General Properties of Aqueous Solutions for AP Chemistry

By — McGraw-Hill Professional
Updated on Feb 9, 2011

Practice problems for these concepts can be found at:

Many of the reactions that you will study occur in aqueous solution. Water is called the universal solvent, because it dissolves so many substances. It readily dissolves ionic compounds as well as polar covalent compounds, because of its polar nature. Ionic compounds that dissolve in water (dissociate) form electrolyte solutions, which conduct electrical current owing to the presence of ions. The ions can attract the polar water molecules and form a bound layer of water molecules around themselves. This process is called solvation. Refer to the Solutions and Periodicity chapter for an in-depth discussion of solvation.

Even though many ionic compounds dissolve in water, many others do not. If the attraction of the oppositely charged ions in the solid for each other is greater than the attraction of the polar water molecules for the ions, then the salt will not dissolve to an appreciable amount. If solutions containing ions such as these are mixed, precipitation will occur, because the strong attraction of the ions for each other overcomes the weaker attraction for the water molecules.

As mentioned before, certain covalent compounds, like alcohols, readily dissolve in water because they are polar. Since water is polar, and these covalent compounds are also polar, water will act as a solvent for them (general rule of solubility: "Like dissolves like"). Compounds like alcohols are nonelectrolytes—substances that do not conduct an electrical current when dissolved in water. However, certain covalent compounds, like acids, will ionize in water, that is, form ions:

There are several ways of representing reactions that occur in water. Suppose, for example, that we were writing the equation to describe the mixing of a lead(II) nitrate solution with a sodium sulfate solution and showing the resulting formation of solid lead(II) sulfate. One type of equation that can be written is the molecular equation, in which both the reactants and products are shown in the undissociated form:

Molecular equations are quite useful when doing reaction stoichiometry problems (see Chapter 7).

Showing the soluble reactants and products in the form of ions yields the ionic equation (sometimes called the total ionic equation):

Writing the equation in the ionic form shows clearly which species are really reacting and which are not. In the example above, Na+ and NO3 appear on both sides of the equation. They do not react, but are simply there in order to maintain electrical neutrality of the solution. Ions like this, which are not actually involved in the chemical reaction taking place, are called spectator ions.

The net ionic equation is written by dropping out the spectator ions and showing only those chemical species that are involved in the chemical reaction:

This net ionic equation focuses only on the substances that are actually involved in the reaction. It indicates that an aqueous solution containing Pb2+ (any solution, not just Pb(NO3)2(aq) will react with any solution containing the sulfate ion to form insoluble lead(II) sulfate. If this equation form is used, the spectator ions involved will not be known, but in most cases this is not a particular problem since the focus is really the general reaction, and not the specific one. You will be expected to write the balanced net ionic equation for many of the reactions on the test.

Practice problems for these concepts can be found at:

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