4.3 Changes State
Changes of State
Learning Objectives
General Understanding of State Changes
Describe melting and boiling as energy input processes without temperature change.
Understand melting and boiling temperatures for water at standard atmospheric pressure.
Describe condensation and solidification in relation to particle behavior.
Explain evaporation as the escape of high-energy molecules from a liquid's surface.
Temperature During State Changes
Melting and Boiling Points of Water
Melting Point: Pure water melts at 0°C under standard atmospheric pressure (1 atm).
Boiling Point: Pure water boils at 100°C under standard atmospheric pressure.
Melting Process
Temperature Changes During Melting
Phase Transition:
From P to Q: Ice temperature increases from -15°C to 0°C.
From Q to R: Temperature remains at 0°C while heat is absorbed as ice melts.
From R to S: Temperature of melted ice rises from 0°C to 10°C.
Energy Transfer During Melting
Energy Absorption: Between Q and R, thermal energy absorbed breaks bonds in solid ice.
Potential vs. Kinetic Energy: Only potential energy increases; kinetic energy remains constant, keeping temperature unchanged.
Solidification Process
Temperature Changes During Solidification
Phase Transition:
From A to B: Naphthalene temperature falls from 90°C to 79°C.
From B to C: Temperature remains at 79°C while heat is released during solidification.
From C to D: Temperature of solid naphthalene drops from 79°C to 60°C.
Energy Transfer During Solidification
Bond Formation: Between B and C, strong bonds form as particles come together during freezing.
Energy Release: Thermal energy is released, reducing potential energy; kinetic energy and temperature stay constant.
Boiling Process
Overview of Boiling
Definition: Thermal energy changes substance from liquid to gas without temperature change.
Energy Absorption: Energy is absorbed to break bonds between liquid particles.
Temperature Changes During Boiling
Phase Transition:
From X to Y: Water temperature rises from 0°C to 100°C; kinetic energy increases.
From Y to Z: Temperature remains at 100°C as water boils to steam; energy separates water molecules and allows them to escape.
Condensation Process
Overview
Definition: Thermal energy removal changes substance from gas to liquid without a temperature change.
Energy Release: At condensation point, the substance releases energy as bonds are formed between particles.
Evaporation Process
Definition of Evaporation
Evaporation in Context: Evaporation explains how wet clothes dry, especially with wind.
Energy Absorption: Requires thermal energy absorbed from surroundings to change from liquid to gas.
Cooling Effect of Evaporation
Example: Getting out of a pool; evaporating water cools the skin due to energy absorption.
Molecular Behavior During Evaporation
Energy Range: Liquid molecules have different energies; energetic molecules escape while lower-energy ones remain.
Temperature Impact: Average energy (temperature) decreases as the more energetic molecules escape.
Factors Affecting Evaporation Rate
Key Factors
Temperature: Higher temperatures increase kinetic energy, enhancing evaporation.
Surface Area: Larger surface area allows more molecules to escape, increasing evaporation rates.
Air Movement: Moving air removes vapor quickly, keeping the air dry, which enhances evaporation.
Comparison of Evaporation and Boiling
Evaporation
Occurs at any temperature
Relatively slow
Takes place only at the surface
No bubbles formed
Temperature may change
No external energy required
Boiling
Occurs at a specific temperature
Relatively fast
Takes place throughout the liquid
Bubbles form within the liquid
Temperature remains constant
External energy source required
Conclusion
Understanding changes of state is crucial in the study of physical science, particularly in thermodynamics and material behavior.