Solubility Guidelines for Ionic Compounds in Water

When an ionic substance such as sodium chloride dissolves in water, the water molecules remove individual ions from the three-dimensional solid stmcture and surround them. This process, called 

Classify each of the following compounds as a nonelectrolyte, a weak electrolyte, or a strong electrolyte (a) methanol (CH3OH), (b) sodium hydroxide (NaOH), (c) ethylamine (C2H5NH2), and (d) hydrofluoric acid (HF). 

Strategy Classify each compound as ionic or molecular. Soluble ionic compounds are strong electrolytes. Classify each molecular compound as an acid, base, or neither. Molecular compounds that ate neither acids nor bases ate nonelectrol)des. Molecular compounds that are bases are weak electrolytes. Finally, classify acids as either strong or weak. Strong acids are strong electrolytes, and weak acids ate weak electrolytes. 

Setup (a) Methanol contains neither a metal cation nor the ammonium ion. It is therefore molecular. Its formula does not begin with H, so it is probably not an acid, and it does not contain a nitrogen atom, so it is not a weak base. Molecular compounds that are neither acids nor bases are nonelectrofytes. 

Practice Problem A Identify the following compounds as nonelectrolytes, weak electrolytes, or strong electrolytes ethanol (C2H5OH), nitrous acid (HNO2), and sodium hydrogen carbonate (NaHCOs, also known as bicarbonate). 

Solubility is defined as the maximum amount of solute that will dissolve in a given quantity of solvent at a specific temperature. Not all ionic compounds dissolve in water. Whether or not an ionic compound is water soluble depends on the relative magnitudes of the water molecules attraction to the ions, and the ions attraction for each other. We will learn more about the magnitudes of attractive forces in ionic compounds in Chapter 8, but for now it is useful to learn some 

The partial charges on the oxygen atom and the hydrogen atoms sum to zero. Water moleciiles, although polar, have no net charge. You wl learn more about partial charges and moteoular polarity in Chapters 8 and 9. 

If the water molecules attraction for the ions exceeds the ions attraction to one another, then the ionic compound will dissolve. If the kms attraction to each other exceeds the water molecules attraction to the ions, then the comfxxnd won t dissolve. 

Some books list fewer exceptions to these solubility rules. In fact, ionic compounds listed here as insoluble are actually very stilly soluble. It is how soluble a compound must be to be called soluble that may vary from book to book, 

Ionic compounds often are classified according to the anions they contain. Compounds that contain the chloride ion are called chlorides, compounds containing the nitrate ion are called nitrstes, and so on. 

---

Solubility guidelines for ionic compounds in water at room temperature... 

Active Figure 9.13 Solubility guidelines for ionic compounds. If a compound contains at least one of the ions in the Solnble Compounds list, apart from the exceptions listed, it is likely to be at least moderately soluble in water. Componnds with at least one ion in the Insoluble Compounds list are poorly soluble in water (again, with the exception of the few compounds in the Exceptions list). Watch this Active Figure at http //now.brookscole.com/cracolice3e. 

Solubility Guidelines for Common Ionic Compounds in Water Generally Soluble Exceptions... 

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. 

Solution (a) Potassium phosphate and silver nitrate are both ionic compounds. Potassium phosphate contains K and PO4 - ions, so its chemical formula is K 4. Silver nitrate contains Ag and NO3- ions, so its chemical formula is AgNOs. Eiecause both reactants are strong electrolytes, the solution contains K, PO4 -, Ag and NO3- ions before the reaction occurs. According to the solubility guidelines in Table 4.1, Ag and P04 form an insoluble compound, so Ag3P04 will precipitate from the solution. In contrast, K" " and NO3- will remain in solution because KNO3 is water soluble. Thus, the balanced molecular equation for the reaction is... 

---