TERMODINÁMICA. CAMBIOS de FASE-ESTADOS de la MATERIA. SUSTANCIAS PURAS. [AL ENTRAR APRENDERÁS TODO]

Understanding Phase Changes in Pure Substances

Introduction to Phase Changes

• The discussion begins with an overview of phase changes in pure substances, specifically focusing on water as a common example.

• It is explained that solid phases have small intermolecular distances but strong intermolecular forces, while liquid phases maintain similar distances but with weaker forces.

Energy and Molecular States

• In the gaseous state, large intermolecular distances exist alongside weak forces, allowing particles to move freely and possess higher kinetic energy.

• The molecular energy hierarchy is established: gas > liquid > solid, indicating that gas has the highest molecular energy.

Processes of Phase Transition

• Key processes are defined:

• Fusion: Solid to liquid (e.g., melting ice).

• Boiling: Liquid to gas (e.g., water boiling).

• Solidification: Gas to solid (e.g., condensation).

Reverse Processes Explained

• The reverse processes include:

• Solidification: Liquid to solid (e.g., freezing water).

• Condensation: Gas to liquid (e.g., dew formation).

• Sublimation: Solid directly to gas without becoming liquid (e.g., dry ice).

Heat Transfer During Phase Changes

• The concept of latent heat is introduced as the amount of energy absorbed or released during these phase transitions.

Analyzing Water's Phase Changes at Standard Conditions

Isobaric Process Overview

• The analysis focuses on water at room temperature and standard atmospheric pressure, defining it as an isobaric process where pressure remains constant throughout phase changes.

Initial State of Water

• At the start, water exists in a compressed liquid state at 25°C, which is below its boiling point of 100°C under one atmosphere.

Heating the Compressed Liquid

• As heat is applied, the temperature rises until it reaches boiling point; however, no vapor bubbles appear yet. This state is termed "liquid saturated."

Transitioning from Liquid Saturation to Vapor

Achieving Boiling Point

• Upon reaching 100°C without any vapor formation indicates that all substance remains in a liquid state despite being at boiling temperature.

Formation of Vapor Bubbles

• When heating continues past this point, vapor bubbles begin forming. This mixed state contains both vapor and liquid phases simultaneously and is referred to as "mixed saturation."

Understanding Mixed Saturation State

Characteristics of Mixed Saturation

• In this mixed saturation state:

• Temperature remains constant at 100°C.

• Energy supplied goes into breaking intermolecular forces rather than increasing temperature.

Composition Dynamics

• Initially composed entirely of liquid (100% liquid), as more heat is added:

• A gradual transition occurs where percentages shift towards more vapor content while reducing liquid content.

Final Stages Towards Complete Vaporization

Progression Towards Vapor State

• As heating continues:

• Percentages can reach states like 75% vapor and 25% liquid before achieving complete vaporization (100% vapor).

Understanding Saturated States in Thermodynamics

Key Concepts of Saturation

• The state of saturated liquid is defined when the last drop of liquid disappears, maintaining a saturation temperature of 100 degrees Celsius. This state is denoted with an index 'f' for properties like specific volume, internal energy, enthalpy, and entropy.

• In contrast, the saturated vapor state is identified with the subscript 'g', representing similar properties (specific volume, internal energy, enthalpy, and entropy) but at a higher value than those in the saturated liquid state.

• As heat is added during phase changes from liquid to vapor, all these properties (specific volume, internal energy, enthalpy, and entropy) increase throughout the process.

Phases of Water and Their Characteristics

• The final state in a mixture of saturated phases is termed as saturated vapor (100% vapor), while the completely liquid phase (0% vapor) is referred to as saturated liquid.

• During mixing states where both liquid and vapor coexist at constant temperature (100 degrees Celsius), any supplied heat goes into breaking intermolecular forces rather than increasing temperature.

Effects of Heat on Vapor States

• When only vapor exists and additional heat is supplied beyond this point (state 4), the temperature increases past 100 degrees Celsius leading to a condition known as superheated vapor (state 5).

• Each pure substance has its unique saturation temperature and pressure based on its chemical composition. For example:

• State 1: Compressed Liquid

• State 2: Saturated Liquid

• State 3: Saturated Mixture

• State 4: Saturated Vapor

• State 5: Superheated Vapor

Temperature Behavior Across Different States

• In extreme states such as compressed liquids or superheated vapors, temperatures rise significantly when heat is applied. However, during mixed states where both phases exist simultaneously, temperatures remain constant due to energy being used for phase change instead.

• The saturation temperature remains fixed at around 100 degrees Celsius under standard atmospheric pressure until boiling occurs; beyond that point in superheated conditions (e.g., above temperatures like 150°C or more), it can exceed this threshold.

Understanding Saturation Pressure Relationships

• The term "temperature of saturation" refers to the constant temperature maintained during phase mixing at which boiling occurs. Similarly, "pressure of saturation" indicates the corresponding pressure level at which this phenomenon takes place.

