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Concept Explaination:

In this story, two important equilibrium concepts are explained.

A common misconception is that liquids turn into gas only when they boil. In fact, liquid is always at equilibrium with its gaseous form: molecules of liquid move from the liquid phase into the gas above it and vice versa. The concentration of vapors above the liquids depends on the volatility of the liquid. In this story the potion evaporates before reaching boiling and when it reaches high enough concentration in the air, the Princess falls asleep.

The second concept in the story is the disruption of an equilibrium state by a change in the system. In this story, the Princess was asleep as long as the concentration of sleeping potion in the air was high. Because she was in a closed room - an isolated system - the concentration of potion in the air didn't change over time. When Tian entered the room and opened a window, he disrupted the equilibrium between the room and the outside. The room is no longer an isolated system. Since the concentration of potion inside the room was higher than the concentration outside, molecules of potion will redistribute between the room and outside. This effectively lowers the concentration of potion inside the room and the Princess wakes up.

This is usually referred to as the Le Chatelier's Principle. If the equilibrium of a system is disrupted, then the system will shift to accommodate the change and restore equilibrium.

Now think: Can you explain how a puddle of water will slowly evaporate even at room temperature? How will wind affect the process?

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