Overview
This lecture introduces the concept of phase changes in matter, focusing on how energy transfers cause substances to change state and how this is represented on temperature vs. energy graphs using water as a key example.
Phase Changes and Energy Transfer
- Adding or removing energy changes the temperature or the phase of a substance.
- Raising temperature increases molecular motion; lowering temperature decreases it, as long as the phase stays the same.
- During a phase change, energy is used to make or break bonds between molecules, not to change temperature.
Phases of Water: Solid, Liquid, Gas
- Water exists as solid (ice), liquid, or gas (steam), each with distinct molecular arrangements.
- Solid: molecules locked in fixed positions, can only vibrate.
- Liquid: molecules can move past each other, with some weaker cohesive bonds.
- Gas: molecules move freely, with little to no cohesive bonding.
Heating and Cooling Curves
- Adding energy to ice below 0°C first raises temperature until melting point (0°C).
- At melting point, added energy breaks bonds, enabling transition to liquid, but temperature stays constant.
- Once all ice has melted, added energy raises the temperature of the liquid until boiling point (100°C).
- At boiling point, added energy breaks more bonds, transitioning liquid to gas, with temperature remaining constant.
- After all water has vaporized, added energy again increases temperature of the gas.
Reverse Process: Cooling
- Removing energy from gas slows molecules until condensation at boiling point, forming liquid.
- Continued cooling lowers liquid temperature until freezing point; further removal of energy causes solidification, forming crystalline ice.
Key Terms & Definitions
- Phase Change — Transition of matter between solid, liquid, and gas states.
- Melting Point — Temperature where solid becomes liquid (0°C for water).
- Boiling Point — Temperature where liquid becomes gas (100°C for water).
- Cohesive Bonds — Forces holding molecules together in a substance.
- Heating/Cooling Curve — Graph showing temperature vs. energy during heating or cooling, with flat regions at phase changes.
Action Items / Next Steps
- Review upcoming material on equations and calculations related to energy and phase changes in section 4.4.
- Prepare to examine mathematical relationships for phase change in the next lecture.