Concentration vs Activity

Quick Answer

Concentration is the literal amount of solute you put in. Activity is the effective concentration the ions actually behave like.

What Each One Really Means

In your introductory chemistry, we treat concentration and activity as if they are the same thing. And for dilute solutions, they nearly are. But once you enter physical chemistry and start dealing with concentrated ionic solutions, the difference becomes very real. Concentration is the straightforward number. If you dissolve 1 mole of NaCl in 1 litre of water, that is 1 molar. Simple. Activity is what the ions actually behave like. You see, in a concentrated solution, the ions are crowded together. Positive ions attract negative ions, and each ion is surrounded by a cloud of oppositely charged particles. This shielding makes each ion behave as if it is less concentrated than it actually is. So the effective concentration, which we call activity, drops below the theoretical concentration. The ratio between the two is the activity coefficient. Now here is the practical point. In equilibrium calculations for concentrated solutions, if you use concentration instead of activity, your answer will be mathematically off. Not small error. Real error. That is exactly why advanced textbooks switch to activities when concentrations get high. For dilute solutions, the activity coefficient is close to 1, so we can get away with using concentration. But know the difference. Examiners love testing this at university level.

Side-by-Side Comparison

Memory Device

Concentration is what you put in. Activity is what the ions actually feel.

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