Phase Changes and Properties of Pure Substances in General Chemistry

Properties of Pure Substances

Definition and Examples

A pure substance is a material with a fixed chemical composition throughout. Examples include water, helium, and carbon dioxide. A mixture of phases (such as ice and water) can still be considered a pure substance if it is homogeneous in composition.

Phases of a Pure Substance

Pure substances exist in three principal phases: solid, liquid, and gas. Each phase has distinct molecular characteristics:

• Solids: Strongest molecular bonds, closely packed three-dimensional crystals, molecules do not move relative to each other.

• Liquids: Intermediate bond strength, molecules can float in groups, some molecular order within groups.

• Gases: Weakest bond strength, molecules are far apart with no ordered structure, move randomly and collide, higher energy levels.

Phase Change Processes

Phase Change in Water (Heating at Constant Pressure)

When water is heated at constant pressure (e.g., 1 atm), it undergoes several phase changes:

1. Compressed Liquid (Subcooled Liquid): Water below boiling point, not about to vaporize.

2. Saturated Liquid: Water at boiling point (100°C at 1 atm), ready to vaporize.

3. Saturated Liquid-Vapor Mixture: Both liquid and vapor coexist as water boils.

4. Saturated Vapor: All water has vaporized at boiling point.

5. Superheated Vapor: Vapor heated above boiling point; temperature increases with added heat.

Phase Change Diagram (T-v Diagram)

The T-v diagram (Temperature vs. Specific Volume) visually represents phase changes at constant pressure. Key points include:

• Compressed liquid region

• Saturated liquid line

• Saturated vapor line

• Superheated vapor region

• Critical point (where saturated liquid and vapor states are identical)

Phase Change Properties and Diagrams

Saturation Temperature and Pressure

Saturation temperature ($T_{sat}$): The temperature at which a pure substance changes phase at a given pressure. Saturation pressure ($P_{sat}$): The pressure at which a pure substance changes phase at a given temperature.

Critical Point

The critical point is where the properties of saturated liquid and vapor become identical. At this point, the substance cannot exist as a distinct liquid or vapor.

Substance	Pcr (MPa)	Tcr (°C)	Tcr (K)	vcr (m³/kg)
Water	22.09	374.145	647.298	0.003155
Air	3.77	132.5	405.65	0.0883

T-v and P-v Diagrams

T-v diagrams plot temperature vs. specific volume, showing phase regions and the critical point. P-v diagrams plot pressure vs. specific volume, and can be extended to include solid phases and triple points.

Triple Point

The triple point is the unique condition where solid, liquid, and vapor phases coexist in equilibrium. For water: $T = 0.01^ ext{o}C = 273.16 ext{ K}$ and $P = 0.6113 ext{ kPa}$.

P-T Diagram (Phase Diagram)

The P-T diagram (Pressure vs. Temperature) shows the regions of stability for each phase and the lines of equilibrium between phases. The triple point is represented as a single point where all three phases meet.

Phase Change Processes

• Solid → vapor: Sublimation

• Solid → liquid: Melting

• Liquid → vapor: Vaporization

Property Tables and Thermodynamic Properties

Key Properties

• Specific internal energy ($u$)

• Specific enthalpy ($h$)

• Specific entropy ($s$)

Enthalpy per unit mass is defined as:

$h = u + Pv$

where $u$ is internal energy, $P$ is pressure, and $v$ is specific volume.

Enthalpy of Vaporization

The enthalpy of vaporization ($h_{fg}$) is the energy required to vaporize a unit mass of saturated liquid at a given temperature or pressure:

• $u_{fg} = u_g - u_f$

• $h_{fg} = h_g - h_f$

• $s_{fg} = s_g - s_f$

Quality and Mixtures

Quality (x)

Quality ($x$) is the ratio of the mass of saturated vapor to the total mass in a saturated liquid-vapor mixture:

$x = \frac{m_g}{m}$

where $m_g$ is the mass of vapor and $m$ is the total mass. Quality ranges from 0 (all liquid) to 1 (all vapor).

Average Specific Volume in Mixtures

For a mixture of saturated liquid and vapor:

• Total volume: $V = V_f + V_g$

• Total mass: $m = m_f + m_g$

• Specific volume: $v = \frac{V}{m}$

• $v = (1-x)v_f + x v_g$

Summary Table: Phase Change Terms

Term	Definition
Compressed Liquid	Liquid not about to vaporize
Saturated Liquid	Liquid about to vaporize
Saturated Vapor	Vapor about to condense
Superheated Vapor	Vapor not about to condense
Critical Point	Point where liquid and vapor properties are identical
Triple Point	Point where solid, liquid, and vapor coexist
Quality (x)	Fraction of mass that is vapor in a mixture

Applications and Examples

• Understanding phase diagrams is essential for predicting phase changes in pure substances under varying temperature and pressure.

• Critical point and triple point data are used in chemical engineering and physical chemistry to design processes involving phase transitions.

• Property tables are used to calculate energy changes, work, and entropy in thermodynamic cycles.

Example: Water

• Critical point: $P_{cr} = 22.09$ MPa, $T_{cr} = 374.145^ ext{o}C$

• Triple point: $T = 0.01^ ext{o}C$, $P = 0.6113$ kPa

Additional info: These concepts are foundational for understanding phase equilibria, thermodynamic cycles, and the behavior of substances under different conditions in general chemistry and physical chemistry.