The model used to describe the behavior of gases is called the kinetic theory. The main difference between the gaseous model and that for condensed states (liquids and solids) is that, in condensed states, there are short range forces between particles which cause them to stay in contactÑto be attracted to one another. In some substances, hydrogen for example, these attractive forces are extremely weak and thus the substance turns liquid and then solid only at very low temperatures. The amount of empty space in the condensed states is small but not zero.
In liquids, particles are envisioned to be in close contact with one another, and with nearest neighbors arranged around one another in fairly regular ways. For a particle in the liquid to escape and become a gaseous particle, the addition of enough energy to overcome the attractive forces between the particles is required.
Crystalline solids differ from liquids in their degree of bonding regularity. Solids are thought to be extraordinarily regular in their arrangement such that, knowing the positions of just a few particles allows the precise prediction of the positions of nearly all others. We speak of a crystal lattice, an imaginary regular geometric array of points such that each lattice point serves as a marker for the location of a solid particle.
In order for a solid to melt and become a liquid, energy must be added to overcome the highly directional attractive forces which hold the solid particles in a regular array. The energy for melting is nearly always lower than the energy for boiling.
This unit can follow gas laws and discussions of kinetic energy. It also can precede or blend easily with thermochemistry. A discussion of the unit cell, coordination numbers, and crystal structures can fit in appropriately. References to phase changes are appropriate when studying LeChatelierÕs principle.
1. Solids and liquids are the two condensed states of matter.
2. Solids have definite shapes and volumes. Liquids have definite volumes but assume the shapes of their containers.
3. Density is an important characteristic property of liquids and solids.
4. As solids or liquids are warmed, the kinetic energy of their particles increases, increasing particle motion. As solids melt or liquids boil, the potential energy of the particles increases as the forces of attraction are disrupted (and vice versa).
5. Liquids in a closed container can achieve a condition known as dynamic equilibrium where the rate of vaporization equals the rate of condensation.
6. Solids in a closed container can achieve a condition known as dynamic equilibrium where the rate of melting equals the rate of freezing.
7. The pressure exerted by the gaseous phase of a liquid (or solid) at dynamic equilibrium is called the equilibrium vapor pressure. Vapor pressure values are influenced by the bond strengths (or attractive forces) between the liquid (or solid) particles.
8. The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the prevailing external (atmospheric) pressure. At the melting point, the vapor pressures of the solid and liquid phases are equal.
9. The surface tension of a liquid is caused by unbalanced forces on the surface particles and makes the surface appear to be elastic. Surface tension is responsible for capillary rise.
10. The viscosity of a fluid is its resistance to flow.
11. Pure crystalline solids have well-defined melting (freezing) points which are greatly influenced by the type and strength of bonding among particles in the crystal.
1. Gases
2. Bonding
3. Atomic and ionic radii
4. Potential and kinetic energy
5. LeChatelierÕs principle
6. Kinetic molecular theory
7. Chemical forces (bonds) vs. physical attractive forces
8. Coordination numbers
9. ArchimedesÕ Principle
1. Preparing a graph.
2. Dealing with three dimensional geometry.
3. Determining the slope of a line.
4. Reading balance, thermometer, and graduated cylinder.
After completing their study of condensed states, students should be able to:
1. list the two condensed states of matter and describe characteristics of each condensed state.
2. graph mass and volume data for an unknown solid sample and determine the density of the solid from the slope of the line.
3. describe the physical changes and energy changes which take place as substances melt, boil, condense, and freeze.
4. offer an appropriate model and explanation for dynamic equilibrium in a closed container where a solid/liquid or liquid/gas system exists.
5. define melting point and boiling point in terms of vapor pressure.
6. relate the vapor pressure of a solid or liquid substance to the attractions between its particles.
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