13.5 Phase Changes
3 min read•june 18, 2024
are visual tools that show how matter behaves under different temperatures and pressures. They help us understand when substances change from to to . These diagrams are crucial for predicting and controlling material states in various applications.
explains how gases mix and interact. It's essential for understanding atmospheric , gas mixtures in industry, and even how we breathe. The and phase equilibrium concepts further deepen our grasp of matter's behavior during transitions.
Phase Changes and Equilibrium
Interpretation of phase diagrams
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- Phase diagrams graphically represent the equilibrium states of matter at various temperatures and pressures
- Solid, liquid, and gas regions are divided by curves on the diagram (water, carbon dioxide)
- Key points on a phase diagram include:
- : Specific and pressure where all three phases (solid, liquid, gas) can simultaneously exist in equilibrium (water: 0.01°C, 611.73 Pa)
- : Maximum temperature and pressure at which the liquid and gas phases can be differentiated (water: 374°C, 22.06 MPa)
- curve separates the solid and gas regions, depicting the equilibrium between these two phases (dry ice, iodine)
- () curve separates the solid and liquid regions, illustrating the equilibrium between these two phases (ice melting, metal casting)
- curve separates the liquid and gas regions, showing the equilibrium between these two phases ( water, evaporation of solvents)
- Phase boundaries represent the conditions where two phases coexist in equilibrium
Dalton's law of partial pressures
- Dalton's law of partial pressures asserts that the total pressure of a gas mixture equals the sum of the partial pressures of each component gas
- , where represents the of the nth component gas
- is the pressure each gas would exert if it alone occupied the of the mixture at the same temperature (nitrogen, oxygen in air)
- Applications of Dalton's law include:
- Determining the composition of gas mixtures, such as air (78% nitrogen, 21% oxygen)
- Investigating the behavior of gases in chemical reactions (combustion, respiration)
- Calculating the pressure of individual components in a gas mixture (scuba diving, anesthesia)
Triple point significance
- The triple point is the unique temperature and pressure where all three phases (solid, liquid, gas) can coexist in equilibrium simultaneously
- At the triple point, the , melting, and vaporization curves intersect (water: 0.01°C, 611.73 Pa)
- The triple point is significant because:
- It signifies the lowest pressure at which the liquid phase can exist (water: 611.73 Pa)
- It serves as a fixed point for calibrating temperature and pressure scales (Kelvin scale, International Temperature Scale)
- Phase transitions at the triple point occur without changes in temperature or pressure, provided all three phases are present (ice-water- system)
Phase equilibrium comparisons
- Equilibrium between phases occurs when the rates of forward and reverse phase transitions are balanced
- Solid-liquid equilibrium (melting/):
- Molecules in the solid and liquid phases possess the same average at the melting point (ice-water at 0°C)
- of fusion is the energy needed to overcome and transform the phase from solid to liquid (ice melting: 334 J/g)
- Liquid-gas equilibrium (vaporization/):
- Molecules in the liquid and gas phases have equal average kinetic energy at the boiling point (water-steam at 100°C)
- Latent heat of vaporization is the energy required to surmount intermolecular forces and change the phase from liquid to gas (water boiling: 2260 J/g)
- Solid-gas equilibrium (sublimation/):
- Molecules in the solid and gas phases possess identical average kinetic energy at the sublimation point (dry ice, iodine)
- Latent is the energy needed to directly change the phase from solid to gas, equaling the sum of the latent heats of fusion and vaporization (carbon dioxide: 573 J/g)
Thermodynamic concepts in phase changes
- changes during phase transitions reflect the energy absorbed or released by the system
- increases as a substance transitions from solid to liquid to gas, due to increased molecular disorder
- affects the amount of energy required to change a substance's temperature during phase transitions
Key Terms to Review (48)
Boiling: Boiling is the process of a liquid transitioning to a gaseous state due to the application of heat. It is a key phase change that occurs when the vapor pressure of a liquid equals the pressure surrounding the liquid, allowing bubbles of vapor to form throughout the liquid.
Change in entropy: Change in entropy is the measure of the disorder or randomness in a system as it undergoes a process. It quantifies the energy dispersal and unavailability for doing work.
Clausius-Clapeyron equation: The Clausius-Clapeyron equation is a fundamental relationship that describes the equilibrium vapor pressure of a substance as a function of temperature. It is a critical tool for understanding phase changes and the behavior of substances in different thermodynamic states.
Condensation: Condensation is the process by which water vapor in the air is converted into liquid water. It is a fundamental phase change that occurs when the temperature of a gas is lowered below its dew point, causing the water vapor to condense into tiny droplets or a thin film of liquid water on a surface.
Critical point: The critical point is the highest temperature and pressure at which a substance can exist as a liquid and gas in equilibrium. Beyond this point, the substance becomes a supercritical fluid with properties of both gas and liquid.
Critical Point: The critical point is a unique point in the phase diagram of a substance where the distinction between the liquid and gas phases disappears, and they become indistinguishable. At the critical point, the properties of the liquid and gas phases converge, marking the end of the phase transition between the two states.
