Phase Changes and Equilibrium fluid dynamics: TEAS Practice Question with Answer & Explanation

Understanding Phase Equilibrium in a Closed System

When you encounter questions about phase changes on the TEAS, particularly regarding closed systems, the core concept is Dynamic Equilibrium. Even if a container looks still to the naked eye, molecules are moving at incredibly high speeds at the microscopic level.

Why Choice B is Correct

In a sealed container, water molecules with high kinetic energy escape the surface of the liquid to become water vapor (evaporation). Simultaneously, water vapor molecules strike the surface of the liquid and are recaptured (condensation).

• Initially, the rate of evaporation is higher than the rate of condensation.
• As the concentration of vapor molecules increases, the rate of condensation increases.
• Eventually, the rate of evaporation equals the rate of condensation. This state is called dynamic equilibrium. At this point, the total amount of liquid and the total amount of gas remain constant, even though individual molecules are constantly switching places.

Analysis of Incorrect Options

• Choice A: This describes a transition in an open container. In a sealed container, the air space becomes saturated, preventing the liquid from disappearing entirely.
• Choice C: Vapor pressure does not rise indefinitely. Once equilibrium is reached, the vapor pressure remains constant (known as the equilibrium vapor pressure) as long as the temperature is unchanged.
• Choice D: Condensation never "stops." In a dynamic system, both evaporation and condensation occur continuously. Choice D ignores the "dynamic" nature of the equilibrium.

TEAS Concept: Kinetic Molecular Theory

The TEAS exam expects you to understand how matter behaves under different conditions. The kinetic molecular theory states that particles are in constant motion. In a liquid, intermolecular forces keep particles close together, but some particles always have enough energy to break free into a gas. In a closed system, these particles cannot escape to the atmosphere, forcing the "returning" process (condensation) to eventually balance out the "leaving" process (evaporation).

Key Terms to Remember

• Evaporation: Liquid turning into gas.
• Condensation: Gas turning into liquid.
• Saturated: When the air above a liquid holds the maximum amount of vapor possible at that temperature.
• Dynamic Equilibrium: A state where two opposing processes occur at the exact same rate, resulting in no net change.

Practice Tip

If a TEAS question mentions a "closed container" or "sealed system," look for answers that involve balance, equality of rates, or "equilibrium." If the container is "open," look for answers involving total evaporation or loss of mass.