Most solids are crystalline, meaning the particles that compose them are arranged in repeated patterns.
Amorphous solids have particles irregularly arranged.
Crystalline solids fall into four classes:
Ionic
Covalent
Metallic
Molecular
Ionic crystals are formed by the attraction between positive and negative ions.
Covalent crystals are formed when pairs of electrons are shared between adjacent atoms.
Some crystals are neither wholly ionic nor wholly covalent but contain bonds of mixed character.
Diamond and graphite are forms of carbon but differ from each other in their physical properties.
10-2. The Metallic Bond
The metallic bond is formed by a "gas" of electrons that moves freely through the assembly of metal ions that form a solid metal.
The metallic bond accounts for the characteristics of metals.
10-3. Molecular Crystals
Some liquids and solids are formed through the action of van der Waals forces, named after the Dutch physicist Johannes van der Waals.
There are several types of van der Waals interactions:
Polar-polar interaction occurs between polar molecules whose positively and negatively charged ends cause them to line up with the ends that have opposite charges adjacent.
Polar-nonpolar interaction occurs between polar and nonpolar molecules because the electric field of the polar molecules causes separations of charge in the nonpolar molecules. The oppositely charged ends of the polar and nonpolar molecules produce an attractive force.
Nonpolar-nonpolar interaction occurs between nonpolar molecules when the molecule's electrons at any given moment are distributed unevenly. This creates temporarily charged molecules whose adjacent ends having opposite signs results in an attractive force.
Van der Waals forces are much weaker than ionic, covalent, and metallic bonds.
10-4. Solubility
In a solution, the substance present in larger amount is the solvent; the other is the solute.
The concentration of a solution is the amount of solute in a given amount of solvent.
The solubility of a substance is the maximum amount that can be dissolved in a given quantity of a particular solvent at a given temperature.
A saturated solution contains the maximum amount of solute possible at a given temperature; a supersaturated solution contains more dissolved solute than is normally possible at a given temperature and is usually unstable.
The solubilities of solids increase with increasing temperatures, while the solubilities of gases in liquids decrease with increasing temperatures.
The boiling point of a solution is usually higher than that of the pure solvent, and its freezing point is lower.
10-5. Polar and Nonpolar Liquids
A polar liquid is a substance whose molecules behave as if negatively charged at one end and positively charged at the other. The molecules of a nonpolar liquid have uniform charge distributions.
Polar liquids dissolve only ionic and polar covalent compounds. Nonpolar liquids dissolve only nonpolar covalent compounds.
Dissociation refers to the separation of a compound into ions when it dissolves.
Electrolytes are substances that dissociate into ions when dissolved in water; nonelectrolytes are soluble covalent compounds that do not dissociate in solution.
Electrolytes in solution are able to conduct electric current.
10-6. Ions in Solution
Ions in solution have their own sets of properties that differ from their original atoms and from the original solute. Dissociation is a type of chemical change.
The properties of a solution of an electrolyte are the sum of the properties of the ions present in the solution.
10-7. Evidence for Dissociation
In 1887, the Swedish chemist Svante Arrhenius proposed that many substances exist as ions in solution.
Arrhenius based his hypothesis on two points:
Reactions between electrolytes in solution occur almost instantaneously, but very slowly or not at all if the electrolytes are dry.
Electrolyte solutions have lower freezing points than comparable solutions of nonelectrolytes.
10-8. Water
Seawater has an average salt content, or salinity, of 3.5 percent.
"Hard" water is freshwater that contains Ca2+ and Mg2+ ions in solution; "soft" water is free of Ca2+ and Mg2+ ions.
10-9. Water Pollution
Sources of water pollution include:
Industrial pollutants
Agricultural fertilizers and pesticides
Thermal pollution
The biochemical oxygen demand, or BOD, is the amount of oxygen needed to completely oxidize the organic material in a sample of water.
10-10. Acids
An acid is a substance that contains hydrogen and whose solution in water increases the number of H+ ions present.
The H+ ions released when an acid dissociates in water combine with water molecules to produce hydronium ions, H3O+.
The characteristic properties of acids are actually the properties of H3O+ ions rather than the properties of H+ ions.
The water solutions of acids taste sour, and acids change the color of litmus dye from blue to red.
10-11. Strong and Weak Acids
Strong acids dissociate completely; weak acids dissociate only slightly.
Some substances, such as carbon dioxide, do not contain hydrogen but produce acidic solutions by reacting with water to liberate H+ ions from water molecules.
10-12. Bases
A base is a substance that contains hydroxide groups and whose solution in water increases the number of OH-ions present.
Strong bases dissociate completely; weak bases dissociate only slightly.
Some substances, such as ammonia, do not contain OH but produce basic solutions because they react with water to release OH-ions from water molecules.
The water solutions of bases have a bitter taste, a soapy feel, and turn red litmus to blue.
The name alkali is sometimes used for a substance that dissolves in water to give a basic solution. The terms alkaline and basic mean the same.
10-13. The pH Scale
Pure water dissociates very slightly into H+ and OH- ions.
In an acidic solution, the concentration of H+ ions is greater than in pure water, and the concentration of OH- ions is lower.
In a basic solution, the concentration of OH- ions is greater than in pure water, and the concentration of H+ ions is lower.
The pH scale expresses the exact degree of acidity or basicity of a solution in terms of its H+ ion concentration.
A solution that is neither acidic nor basic is said to be neutral and has a pH of 7.
Acidic solutions have pH values of less than 7.
Basic solutions have pH values of more than 7.
A change in pH of 1 means a change in H+ ion concentration by a factor of 10.
10-14. Salts
When a basic solution is mixed with an acidic solution, the base destroys, or neutralizes, the properties of the acid and vice versa. The process is called neutralization.
In neutralization reactions, H+ and OH- ions join to form water molecules.
Ions left in solution as a result of neutralization can combine to form a salt when the solution is evaporated to dryness.
Most salts are crystalline solids that consist of positive metal ions and negative nonmetal ions.