Critical pressure: Critical pressure is the minimum pressure required to liquefy a gas at its critical temperature. It marks the endpoint of the liquid-vapor phase boundary.
Critical temperature: Critical temperature is the highest temperature at which a substance can exist as a liquid, regardless of pressure. Beyond this temperature, the substance becomes a supercritical fluid.
Dalton's Law of Partial Pressures: Dalton's Law of Partial Pressures states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of each individual gas in the mixture. This principle is crucial in understanding the behavior of gases and their interactions during phase changes.
Deposition: Deposition is the process by which a gas or vapor transitions directly into a solid state, bypassing the liquid phase. This phase change occurs when the temperature and pressure conditions are such that the gas cannot remain in its gaseous form and instead condenses onto a surface or forms solid crystals.
Enthalpy: Enthalpy is a measure of the total energy of a thermodynamic system, including its internal energy and the work done by or on the system as a result of changes in pressure and volume. It represents the sum of a system's internal energy and the work done on the system by its surroundings or the work done by the system on its surroundings.
Entropy: Entropy is a measure of the disorder or randomness in a system. It represents the unavailability of a system's energy to do useful work and the natural tendency of the universe towards increased disorder and chaos. This concept is central to the understanding of thermodynamics and the second law of thermodynamics, which governs the flow of energy and heat in physical systems.
Freezing: Freezing is the process in which a liquid transitions into a solid as a result of temperature decrease, where the molecules lose kinetic energy and form a structured lattice. This phase change is crucial in understanding the behavior of materials and energy transfer during phase transitions, as it illustrates how substances behave differently at various temperatures and states.
Fusion: Fusion is the process of combining two or more atomic nuclei to form a single, heavier nucleus. This process releases a large amount of energy and is the fundamental source of energy in the Sun and other stars.
Gas: A gas is one of the four fundamental states of matter, characterized by its ability to expand and fill any container it is placed in. Gases have no fixed shape or volume and are highly compressible, making them the most fluid and least dense of the four states of matter.
Heat Capacity: Heat capacity is a measure of the amount of energy required to raise the temperature of a substance by a certain amount. It quantifies how much heat a material can absorb or release without undergoing a significant change in temperature. This concept is crucial in understanding the thermal properties of materials and their behavior during various thermodynamic processes.
Heat of sublimation: Heat of sublimation is the amount of energy required to change a substance from a solid phase directly to a gas phase without passing through the liquid phase. It is usually measured in joules per gram (J/g) or kilojoules per mole (kJ/mol).
Ideal gas law: The Ideal Gas Law is a fundamental equation in physics that relates the pressure, volume, temperature, and number of moles of an ideal gas. It is expressed as $PV = nRT$, where $P$ is pressure, $V$ is volume, $n$ is the number of moles, $R$ is the universal gas constant, and $T$ is temperature.
Intermolecular Forces: Intermolecular forces are the attractive or repulsive forces that exist between molecules, which determine the physical and chemical properties of substances. These forces play a crucial role in understanding the behavior of fluids, the surface tension and capillary action of liquids, as well as the phase changes that occur in matter.
Internal kinetic energy: Internal kinetic energy is the sum of the kinetic energies of all particles within a system. It plays a crucial role in understanding how energy is distributed and conserved during elastic collisions.
Johannes van der Waals: Johannes van der Waals was a Dutch physicist who made significant contributions to the understanding of the behavior of gases and liquids, particularly in the context of phase changes. His work laid the foundation for the development of the van der Waals equation, which describes the relationship between pressure, volume, and temperature for real gases.
Kinetic Energy: Kinetic energy is the energy of motion possessed by an object. It is the energy an object has by virtue of being in motion and is directly proportional to the mass of the object and the square of its velocity. Kinetic energy is a crucial concept in physics, as it relates to the work done on an object, the conservation of energy, and various other physical phenomena.
Latent Heat: Latent heat is the energy released or absorbed by a substance during a phase change, such as the transition from solid to liquid or liquid to gas, without a change in temperature. It is the energy required to change the physical state of a substance while maintaining a constant temperature.
Latent heat coefficients: Latent heat coefficients represent the amount of heat required to change the phase of a unit mass of a substance without changing its temperature. They are crucial in understanding phase changes like melting, freezing, boiling, and condensation.
Liquid: A liquid is one of the three primary states of matter, characterized by a definite volume but no fixed shape, allowing it to flow and take the shape of its container. This state occurs when molecules have enough energy to move past each other while remaining close enough to exert intermolecular forces. The behavior and properties of liquids are central to understanding phase changes and the energy involved during these transformations.
Melting: Melting is the process in which a solid turns into a liquid due to the addition of heat, breaking the bonds that hold its particles in a fixed structure. This phase change is crucial for understanding how materials transition between solid and liquid states, influencing their physical properties and behaviors during temperature changes. The melting point is the specific temperature at which this transformation occurs, depending on the material involved.
P-T Diagram: A P-T diagram, also known as a phase diagram, is a graphical representation that depicts the relationship between the pressure (P) and temperature (T) of a substance, allowing for the identification of its different phases (solid, liquid, and gas) and the boundaries between them.
