# 7.5: Aqueous Solutions and Solubility - Compounds Dissolved in Water

Learning Objectives

• Define and give examples of electrolytes.

When some substances are dissolved in water, they undergo either a physical or a chemical change that yields ions in solution. These substances constitute an important class of compounds called electrolytes. Substances that do not yield ions when dissolved are called nonelectrolytes. If the physical or chemical process that generates the ions is essentially 100% efficient (all of the dissolved compound yields ions), then the substance is known as a strong electrolyte. If only a relatively small fraction of the dissolved substance undergoes the ion-producing process, it is called a weak electrolyte.

Substances may be identified as strong, weak, or nonelectrolytes by measuring the electrical conductance of an aqueous solution containing the substance. To conduct electricity, a substance must contain freely mobile, charged species. Most familiar is the conduction of electricity through metallic wires, in which case the mobile, charged entities are electrons. Solutions may also conduct electricity if they contain dissolved ions, with conductivity increasing as ion concentration increases. Applying a voltage to electrodes immersed in a solution permits assessment of the relative concentration of dissolved ions, either quantitatively, by measuring the electrical current flow, or qualitatively, by observing the brightness of a light bulb included in the circuit (Figure $$\PageIndex{1}$$).

## Ionic Electrolytes

Water and other polar molecules are attracted to ions, as shown in Figure $$\PageIndex{2}$$. The electrostatic attraction between an ion and a molecule with a dipole is called an ion-dipole attraction. These attractions play an important role in the dissolution of ionic compounds in water.

When ionic compounds dissolve in water, the ions in the solid separate and disperse uniformly throughout the solution because water molecules surround and solvate the ions, reducing the strong electrostatic forces between them. This process represents a physical change known as dissociation. Under most conditions, ionic compounds will dissociate nearly completely when dissolved, and so they are classified as strong electrolytes.

Let us consider what happens at the microscopic level when we add solid KCl to water. Ion-dipole forces attract the positive (hydrogen) end of the polar water molecules to the negative chloride ions at the surface of the solid, and they attract the negative (oxygen) ends to the positive potassium ions. The water molecules penetrate between individual K+ and Cl ions and surround them, reducing the strong interionic forces that bind the ions together and letting them move off into solution as solvated ions, as Figure $$\PageIndex{2}$$ shows. The reduction of the electrostatic attraction permits the independent motion of each hydrated ion in a dilute solution, resulting in an increase in the disorder of the system, as the ions change from their fixed and ordered positions in the crystal to mobile and much more disordered states in solution. This increased disorder is responsible for the dissolution of many ionic compounds, including KCl, which dissolve with absorption of heat.

In other cases, the electrostatic attractions between the ions in a crystal are so large, or the ion-dipole attractive forces between the ions and water molecules are so weak, that the increase in disorder cannot compensate for the energy required to separate the ions, and the crystal is insoluble. Such is the case for compounds such as calcium carbonate (limestone), calcium phosphate (the inorganic component of bone), and iron oxide (rust).

## Solubility Rules

Some combinations of aqueous reactants result in the formation of a solid precipitate as a product. However, some combinations will not produce such a product. If solutions of sodium nitrate and ammonium chloride are mixed, no reaction occurs. One could write a molecular equation showing a double-replacement reaction, but both products, sodium chloride and ammonium nitrate, are soluble and would remain in the solution as ions. Every ion is a spectator ion and there is no net ionic equation at all. It is useful to be able to predict when a precipitate will occur in a reaction. To do so, you can use a set of guidelines called the solubility rules (Tables $$\PageIndex{1}$$ and $$\PageIndex{2}$$).

Table $$\PageIndex{1}$$: Solubility Rules for Soluble Substances
Soluble in Water Important Exceptions (Insoluble)
All Group IA and NH4+ salts none
All nitrates, chlorates, perchlorates and acetates none
All sulfates CaSO4, BaSO4, SrSO4, PbSO4
All chlorides, bromides, and iodides AgX, Hg2X2, PbX2 (X= Cl, Br, or I)

Table $$\PageIndex{2}$$: Solubility Rules for Sparingly Soluble Substances

Sparingly Soluble in Water Important Exceptions (Soluble)
All carbonates and phosphates Group IA and NH4+ salts
All hydroxides Group IA and NH4+ salts; Ba2+, Sr2+, Ca2+ sparingly soluble
All sulfides Group IA, IIA and NH4+ salts; MgS, CaS, BaS sparingly soluble
All oxalates Group IA and NH4+ salts
Special note: The following electrolytes are of only moderate solubility in water: CH3COOAg, Ag2SO4, KClO4. They will precipitate only if rather concentrated solutions are used.

As an example on how to use the solubility rules, predict if a precipitate will form when solutions of cesium bromide and lead (II) nitrate are mixed.

$\ce{Cs^+} \left( aq \right) + \ce{Br^-} \left( aq \right) + \ce{Pb^{2+}} \left( aq \right) + 2 \ce{NO_3^-} \left( aq \right) \rightarrow ? \nonumber$

The potential precipitates from a double-replacement reaction are cesium nitrate and lead (II) bromide. According to the solubility rules table, cesium nitrate is soluble because all compounds containing the nitrate ion, as well as all compounds containing the alkali metal ions, are soluble. Most compounds containing the bromide ion are soluble, but lead (II) is an exception. Therefore, the cesium and nitrate ions are spectator ions and the lead (II) bromide is a precipitate. The balanced net ionic reaction is:

$\ce{Pb^{2+}} \left( aq \right) + 2 \ce{Br^-} \left( aq \right) \rightarrow \ce{PbBr_2} \left( s \right) \nonumber$

Example $$\PageIndex{1}$$: Solubility

Classify each compound as soluble or insoluble

1. Zn(NO3)2
2. PbBr2
3. Sr3(PO4)2

Solution

1. All nitrates are soluble in water, so Zn(NO3)2 is soluble.
2. All bromides are soluble in water, except those combined with Pb2+, so PbBr2 is insoluble.
3. All phosphates are insoluble, so Sr3(PO4)2 is insoluble.

Exercise $$\PageIndex{1}$$: Solubility

Classify each compound as soluble or insoluble.

1. Mg(OH)2
2. KBr
3. Pb(NO3)2
insoluble