Question

A system consists of liquid water in equilibrium with a gaseous mixture of air and water vapor. How many phases are present? Does the system consist of a pure substance? Explain. Repeat for a system consisting of ice and liquid water in equilibrium with a gaseous mixture of air and water vapor.

Short answer

First system: 2 phases, not pure. Second system: 3 phases, not pure.

Step-by-step solution

- Identify Phases in the First System

The first system consists of liquid water in equilibrium with a gaseous mixture of air and water vapor. In this case, there are two distinct phases: one liquid phase (liquid water) and one gas phase (mixture of air and water vapor).

- Determine If the First System Is a Pure Substance

A pure substance has a homogeneous and invariable chemical composition. Since the first system consists of a mixture of air (a combination of various gases) and water vapor along with liquid water, it does not qualify as a pure substance.

- Identify Phases in the Second System

The second system consists of ice and liquid water in equilibrium with a gaseous mixture of air and water vapor. In this case, there are three distinct phases: one solid phase (ice), one liquid phase (liquid water), and one gas phase (mixture of air and water vapor).

- Determine If the Second System Is a Pure Substance

Similar to the first system, the second system contains a mixture of air and water vapor, along with ice and liquid water. Thus, it also does not qualify as a pure substance due to the presence of the air mixture.

Key concepts

phase equilibrium

Phase equilibrium refers to the state where different phases of a substance coexist without changing over time. For example, when liquid water is in equilibrium with its vapor, the rate at which water molecules are evaporating equals the rate at which water vapor molecules condense back into the liquid. This balance maintains a steady state of the phases involved.

Phase equilibrium is crucial in understanding various natural processes and industrial applications. It helps in predicting how substances behave under different conditions of temperature and pressure. In our exercise, both systems have achieved phase equilibrium:

• First system: liquid water and a gaseous mixture of air and water vapor
• Second system: ice, liquid water, and a gaseous mixture of air and water vapor

Both systems demonstrate phase equilibrium by having their respective phases coexist without any net change over time.

pure substances

A pure substance is one that has a uniform and invariable chemical composition. It can exist in different phases, but its chemical makeup stays identical across these phases. Examples of pure substances include distilled water, pure oxygen, or gold.

In the context of the given exercise:

• The first system is not a pure substance because it involves a mixture of air (a combination of various gases) and water vapor along with liquid water.
• The second system also does not qualify as a pure substance. This system includes ice, liquid water, and a gaseous mixture of air and water vapor.

The critical factor here is the presence of air, a mixture of various gases, which disqualifies both systems from being considered pure substances.

thermodynamic systems

A thermodynamic system is a specific portion of matter under study, separated by boundaries from its surroundings. In analyzing such systems, we can better understand energy and mass exchanges due to physical and chemical processes occurring within.

There are three types of thermodynamic systems:

• Open System: Can exchange both energy and matter with its surroundings.
• Closed System: Can exchange energy but not matter with its surroundings.
• Isolated System: Does not exchange energy or matter with its surroundings.

In the exercise, the systems in question are more likely open systems. They involve exchanges of energy (through heat and work) and matter (air and water vapor) with their surroundings.

liquid-vapor equilibrium

Liquid-vapor equilibrium is a common type of phase equilibrium. It occurs when liquid and vapor phases of a substance coexist in a balanced state. An excellent everyday example is boiling water, where liquid water is in equilibrium with water vapor.

In our exercise, we see liquid-vapor equilibrium in both systems:

• In the first system, liquid water coexists with a gaseous mixture of air and water vapor.
• In the second system, ice and liquid water coexist alongside a gaseous mixture of air and water vapor.

In these scenarios, molecules are continually moving between the liquid and vapor phases. However, the overall amount of each phase does not change over time, demonstrating liquid-vapor equilibrium.