Pure Substances

A substance is any form of matter that has a definite chemical composition and identifiable thermodynamic properties.

Definition

A substance that has a fixed chemical composition throughout its mass. A mixture of 2 or more phases of a pure substance is also a pure substance. Homogenous mixture of 2 or more chemical substances is also a pure substance.

Phases

There are 3 principle phases:

• Solid
• Liquid
• Gas

There can be multiple phases available within a principle phase.

Solid

Molecules are arranged in 3-dimensional pattern; stay at fixed positions; cannot move relative to each another; oscillate continually. Molecular attraction is strong.

Liquid

Molecules rotate and translate freely. Intermolecular forces are weaker relative to solids.

Gas

Molecules move at random; have higher energy level compared to the other 2. Intermolecular distances are high. Intermolecular forces are very weak.

Phase Transition

Happens due to changes in temperature and/or pressure. In a phase transition, temperature of the substance remains constant. Because the substance absorbs or releases energy as latent heat. The amount of energy required for a phase change depends on the substance and the specific process and its pressure.

Melting

Aka. fusion. Transition from solid to liquid when heat is added at constant pressure.

Freezing

Transition from liquid to solid when heat is removed.

Vaporization

Aka. boiling or evaporation. Transition from liquid to gas when heat is added.

Condensation

Transition from gas to liquid when heat is removed.

Sublimation

Direct transition from solid to gas without passing through the liquid phase.

Deposition

Direct transition from gas to solid.

Saturation Temperature

At a given pressure, the temperature at which a pure substance changes phase. Denoted by $T_\text{sat}$. $$

Saturation Pressure

At a given temperature, the pressure at which a pure substance changes phase. Denoted by $P_\text{sat}$. $$

Terminology

Subcooled Liquid

Aka. compressed liquid. Only liquid present. Behaves almost incompressibly.

Condition: $T < T_\text{sat}(P)$.

Saturated Liquid

A liquid just ready to vaporize. Any heat addition initiates phase change.

Condition: $T = T_\text{sat}(P)$.

Specific enthalpy and specific entropy of saturated liquid are denoted by $h_f$ and $s_f$ respectively.

Liquid–Vapour Mixture

Aka. boiling region. Liquid and vapour coexist in equilibrium. Occurs when heat is added at constant pressure and the temperature remains constant.

Temperature stays at $T_\text{sat}(P)$ during phase change.

Dryness Fraction

Aka. quality factor. Defines vapour proportion:

$$X = \frac{\text{mass of vapour}}{\text{total mass}} \quad \in (0, 1)$$

$X=0$ means it’s saturated liquid. $X=1$ means it’s saturated vapour.

Specific enthalpy, or specific volume, or specific entropy of the mixture can be found using:

$$h = h_f + X \, h_{fg}$$

Saturated Vapour

A vapour just ready to condense if heat is removed. No liquid remains; any cooling starts condensation.

Condition: $T = T_\text{sat}(P)$ and $x = 1$.

Specific enthalpy and specific entropy of saturated liquid are denoted by $h_g$ and $s_g$ respectively.

Superheated Vapour

A vapour heated beyond the saturated vapour state. Temperature rises with heat addition (unlike in boiling region). No liquid present.

Condition: $T > T_\text{sat}(P)$.

At high superheat, behaves more like an ideal gas.

Supercritical Fluid

No distinction between liquid and vapour phases.

Condition: $T > T_c$ and $P > P_c$.

Property tables

Thermodynamic properties are often represented in the form of tables called property tables. Because they are too complex to be expressed by simple equations.

Includes values for specific volume, pressure, temperature, specific enthalpy and specific entropy.

Saturated Water Table

Used when the substance is at the saturation condition. Include values for $T$, $P$, $v_f$, $v_g$, $h_f$, $h_g$, $h_{fg}$, $s_f$, $s_g$.

• $h_f, s_f$ - properties of a saturated liquid
• $h_g, s_g$ - properties of saturated vapor
• $h_{fg}, s_{fg}$ - difference between the saturated vapor and saturated liquid value

Liquid-vapour mixture properties can be found using quality factor $X$.

Superheated Vapour Table

Used when vapour temperature is above the saturation temperature at given pressure. Organised by pressure, then by temperature. Includes $v$, $h$, $s$.

Subcooled Liquid Table

Used when liquid temperature is below the saturation temperature at given pressure. Often sparse; liquid properties change slowly with pressure.

Approximated by saturated liquid values at same temperature: $h(T,P) \approx h_f(T)$.

Table for Refrigerants and Other Fluids

Refrigerants and gases have similar formats. Saturated and superheated sections. Used in refrigeration and heat pump calculations.