A pure substance is defined as a substance with a fixed chemical composition, such as water, nitrogen, helium, and carbon dioxide. Even a mixture of various chemical elements can be considered a pure substance if it's homogeneous and has uniform concentration, like air. Pure substances can exist as elements or compounds, while mixtures can be homogeneous (uniform concentration) or heterogeneous (different phases). The video illustrates the molecular characteristics of solid, liquid, and gaseous phases, highlighting how intermolecular bonds vary in strength across these states. Solids have molecules in fixed positions, liquids allow molecules to move past each other, and gases have molecules moving randomly with weak intermolecular bonds.

The video explains the process of phase change using water as an example. When heat is added to liquid water at a constant pressure, its temperature rises until it reaches the saturation temperature (e.g., 100°C at 1 atmosphere), where it becomes a saturated liquid. Further heating at this constant temperature leads to a saturated liquid-vapor mixture as vaporization occurs. Once all the liquid turns into vapor, it becomes saturated vapor. Continuous heating beyond this point results in superheated vapor, where the temperature increases again. This process is illustrated on a temperature-specific volume (T-v) diagram, showing the compressed liquid, saturated liquid, saturated liquid-vapor mixture, saturated vapor, and superheated vapor regions.

Important terms related to phase change are introduced: saturation temperature is the temperature at which a pure substance changes phase, and saturation pressure is the pressure at which it changes phase. These are interdependent; as saturation temperature increases, saturation pressure also increases. The video also defines latent heat of fusion as the energy absorbed during melting (e.g., 333.7 kJ/kg for water at 0°C) and latent heat of vaporization as the energy required for vaporization or released during condensation (e.g., 2257 kJ/kg for water at 100°C). These concepts are crucial for understanding the energy changes involved in phase transitions.

The video discusses various property diagrams used to visualize the states and phase changes of pure substances. The T-v diagram shows temperature versus specific volume at constant pressure, illustrating different regions like compressed liquid, saturated liquid-vapor mixture, and superheated vapor. The P-v diagram shows pressure versus specific volume at constant temperature, while the P-T diagram shows pressure versus temperature at constant specific volume, including the sublimation line (solid to vapor) and the triple point where solid, liquid, and vapor phases coexist. The critical point, where saturated liquid and saturated vapor become identical, is also highlighted. Enthalpy (H = U + PV) is introduced as the heat content of a system, consisting of internal energy (U) plus the product of pressure (P) and volume (V).

The video explains how to use steam tables (also referred to as water tables or international steam tables) to find properties like specific volume, internal energy, enthalpy, and entropy for saturated liquid, saturated vapor, and mixtures. It demonstrates how to interpret values like Vf (specific volume of saturated liquid), Vg (specific volume of saturated vapor), and Vfg (difference between Vg and Vf). The concept of 'quality' (x) is introduced for saturated liquid-vapor mixtures, defining it as the mass of vapor divided by the total mass of the mixture. Quality ranges from 0 (saturated liquid) to 1 (saturated vapor), and it's used to calculate properties like specific volume, internal energy, and enthalpy for mixtures.

Several example problems are solved to illustrate the application of thermodynamic tables. The first problem determines the pressure and volume of a tank containing saturated liquid water at a given temperature, using the saturated water temperature table (Table A4). The second problem finds the temperature and mass of saturated water vapor inside a cylinder at a given pressure, utilizing the saturated water pressure table (Table A5). A third problem calculates the volume change and energy transferred during the complete vaporization of saturated liquid water at a constant pressure, again using thermodynamic tables to find specific volumes and enthalpy changes.

Another problem demonstrates how to analyze a rigid tank containing a mixture of liquid and vapor water. Given the total mass, temperature, and the mass of the liquid, the task is to determine the pressure in the tank and its total volume. This involves calculating the quality of the mixture and then using it along with the specific volumes of saturated liquid and saturated vapor from the tables to find the average specific volume and subsequently, the total volume. A similar problem involves calculating the quality, enthalpy, and volume occupied by the vapor for a refrigerant at a specific temperature and volume.

The video moves on to superheated vapor, defining it as water vapor not about to condense, with a temperature above the saturation temperature. For superheated vapor, pressure and temperature are no longer dependent properties, and a separate superheated water table (Table A6) is used. An example involves finding the internal energy of water at a given pressure and temperature, confirming it's superheated by comparing it to the saturation temperature. The concept of compressed liquid is also discussed, where the pressure is higher than the saturation pressure at a given temperature, or the temperature is lower than the saturation temperature at a given pressure. Table A7 for compressed liquid water is used, and it's noted that for most practical purposes, properties of compressed liquid can be approximated by those of saturated liquid at the same temperature due to small differences.

A comprehensive problem requires determining missing properties and phase descriptions for various water states (saturated liquid-vapor mixture, superheated vapor, and compressed liquid) using thermodynamic tables and interpolation techniques. This exercise reinforces the understanding of how to use the different tables (saturated water temperature table, saturated water pressure table, superheated water table, compressed liquid table) and the formulas for quality, average internal energy, and average enthalpy to characterize different thermodynamic states of water.