Pure Substances

Definition

A pure substance is defined as a substance that has a fixed chemical composition throughout, meaning it is homogeneous and does not change its chemical makeup regardless of its physical state (solid, liquid, or gas). In thermodynamics, this applies even to mixtures of different chemical elements or compounds, provided the mixture is uniform.

Main Content

1. Thermodynamic Homogeneity

A substance is considered pure if it is chemically homogeneous and uniform.
Example: Liquid water, a mixture of liquid water and water vapor, or nitrogen gas are all pure substances because they have the same chemical formula ($H_2O$ or $N_2$) throughout.

2. The Phase Rule and State

A pure substance’s state is determined by two independent, intensive properties (such as pressure and temperature).
Example: If you know the pressure and temperature of steam, you can uniquely determine its specific volume, internal energy, and enthalpy.

3. Non-Pure Substances

Mixtures that do not maintain a uniform chemical composition are not pure substances.
Example: Air is generally considered a "pseudo-pure" substance in some engineering calculations, but liquid air mixed with gaseous air is not a pure substance because the composition of the liquid phase differs from the vapor phase.

Working / Process

1. Identifying the Phase

Check if the substance consists of a single chemical element or a stable compound.
Verify that the mixture remains uniform throughout the process; for instance, a mixture of ice and water is a pure substance, but a mixture of oil and water is not.

2. State Determination

Define two independent intensive properties, such as Temperature ($T$) and Pressure ($P$).
Use these properties to locate the state on a phase diagram, such as the $T-v$ (Temperature-specific volume) diagram.

Temperature (T)
|          / (Superheated Vapor)
|         /
|   _____/ (Saturated Vapor line)
|  /    /
| /____/ (Saturation Dome)
|/ (Compressed Liquid)
|__________________________
        Specific Volume (v)

(Diagram: Temperature vs. Specific Volume for a pure substance like water)

3. Thermodynamic Analysis

Apply steam tables based on the identified state to find enthalpy ($h$), entropy ($s$), and internal energy ($u$).
If the substance is in the saturation region (wet steam), calculate the quality ($x$) to determine the ratio of vapor to liquid.

Advantages / Applications

Fundamental to the design of steam power plants, where water undergoes phase changes to drive turbines.
Essential for refrigeration and HVAC systems, as the working fluids (refrigerants) act as pure substances during compression and expansion.
Simplifies complex engineering calculations by allowing the use of standardized steam tables rather than experimental testing for every condition.

Summary

A pure substance is a matter with a fixed chemical composition that remains uniform regardless of its phase. It serves as the foundation for analyzing thermodynamic cycles, particularly in steam power engineering. Engineers rely on the predictable relationships between temperature, pressure, and volume for these substances to optimize energy conversion processes.

Important terms to remember: - Saturation: The point where a phase change begins or ends. - Quality ($x$): The mass fraction of vapor in a mixture of liquid and vapor. - Steam Tables: Data sets providing properties of water/steam at various states.