Year 10 Science · Unit 2 · Lesson 2
Challenge Worksheet
Learning Goals
What if…?
Scenario
Scientists monitoring the Great Barrier Reef have recorded that average ocean pH has fallen from 8.2 to approximately 8.1 since pre-industrial times (1850s), and models predict a further fall to 7.9 by 2100 if CO₂ emissions continue. The pH scale is logarithmic, each unit change represents a 10-fold change in H⁺ ion concentration. Reef-building corals, oysters farmed in Port Stephens, and pteropod sea snails (tiny animals at the base of Antarctic food chains) all rely on carbonate ions (CO₃²⁻) to build their calcium carbonate (CaCO₃) shells and skeletons.
(a) Explain the chemical reactions that occur when CO₂ dissolves in seawater. Name the acid formed and describe how H⁺ ions are released. Use correct chemical terminology.
(b) A drop from pH 8.2 to 7.9 represents a 0.3 unit change. Using the logarithmic nature of the pH scale, explain why this small number actually represents approximately a 100% increase in ocean acidity. Show your reasoning clearly.
(c) Explain which organisms (corals, oysters, or pteropods) are most threatened by ocean acidification and why. In your answer, connect the chemistry of lower pH to the availability of carbonate ions for shell-building.
1. The ocean is alkaline (pH ~8.1), yet scientists describe increasing CO₂ absorption as "acidification." Is it accurate to call this process acidification? Justify your answer using the definition of an acid and the direction of pH change.
2. Australian oyster farmers in Port Stephens and Coffin Bay have reported that oyster spat (baby oysters) struggle to form shells in years when ocean pH is lower. Using your knowledge of acids, bases, and carbonate chemistry, propose one chemical intervention that could protect their oyster-growing tanks. Explain the chemistry behind your solution.
Wrap Up
In one sentence, what was the main idea of this lesson?