States of Matter and Phase Changes

1. States of Matter and Phase Changes Kinetic theory of matter

2. Dalton’s Model (1803) • All matter is ultimately composed of atoms. • Atoms can be neither subdivided nor changed into one another. • Atoms can be neither created or destroyed. • Atoms of a particular substance are identical. • Chemical change is the union or separation of atoms.

3. Phases of Matter • Dalton based his atomic theory of matter on his life-long study of gases. • Dalton’s studies are extensible to the inclusion of molecular substances and phases of matter. • A phase is the name we give to a state of aggregation of matter. • Common phases are gas, liquid, and solid. (Many substance exhibit more than one solid phase, but these usually exist at high pressure. Ice has more than a half dozen solid forms.)

4. Gases • The behavior of phases are a result of a balance of different forces. • In gases, molecules are highly active, large volume, very low attraction among molecules, no structure. A sample of gas takes its form from the shape of the container. Tends to expand indefinitely if unconfined.

5. Liquids • In liquids, volume does not change because the intermolecular forces are strong enough to keep the molecules in close association with each other. However, the molecules are sufficiently energetic to easily break associations and form new ones with other molecules giving the fluid behavior. Volume of liquid does not change, but shape of sample is the shape of the container.

6. Solids • Solids have well characterized geometries. They exhibit crystal structures – atoms, molecules, or ions in regular lattice work. Shape of a crystal does not depend on the shape of the container. Atoms not very energetic and do not move around. Stay in fixed locations. Strong forces among particles to keep them in place overwhelming molecular motions.

7. Balance of forces • Key to understanding phases is the balance between the kinetic energy of the molecules that tend to disorder the sample and the forces of attraction among molecules that hold them together.

8. Role of Heat • Application or removal of heat is necessary to covert one phase into another. • Addition of heat – solid to liquid to gas. • Removal of heat – gas to liquid to solid. • The heat at a phase transition is a well know quantity and varies with substance. • Heat of melting (addition) or solidification (removal) of water: 80 cal / g. • Heat of vaporization (addition) or condensation (removal) of water: 540 cal / g. • Uncommon process – sublimation, solid to gas. Example, carbon dioxide as a solid is dry ice.

9. Cooling and Heating Curves • As a sample cools (as long as it is in the same phase), temperature falls as heat is withdrawn. • When it comes to a phase transition – condensation from gas to liquid or solidification from liquid to solid, temperature is constant until entire phase transition is completed.

10. Example • Phenol has a melting point at 43 C and a boiling point of 182 C. Sketch the heating curve of a sample of phenol that starts at room temperature of 25 C and continues to 200 C.

12. example • Sketch the cooling curve of a 10 g sample of steam that begins at 150 C and is cooled to a temperature of -20 C. Calculate the amount of heat that must be removed in calories to do this.