Solids, Liquids and Solutions
By comparison with gases, solids and liquids have microscopic structures in which the constituent particles are very close together. The volume occupied by a given amount of a solid or liquid is much less than that of the corresponding gas. Consequently solids and liquids collectively are called condensed phases. The properties of solids and liquids are much more dependent on intermolecular forces and on atomic, molecular, or ionic sizes and shapes than are the properties of gases.
Despite their greater variation with changes in molecular structure, some properties of condensed phases are quite general. In a solid, for example, microscopic particles are arranged in a regular, repeating crystal lattice. The microscopic structure of a solid may thus be described in terms of a unit cell—the smallest parallel-sided shape from which the overall structure can be constructed—and a space lattice—the pattern of points made by the corners of many unit cells packed together . The seven crystal systems include all the basic types of unit cells. Closest-packed structures are important because they are ways of squeezing the maximum number of atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. into the minimum volume. There are two types: hexagonal closest packedOne of two schemes for closest packing of spheres; described as "aba" to indicate that the atoms of the third layer lie directly above the atoms of the first layer. (hcpAbbreviation for hexagonal closest packed; one of two schemes for closest packing of spheres; described as "aba" to indicate that the atoms of the third layer lie directly above the atoms of the first layer.) and cubic closest packedOne of two schemes for closest packing of spheres; described as "abca" to indicate that the atoms of the second and third layers do not lie directly above the atoms of the first layer, but the atoms of the fourth layer are directly above the atoms of the first layer. Abbreviated ccp. (ccpAbbreviation for cubic closest packed; one of two schemes for closest packing of spheres; described as "abca" to indicate that the atoms of the second and third layers do not lie directly above the atoms of the first layer, but the atoms of the fourth layer are directly above the atoms of the first layer.). Most metals adopt hcp, ccp, or body-centered cubicA crystal lattice structure whose cubic unit cell has one atom in the center and one atom at each corner (only 1/8 of each corner atom is within the unit cell); abbreviated bcc. crystal lattices.
The particles at the lattice points which define crystals may be atoms, molecules, or ions. NaCl has a face-centered cubic lattice with alternating Na+ and Cl- ions (so that each ion is surrounded by 6 other of opposite charge), while solid CO2 may form several types of lattice , all with CO2 molecules at lattice points.
Quartz (one form of SiO2), however, is a network crystal with atoms at lattice points, connected by ~Si-O-Si-O~ covalent bonds, so that the whole crystal could be considered one big molecule. The crystal may be rhombohedral (α-quartz) or hexagonal (β-quartz), and other forms of silicon dioxide have other crystal structures. Many other silicates are ionic, with SiO44- or more complex ions occupying lattice points.
Similarly, useful generalizations can be made regarding the properties of liquids. Liquids differ from solids in that their constituent particles have sufficient energyA system's capacity to do work. to move past one another rather than vibrating about average location. Some liquids become so viscous just above their melting points that their particles no longer can move. In such a case a glass or amorphous material, in which there is no long-range order, is formed.
Generalizations about phase transitions, where a solid changes to liquid, liquid to gas, or solid to gas can also be made. All require energy because they involve separation of particles which attract one another. Each phase transition can reach a state of equilibriumA state in which no net change is occurring, that is, in which the concentrations of reactants and products remain constant; chemical equilibrium is characterized by forward and reverse reactions occurring at the same rate. under certain conditions of temperatureA physical property that indicates whether one object can transfer thermal energy to another object. and pressureForce per unit area; in gases arising from the force exerted by collisions of gas molecules with the wall of the container.. Such conditions are summarized in a phase diagram. The liquid-vapor equilibrium differs from the other two in that there is no distinction between liquid and vapor above the critical temperature. Hence the liquid-vapor curve in a phase diagram ends at the critical pointThe temperature and pressure above which no distinction exists between a liquid and its vapor..
The liquid phase, where microscopic particles are close together but can still move past one another, provides an ideal medium for chemical reactions. ReactantA substance consumed by a chemical reaction. molecules can move toward one another because they are not held in fixed locations as in a solid, and a great many more collisions between molecules are possible because they are much closer together than in a gas. Such collisions lead to breaking of some bonds and formation of new ones, that is, to chemical reactions. This molecular intimacy without rigidity, combined with ease of handling of liquids in the laboratory, leads chemists to carry out many reactions in the liquid phase. Usually such reactions involve solutions of reactants in liquid solvents. Consequently we shall examine some general properties of solutions as well.
A saturated solution is one in which soluteThe substance added to a solvent to make a solution. is in equilibrium with solution at a given temperature. If a supersaturated solution is prepared, it is unstable. Adding a seed crystal permits solute molecules to precipitate until the rate of solution equals the rate of precipitationThe formation of a solid within a solution, often by the combination of cations and anions to form an insoluble ionic compound.. The solubilityThe extent to which a solute dissolves in a solvent; often expressed as the mass of a substance that will dissolve in 100 mL of solvent. of a solute or the composition of any solution which is to be used quantitatively is usually reported in terms of concentrationA measure of the ratio of the quantity of a substance to the quantity of solvent, solution, or ore. Also, the process of making something more concentrated. (amount of solute/volume of solution), massA measure of the force required to impart unit acceleration to an object; mass is proportional to chemical amount, which represents the quantity of matter in an object. fraction (mass of solute/mass of solution), or mole-fraction (amount of solute/amount of solution).
The solubilities of various substances can be related to the molecular structures of solute and solvent. The most important rule is that like dissolves like. When solute and solvent molecules are nearly identical, a solution behaves ideally and obeys Raoult’s law. However, nonideal behavior of solutions is much more common than nonideal behavior of gases, because molecules are much closer together in the liquid phase and intermolecular forces are much more important.
The components of liquid solutions may be separated by distillation, provided deviations from ideal behavior are not so large as to produce azeotropes. A second very important method of separation is chromatography. It depends on the relative abilities of different substances to dissolve in or be adsorbed on one phase in preference to another. Thus if intermolecular forces are such that a component of a mixtureA combination of two or more substances in which the substances retain their chemical identity. is strongly held by the mobile phase, while another component is held by the stationary phase, the two can be separated chromatographically.
Colligative properties include lowering of vapor pressureThe pressure (or partial pressure) exerted by the gaseous form of a substance in equilibrium with the liquid form., elevation of boiling point, depression of freezing point, and osmotic pressure. They depend on the number of particles (and hence the amount) of solute dissolved in a given quantity of solvent, irrespective of the nature of the particles. Colligative properties can be used to determine molar masses, although the only one commonly used for that purpose today is osmotic pressure.