Ocean Acidification Notes Before a Scope-Limited Quiz
You are a first-year environmental science student reviewing recorded lecture notes before a quiz. The lecture topic is ocean acidification. You wrote: 'Ocean pH has fallen by 0.1 units since 1850, which is a 30% increase in acidity. Pteropods dissolve when pH drops below 7.8. The professor showed a graph of shell mass declining by 12% in the high-CO2 tank.' A classmate's notes say: 'pH fell 0.1 = 10% more acidic; pteropods affected at pH 7.8; shell mass fell 20%.' You also remember the professor saying, 'The 12% figure is from the 2018 experiment, not the 2021 replication,' and a teaching assistant later announced that the quiz will only cover the 2021 replication, which found a 7% decline. You have 20 minutes before the quiz and cannot rewatch the full 50-minute lecture; you can rewind only two 30-second segments.
1. Which number would you use for shell mass decline on the quiz, and why? Explain how you would resolve the conflict between your notes and your classmate's.
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I would use 7%, the 2021 replication figure, because the teaching assistant defined the quiz scope. The 12% in my notes belongs to the 2018 experiment, which the professor explicitly distinguished, so it is real but out of scope. My classmate's 20% is the least supported: neither my notes nor the professor's distinction mentions it, and it may be a misheard version of 12% or 7%. I would not average 12% and 7%-that would invent a number no source reported. With 20 minutes left, I would spend two 30-second rewinds: one around the phrase '2021 replication' to confirm the 7%, and one around 'percent decline' to check whether the professor said 'seven' or 'seventeen.' If the rewinds conflict, the TA's written announcement outranks memory because it is the assessment scope. I would also write a note: '12% = 2018; 7% = 2021 replication; quiz covers 2021 only.' That resolves the conflict by source authority and scope rather than by majority vote.
2. Your classmate says, 'A 0.1 pH drop is only 10%, so acidification is minor.' How would you correct this using the lecture's definition, and what would you say about the 30% figure?
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I would correct the logarithmic misunderstanding first. pH is a negative log scale, so a drop from 8.2 to 8.1 is not a 10% change in acidity. The hydrogen-ion concentration rises by a factor of about 1.26, or roughly 26%, because 10^0.1 is about 1.26. A 10% figure would be right only if 'acidity' meant pH units directly, which it does not. On the 30% figure, I would be precise: my note says the professor used '30% increase in acidity' since 1850, but a 0.1-unit drop alone calculates to about 26%. The 30% could reflect a slightly larger pH change, rounding, or a different baseline; I would not call it exact without the lecture's definition. For the quiz, I would write 'about a 26% increase for a 0.1-unit drop, often reported as roughly 30% in the lecture' only if the lecture explicitly allowed the approximation. Otherwise I would use the calculated figure and flag the discrepancy.