What's the difference between activity and concentration?
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Concentration is how much of a species is present; activity is its effective concentration — what the species actually "does" in a real solution. They're linked by the activity coefficient:
{i} = γi · [i]
In dilute solutions γi ≈ 1, so activity ≈ concentration. As ionic strength rises, ion–ion interactions lower γi below 1.
Lecture 7: Activity & Activity Coefficients
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Is this idea used anywhere outside our course?
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Beyond the lecture
Yes — in seawater and brine modeling, frameworks like the Pitzer equations extend activity corrections to very high ionic strengths, well beyond the dilute-solution methods we cover in this course.
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