Question

which of these examples represents a system at equilibrium? three fish swim under a plant and five other fish come out from the plant. in a sealed flask, 500 water molecules vaporize while 500 other water molecules condense. a chef puts two pizzas into the oven and takes five pizzas out.

Explanation:

A system at equilibrium has equal rates of forward and reverse processes (or equal input and output, in a broader sense, with no net change).

• For the first option: 3 fish go under the plant and 5 come out – the numbers are unequal, so no equilibrium.
• For the second option: In a sealed flask, 500 water molecules vaporize (liquid to gas) and 500 condense (gas to liquid). The rates of vaporization and condensation are equal, so the amount of liquid and gas water remains constant – this is equilibrium.
• For the third option: 2 pizzas go into the oven, 5 come out – unequal numbers, so no equilibrium.

Answer:

B. In a sealed flask, 500 water molecules vaporize while 500 other water molecules condense.