Solubility rules for ionic compounds in water

TABLE 2.1 Solubility Rules for Ionic Compounds in Water... 

Water. It should come as no surprise that ordinary water can be an excellent solvent for many samples. Due to its extremely polar nature, water will dissolve most substances of likewise polar or ionic nature. Obviously, then, when samples are composed solely of ionic salts or polar substances, water would be an excellent choice. An example might be the analysis of a commercial iodized table salt for sodium iodide content. A list of solubility rules for ionic compounds in water can be found in Table 2.1. 

Table 9.6 lists the solubility rules for ionic compounds in water. 

Suppose you are trying to help your friend understand the general solubility rules for ionic substances in water. Explain in general terms to your friend what the solubility rules mean, and give an example of how the rules could be applied in determining the identity of the precipitate in a reaction between solutions of two ionic compounds. 

We can assume that the precipitate that formed in our test tube must be one (or both) of the possible products from our word equation. In order to identify which of the products formed, we would need to find out which of the products is insoluble in water and would therefore precipitate out. Our most handy reference tables for this type of problem will contain the solubility rules for ionic compounds, shown here, and/or a solubility table (see Figure 6-3a). 

TABLE 4.2 Solubility Rules for Common Ionic Compounds in Water at 25°C... 

The general rules for predicting the solubiUty of ionic compounds in water were introduced in Section 4.2. Although useful, these solubility rules do not enable us to make quantitative predictions about how much of a given ionic compound will dissolve in water. To develop a quantitative approach, we start with what we already know about chemical equilibrium. Unless otherwise stated, in the following discussion the solvent is water and the temperature is 25°C. 

You should refer to Table 4.1 for the solubility rules of ionic compounds to see which compounds are soluble in water. The soluble ionic compounds are those that readily dissolve in water. The insoluble compounds hardly dissolve at all. 

Recall from Section 7.5 that a compound is considered soluble if it dissolves in water and insoluble if it does not. Recall also that, by applying the solubility rules (Table 7.2), we can classify many ionic compounds as soluble or insoluble. We can better understand the solubility of an ionic compound with the concept of equilibrium. The process by which an ionic compound dissolves is an equilibrium process. For example, we can represent the dissolving of calcium fluoride in water with the chemical equation ... 

To predict what will happen without doing experiments, we need some information about which sorts of ionic compounds are water soluble and which are water insoluble. We expect the insoluble ones to form when the appropriate ions are present in solution. We don t have all-encompassing rules for predicting solubilities, but a few guidelines work for the majority of common ionic solutes. A concise form of these guidelines is presented in Table 5.1. 

The general criterion for solubility is the rule that like dissolves like . In other words polar solvents dissolve polar and ionic solutes, non-polar solvents dissolve non-polar solutes. In the case of water, this means that ionic compounds such as sodium chloride and polar compounds such as sucrose are soluble, but non-polar compounds such as paraffin wax are not. 

Notice that the title of the rules is general solubility rules. There are exceptions to the rules listed there. For example, although ionic compounds that contain halogens tend to be soluble, when the cation is lead (II) (as in lead (II) chloride or lead (II) iodide) the ionic compound will be insoluble in water. The rules listed are really meant to be general trends, which you may want to memorize. They will give you the ability to make fast predictions about the identity of a precipitate that is found after many ionic reactions, just not the one in our first example. 

General Rules for Solubility of Ionic Compounds (Salts) in Water at 25 °C... 

In the following sections we will look at how a compound s composition lets us predict whether it is a strong electrolyte, weak electrolyte, or nonelectrolyte. For the moment, you need only to remember that water-soluble ionic compounds are strong electrolytes. Ionic compounds can usually be identified by the presence of both metals and nonmetals [for example, NaCl, FeS04, and A1(N03)3]. Ionic compounds containing the ammonium ion, NH [for example, NH4Br and (NH4)2C03], are exceptions to this rule of thumb. 

In a precipitation reaction, an insoluble ionic compound forms when solutions of two soluble ones are mixed. The electrostatic attraction between certain pairs of solvated ions is strong enough to overcome the attraction of each ion for water. Based on a set of solubility rules, we can predict whether a precipitate will form by noting which of all possible cation-anion combinations is insoluble. 

With forensic analyses of toxicological and drug evidence, water solubility of the analytes is of paramount concern for sample preparation and extraction. Solubility is also important toxicologically, since it plays a role in determining how, where, and how quickly a drug is absorbed. The like-dissolves-like rule stiU applies but is broadened to include acid-base character and solubility of salts. Thus, target analytes may be ionic compounds, molecular compounds, or in the case of many drugs, salts like cocaine hydrochloride (cocaine HCl). 

---