Overview
This lecture explains the phases of matter (solid, liquid, gas), how their properties differ due to molecular bonds, and introduces the kinetic molecular theory linking these phases to molecular motion and energy.
Phases of Matter
- Solids have a fixed shape and volume due to strong cohesive bonds between molecules.
- Liquids have a fixed volume but take the shape of their container because of weaker cohesive bonds.
- Gases have very weak or minimal cohesive bonds, take the container’s shape, and expand to fill the entire volume.
- Solids do not flow, while liquids and gases (fluids) flow and do not have a fixed shape.
- Gases can be compressed easily; solids and liquids cannot be compressed into a smaller volume.
Molecular Motion and Cohesion
- In solids, molecules are locked in place but still vibrate in position due to strong cohesive bonds.
- In liquids, molecules are loosely bonded, allowing them to slide past each other and mix (e.g., stirring dye into water).
- In gases, molecules move freely and spread out quickly due to negligible cohesive forces, resulting in phenomena like diffusion.
- Diffusion is the process by which molecules spread from an area of high concentration to low concentration (e.g., smell traveling through air).
Energy and Phase Changes
- Moving from solid to liquid to gas requires input of energy.
- Solid phase is the lowest energy; molecules are tightly bound but still move.
- Adding energy breaks cohesive bonds, turning solids into liquids (melting) and liquids into gases (evaporation/boiling).
- Gas phase is the highest energy, with molecules moving freely and rapidly.
Kinetic Molecular Theory
- Kinetic molecular theory states that matter consists of particles in constant motion, and their motion increases with added energy.
- Phase of matter is determined by the energy of the molecules and the strength of their cohesive bonds.
Key Terms & Definitions
- Cohesive bond — The attraction holding molecules together in a substance.
- Diffusion — The movement of molecules from high to low concentration.
- Kinetic molecular theory — Theory stating that particles of matter are always in motion, and energy determines their phase.
Action Items / Next Steps
- Review and understand section 4.1 concepts before moving on to section 4.2.