Interpret equilibrium

Supplied materials

• All measurements and observations below are supplied classroom examples. They are not data from actual learners or a claim about a particular real site.
• Paper or a browser document for a labeled drawing, table or explanation.
• Worked problem inputs: A membrane remains at unequal concentrations because a pump uses energy. Explain why observing a stable difference does not show equilibrium.
• Assessment case 1: At equal solute concentration, ten modeled particles move each way per interval. What follows? Which inference or prediction is best supported?
• Assessment case 2: Which scientific label fits the specifically described step or feature? Supplied case: Entry exceeds exit.
• Assessment case 3: Which scientific label fits the specifically described step or feature? Supplied case: Exit exceeds entry.
• Assessment case 4: Which scientific label fits the specifically described step or feature? Supplied case: Pump sustains unequal concentrations.
• Assessment case 5: Which scientific label fits the specifically described step or feature? Supplied case: No net change despite microscopic motion.
• Assessment case 6: A pump consumes energy to maintain unequal concentrations and steady counts. What is warranted? Which inference or prediction is best supported?

Learn the idea

At dynamic equilibrium, opposing microscopic movements can balance so there is no net change. It does not mean every particle stops. A cell may maintain gradients using ongoing processes.

Dynamic equilibrium means opposing flows balance on average. Net influx means more enters than leaves. Net efflux means more leaves than enters. Maintained gradient means ongoing processes preserve a difference.

For the supplied mechanism problem, A membrane remains at unequal concentrations because a pump uses energy. Explain why observing a stable difference does not show equilibrium. Ongoing active transport can maintain a gradient. Stable concentrations may be a steady state with continuing flows, not passive dynamic equilibrium at equal concentration.

1. Read the supplied problem and mark the structures, processes or quantities named: A membrane remains at unequal concentrations because a pump uses energy. Explain why observing a stable difference does not show equilibrium.
2. Draw the relevant boundary, pathway or comparison and label it using the scientific ideas above.
3. Write a causal explanation or calculation, showing how each observation supports the result.
4. Compare your reasoning with the worked solution; name a condition that would change or limit the inference.

Check your reasoning

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Take it further

New equilibrium model: equal concentrations persist with equal average flows in both directions. Expected: dynamic equilibrium, not motionless particles; draw opposing equal flow arrows.

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