Question

A dish of liquid water is placed on a table in a room. After a while, all of the water evaporates. Taking the water and the air in the room to be a closed system, can the system be regarded as a pure substance during the process? After the process is completed? Discuss.

Short answer

The system cannot be regarded as a pure substance during or after the process.

Step-by-step solution

Define the System

Consider the initial state where the dish contains liquid water and the room contains air. Define the 'closed system' which includes both the water and the air.

Understand the Concept of a Pure Substance

A pure substance is one that has a uniform and consistent chemical composition throughout.

Analyze the Evaporation Process

During the evaporation process, water molecules transition from the liquid phase to the vapor phase and mix with the air molecules in the room.

Determine if the System is a Pure Substance During the Process

During the process, the composition of the system is changing. The air has nitrogen, oxygen, and now water vapor. Hence, the system cannot be regarded as a pure substance.

Determine if the System is a Pure Substance After the Process

After all the water has evaporated, the water vapor is mixed uniformly with the air. The system is a mixture of different gases and thus, it cannot be regarded as a pure substance.

Key concepts

closed system

In thermodynamics, a 'closed system' is one where no matter is exchanged with the surrounding environment, but energy in the form of heat or work can be transferred. In the exercise, the closed system includes both the air in the room and the water in the dish. This means the total mass of water and air remains constant throughout the process, even though the water changes phase from liquid to vapor. This property helps us understand the behavior of the system and analyze the processes occurring within it.

evaporation process

The 'evaporation process' involves the transformation of water from its liquid state to a gas. This happens when water molecules at the surface gain enough energy to enter the vapor phase and turn into water vapor. In the given problem, the dish of water gradually evaporates into the air in the room. During evaporation, the room's air starts to contain more water vapor. Important points to remember about evaporation:

• The rate of evaporation increases with temperature.
• Evaporation is a surface phenomenon; it occurs at the surface of the liquid.
• It's an endothermic process, meaning it absorbs heat from the surroundings.

mixture of gases

A 'mixture of gases' occurs when different gases are present together but do not chemically react with each other. After the water evaporates in the given problem, the room's air consists of nitrogen, oxygen, and water vapor. Since these gases are mixed but not chemically bonded:

• The system's composition is no longer uniform.
• Partial pressures of gases add up to make the total pressure (Dalton's Law).
• The mixture is not a pure substance because it doesn't have a consistent chemical composition.

chemistry in thermodynamics

Understanding 'chemistry in thermodynamics' involves studying how chemical processes are affected by energy transfers and phase changes. In the exercise, the chemical composition of the system changes during evaporation. This process can be better understood using the principles of thermodynamics:

• In a closed system, the amount of matter remains constant.
• Phase changes, like evaporation, involve energy changes (latent heat of vaporization).
• Mixtures formed, like the air with water vapor, illustrate how chemical behavior impacts thermodynamic properties (e.g., changes in total pressure and volume).