• If the system's temperature falls below saturation levels (<100°C), it indicates a compressed liquid state; conversely if it exceeds saturation (>100°C), it signifies superheated steam conditions.

• It's crucial to note that saturation temperatures are dependent on their respective pressures; variations in ambient pressure will alter boiling points—higher altitudes lead to lower boiling points due to decreased atmospheric pressure.

Thermodynamic Properties of Water

Understanding Saturated States

• A thermodynamic table for water is displayed, focusing on the saturated state, which indicates a mixture of liquid and vapor. Different saturation pressures correspond to various temperatures; for instance, at 20°C, the saturation pressure is 2.33 kPa.

• The subscript "f" denotes saturated liquid properties in the table, including specific volume, internal energy, enthalpy, and entropy when fully liquid (100% liquid).

• The letter "g" identifies saturated vapor states where there is 100% vapor present. At this point, no liquid remains.

Key Thermodynamic Concepts

• During the phase change process from liquid to vapor at saturation temperature, both phases coexist at constant temperature while transitioning between states.

• Internal energy (u_fg), enthalpy (h_fg), and entropy (s_fg) differences are crucial for calculations; these values represent the difference between saturated vapor and saturated liquid states.

• For example, calculating internal energy involves subtracting the value of saturated liquid from that of saturated vapor: u_fg = u_g - u_f.

Pressure and Temperature Relationships

• Different saturation temperatures correlate with distinct saturation pressures; for instance, at 40°C, the saturation pressure is 7.38 kPa.

• Units for specific volume are m³/kg; internal energy is expressed in kJ/kg; enthalpy and entropy also use kJ/kg but include Kelvin in their units.

Graphical Representation of States

• A graph illustrates various thermodynamic states: compressed liquid, saturated liquid mixture, saturated vapor, and superheated vapor. The vertical axis represents temperature (°C), while the horizontal axis shows specific volume (m³/kg).

• Specific volume can be calculated as one divided by density (ρ). Density itself is mass per unit volume.

Isochoric Process Insights

• A purple line on the graph indicates an isobaric process where pressure remains constant across different points on this line.

• As you move along this line during heating or cooling processes in a thermodynamic system—like moving from point 1 to point 5—temperature increases while maintaining constant pressure conditions.

Phase Transition Dynamics

• In state transitions illustrated on the graph: starting from a compressed state at point 1 with a certain volume and increasing temperature until reaching boiling point or saturation temperature (100°C under standard atmospheric pressure).

Understanding Phase Changes in Water

Liquid Saturation and Initial Heating

• The water reaches 100 degrees Celsius, nearing the appearance of the first vapor bubble. This state is identified as saturated liquid, denoted by "f" with a subscript.

• At state 3, there exists a mixture of liquid and vapor phases, indicating that both forms coexist at this temperature.

Characteristics of Mixture States

• During the mixing process between liquid and vapor, the temperature remains constant while properties like volume increase. This is represented by a horizontal line on phase diagrams.

• State 4 represents fully saturated vapor (100% vapor), where no liquid remains. It is identified with "g."

Transition to Superheated Vapor

• In state 5, the temperature increases again as only vapor is present. This indicates that when a pure substance exists in one phase, its temperature can rise.

• The transition from states 2 to 4 shows that during mixing (states 2 and 3), temperature remains constant due to the coexistence of two phases.

Understanding Isobaric Processes

• The area known as compressed liquid contains 100% liquid. All processes occur along a purple line representing constant pressure (isobaric).

• Isobaric means transformations happen at constant pressure; thus, pressures across all states remain equal despite increasing temperatures.

Exploring Isothermal Conditions

• If an experiment were conducted under isothermal conditions (constant temperature), various temperatures could be used while still achieving similar states: compressed liquid, saturated liquid, mixed state, saturated vapor, and superheated vapor.

• An isotherm graph illustrates these states against pressure in kilopascals on the vertical axis and specific volume on the horizontal axis.

Graphical Representation of States

• The red line on graphs denotes an isotherm where all points maintain a consistent temperature; deviations from this line indicate different temperatures.

Understanding the States of Water

Compressed Liquid at Initial Conditions

• At a temperature of 25 degrees Celsius and an initial pressure of 5000 kPa, the liquid is in a compressed state. This pressure value is arbitrary for the experiment.

Transition to Saturated Liquid

• To achieve saturated liquid status at 25 degrees Celsius, the pressure must be decreased to the saturation pressure of 317 kPa. This represents a critical point where liquid can exist without vapor.

Behavior During Phase Changes

• In state three, where there is a saturated mixture, both temperature and pressure remain constant. The horizontal line on a graph indicates that these parameters do not vary during this phase transition.

Coexistence of Phases

• The constant nature of temperature and pressure during this horizontal trajectory signifies the coexistence of liquid and gas phases until reaching vapor saturation.

Achieving Superheated Vapor

• Upon further decreasing the pressure to around 0.1 kPa, water transitions into superheated vapor at 25 degrees Celsius. This marks a significant change from saturated states to superheated conditions.

Summary of Water States