A solution is a homogeneous mixture of two or more substances. One of these substances is called the solvent; it is usually the component that is present in greater quantity. The other substances in the solution are known as solutes; they are said to be dissolved in the solvent. For example, when a small amount of sodium chloride, NaCl, is dissolved in a large quantity of water, we refer to water as the solvent and sodium chloride as the solute.
Imagine preparing two aqueous solutions—one by dissolving a teaspoon of table salt (sodium chloride) in a cup of water and the other by dissolving a teaspoon of table sugar (sucrose) in a cup of water. Both solutions are clear and colorless. How do they differ? One way, which might not be immediately obvious, is in their electrical conductivity: The salt solution is a good conductor of electricity, whereas the sugar solution is not.
Whether or not a solution conducts electricity can be determined by using a device such as that shown in Figure 4.2. To light the bulb, an electric current must flow between the two electrodes that are immersed in the solution. Although water itself is a poor conductor of electricity, the presence of ions causes aqueous solutions to become good conductors. Ions carry electrical charge from one electrode to another, completing the electrical circuit. Thus, the conductivity of NaCl solutions can be attributed to the presence of ions in the solution.
A substance (such as NaCl) whose aqueous solutions contain ions and hence conduct electricity is called an electrolyte. A substance (such as sucrose, C12H22O11) that does not form ions in solution is called a nonelectrolyte. The difference between the two substances is due largely to the fact that sodium chloride is an ionic compound, whereas sugar is a molecular one. Let's examine solutions of each of these kinds of compounds.
Recall that solid NaCl consists of Na+ and Cl– ions in an orderly arrangement (Figure 2.19). When NaCl is dissolved in water, each ion is separated from the solid structure and dispersed throughout the solution. The ionic solid is said to dissociate into its component ions as it dissolves.
Water is a very effective solvent for ionic compounds. Although water is an electrically neutral molecule, one end of the molecule (the O atom) is rich in electrons and thus possesses a partial negative charge. The other end (the H atoms) has a partial positive charge. Positive ions (cations) are attracted by the negative end of H2O, and negative ions (anions) are attracted by the positive end. As an ionic compound dissolves, the ions become surrounded by H2O molecules as shown in Figure 4.3(a). This process helps stabilize the ions in solution and prevents cations and anions from recombining. Furthermore, because the ions and their shells of surrounding water molecules are free to move about, the ions become dispersed uniformly throughout the solution.
Figure 4.3 (a) Dissolution of an ionic compound. When an ionic compound dissolves in water, H2O molecules separate, surround, and disperse the ions into the liquid. (b) Methanol, CH3OH, a molecular compound, dissolves without forming ions.
We can usually predict the nature of the ions present in a solution of an ionic compound from the chemical name of the substance. For example, sodium sulfate, Na2SO4, dissociates into sodium ions, Na+, and sulfate ions, SO42–. It is important that you remember the formulas and charges of common ions (Tables 2.4 and 2.5) to understand the forms in which ionic compounds exist in aqueous solution.
When a molecular compound dissolves in water, the structural integrity of the dissolving molecule is usually maintained, so that the solution consists of individual molecules dispersed throughout the solution. In other words, most molecular compounds do not form ions when they dissolve; consequently, they are nonelectrolytes. For example, a solution of methanol, CH3OH, in water consists of CH3OH molecules dispersed throughout the water. There are no ions formed from the solute [Figure 4.3(b)].
There are, however, important exceptions to this behavior. Some molecular solutes interact strongly with water, and ions are formed in the process. In particular, acids and a few other molecular compounds such as ammonia, NH3, react with water to form ions and are consequently electrolytes. For example, when HCl(g) dissolves in water to form hydrochloric acid, HCl(aq), it ionizes or breaks apart into H+(aq) and Cl–(aq) ions in solution.
There are two categories of electrolytes. Essentially all ionic compounds (such as NaCl) and a few molecular compounds (such as HCl) exist in solution completely or nearly completely as ions. Such compounds are called strong electrolytes. There are also some molecular compounds that produce a small concentration of ions when they dissolve; they are called weak electrolytes. For example, in a solution of acetic acid, HC2H3O2, most of the solute is present as HC2H3O2 molecules. Only a small fraction (about 1 percent) of the HC2H3O2 is present as H+(aq) and C2H3O2–(aq) ions.
We must be careful not to confuse the extent to which an electrolyte dissolves with whether it is strong or weak. For example, HC2H3O2 is extremely soluble in water but is a weak electrolyte. In contrast, Ba(OH)2 is not very soluble, but the amount of the substance that does dissolve dissociates almost completely. Therefore, Ba(OH)2 is a strong electrolyte.
When a weak electrolyte such as acetic acid ionizes in solution, we write the reaction in the following manner:
The double arrow means that the reaction is significant in both directions. At any given moment some HC2H3O2 molecules are ionizing to form H+ and C2H3O2–. At the same time, H+ and C2H3O2– ions are recombining to form HC2H3O2. The balance between these opposing processes determines the relative concentrations of ions and neutral molecules. This balance produces a state of chemical equilibrium that varies from one weak electrolyte to another. Chemical equilibria are extremely important, and we will devote several later chapters to examining them.
Chemists use a double arrow to represent the ionization of weak electrolytes and a single arrow to represent the ionization of strong electrolytes. For example, because HCl is a strong electrolyte, we write the equation for the ionization of HCl as follows:
The single arrow indicates that the H+ and Cl– ions have no tendency to recombine in water to form HCl molecules.
In the sections ahead we will begin to look more closely at how we can use the composition of a compound to predict whether it is a strong electrolyte, weak electrolyte, or nonelectrolyte. For the moment it is important only to remember that soluble ionic compounds are strong electrolytes. We identify ionic compounds as being ones composed of metals and nonmetals [such as NaCl, FeSO3, and Al(NO3)3], or compounds containing the ammonium ion, NH4+, such as NH4Br and (NH4)2CO3.