Since pH went DOWN, acidity went UP."
Step 3: Apply the relationship
💭 "Change is exactly 0.1 units,
so 26% more acidic = acidity increased by factor of 1.26"
Step 4: State the answer clearly
💭 "Answer: The ocean is 26% more
acidic now than in 1950 (1.26 times as acidic)."
Step 5: Check if it makes sense
💭 "pH decreased (8.20 → 8.10), so
acidity should increase. 26% is significant but not extreme.
✓"
✏️ Self-Explanation Prompt:
Why does pH decrease when CO₂ increases? Explain in your own words
before continuing.
📗 Need extra support? Click here for
hints and sentence starters
Key Concept Reminder:
pH going DOWN = becoming MORE acidic
Logarithmic means small changes in pH = BIG changes in acidity
0.1 pH change = 26% more acidic
Sentence Starters:
"Even though 0.1 seems small, it actually means..."
"For marine organisms, this is significant because..."
"At pH 7.95, all three organisms would be affected because..."