P-V-T Surface: The P-V-T (Pressure-Volume-Temperature) surface is a three-dimensional graphical representation that depicts the relationship between the pressure, volume, and temperature of a substance, particularly in the context of phase changes. This surface provides a comprehensive understanding of the thermodynamic behavior of a substance under various conditions.
Partial pressure: Partial pressure is the pressure exerted by a single type of gas in a mixture of gases. It is proportional to its mole fraction and the total pressure of the mixture.
Partial Pressure: Partial pressure is the contribution of a specific gas to the total pressure of a mixture of gases. It represents the pressure that each individual gas would exert if it were the only gas present in the same volume. The concept of partial pressure is essential in understanding the behavior of gas mixtures and their applications in various fields, including the Ideal Gas Law, phase changes, and humidity.
Phase Boundary: A phase boundary is the interface or dividing line between two distinct phases of a substance, such as solid, liquid, and gas. It represents the point at which a change in the physical state or phase of a material occurs.
Phase Changes: Phase changes refer to the transitions between the different physical states of matter, such as solid, liquid, and gas. These transitions occur when the input or removal of energy, typically in the form of heat, causes a substance to change from one state to another.
Phase Diagrams: A phase diagram is a graphical representation that shows the various phases (solid, liquid, gas) of a substance or a mixture of substances under different conditions of temperature and pressure. It provides a visual understanding of the relationships between the different phases and the conditions under which they exist.
Phase Transition: A phase transition is a physical transformation of a substance from one state of matter (solid, liquid, or gas) to another, often accompanied by the absorption or release of energy. These changes in the fundamental structure of a material occur at specific temperature and pressure conditions.
Plasma: Plasma is the fourth state of matter, distinct from the solid, liquid, and gaseous states. It is an ionized gas composed of free-moving electrons, ions, and neutral atoms or molecules. Plasma plays a crucial role in various physical phenomena, including phase changes, latent heat, and magnetic forces between parallel conductors.
Pressure: Pressure is the force exerted per unit area on a surface. It is a fundamental concept in physics that describes the amount of force applied to a given area, and it plays a crucial role in understanding the behavior of fluids, gases, and various physical systems.
Solid: A solid is one of the fundamental states of matter, characterized by structural rigidity and resistance to changes in shape or volume. Solids are composed of densely packed atoms or molecules that are held together by strong intermolecular forces, resulting in a fixed shape and volume.
Solid-state radiation detectors: Solid-state radiation detectors are devices that use semiconductor materials to detect ionizing radiation. They convert incoming radiation into electrical signals, which can then be measured and analyzed.
Sublimation: Sublimation is the phase change where a substance transitions directly from a solid to a gas without passing through the liquid phase. It occurs when the molecules of the solid gain enough energy to overcome intermolecular forces and enter the gas phase.
Sublimation: Sublimation is the process by which a substance transitions directly from a solid state to a gaseous state, without passing through the intermediate liquid phase. This phase change occurs when the vapor pressure of the solid exceeds the pressure of the surrounding environment, allowing the solid to vaporize.
Temperature: Temperature is a physical quantity that measures the average kinetic energy of the particles, such as atoms or molecules, in a substance. It is a fundamental concept that is closely related to the behavior of matter and energy in various contexts, including vectors, scalars, coordinate systems, the ideal gas law, kinetic theory, and phase changes.
Thermodynamics: Thermodynamics is the branch of physics that deals with the relationships between heat, work, temperature, and energy. It explores how energy is transformed and transferred within physical systems, governing the principles that dictate the behavior of matter in various states. The laws of thermodynamics play a crucial role in understanding how energy flows and changes form, influencing everything from engines to biological processes.
Triple point: The triple point is the unique set of conditions at which all three phases (solid, liquid, and gas) of a substance coexist in thermodynamic equilibrium. It is characterized by a specific temperature and pressure for each substance.
Triple Point: The triple point is a unique condition in which the solid, liquid, and gaseous phases of a substance can coexist in equilibrium. It represents the specific temperature and pressure at which all three phases of a material can be present simultaneously.
Vapor: Vapor is the gaseous state of a substance that is typically liquid or solid under standard conditions. It occurs when molecules escape from the liquid or solid phase into the gas phase.
Vapor pressure: Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature. It reflects how readily molecules escape from the liquid or solid phase into the gas phase.
Vaporization: Vaporization is the process by which a liquid transforms into a gas, occurring when molecules at the surface of the liquid gain enough energy to break free from intermolecular forces. This process can happen in two main ways: evaporation, which occurs at any temperature, and boiling, which happens at a specific boiling point. Understanding vaporization helps explain phenomena like humidity, heat transfer during phase changes, and the energy required for substances to change from one state to another.
Volume: Volume is the amount of three-dimensional space an object or substance occupies, typically measured in liters, cubic meters, or other units. It plays a crucial role in understanding how substances behave in different contexts, including their density, buoyancy, and how they respond to changes in temperature and pressure.