Reactions in Aqueous Solution: Electrolytes, Dissolution, and Solution Chemistry

Solutions and Aqueous Solutions

Definition and Components of Solutions

Solutions are homogeneous mixtures composed of two or more pure substances. The component present in the greatest amount is called the solvent, while the other components are called solutes. When water acts as the solvent, the solution is referred to as an aqueous solution.

• Solvent: Substance in greatest abundance in a solution.

• Solute: Substance(s) dissolved in the solvent.

• Aqueous solution: Solution where water is the solvent.

• Example: Salt water is an aqueous solution where water is the solvent and sodium chloride is the solute.

Dissolution Processes in Water

How Substances Dissolve in Water

Substances dissolve in water through different mechanisms depending on their chemical nature. Ionic compounds and molecular compounds behave differently when interacting with water.

• Dissociation: Ionic compounds separate into their constituent ions when dissolved in water. Water molecules surround and stabilize these ions.

• Solvation: The process by which solvent molecules surround solute particles. All substances dissolve by solvation.

• Molecular compounds: Most do not dissociate into ions but may interact with water through hydrogen bonding or other intermolecular forces. Some molecular substances may react with water upon dissolution.

• Example: Sodium chloride (NaCl) dissociates into Na+ and Cl- ions in water. Methanol (CH3OH) dissolves without forming ions.

Electrolytes and Nonelectrolytes

Classification and Behavior in Solution

Substances dissolved in water can be classified based on their ability to conduct electricity, which depends on the presence of ions in solution.

• Electrolyte: A substance that dissociates into ions when dissolved in water, allowing the solution to conduct electricity.

• Nonelectrolyte: A substance that may dissolve in water but does not dissociate into ions, so the solution does not conduct electricity.

Type	Strong Electrolyte	Weak Electrolyte	Nonelectrolyte
Ionic	All	None	None
Molecular	Strong acids (see Table 4.2)	Weak acids, weak bases	All other compounds

Strong, Weak, and Nonelectrolytes

The degree of dissociation in water determines whether a substance is a strong, weak, or nonelectrolyte.

• Strong electrolyte: Dissociates completely in water, producing a high concentration of ions. Example: NaCl.

• Weak electrolyte: Dissociates only partially in water, producing a low concentration of ions. Example: Acetic acid (CH3COOH).

• Nonelectrolyte: Does not dissociate in water; no ions are produced. Example: Sucrose (C12H22O11).

Electrical Conductivity of Solutions

The ability of a solution to conduct electricity is a direct measure of the presence of ions.

• Pure water: Does not conduct electricity (no ions).

• Nonelectrolyte solution: Does not conduct electricity (no ions).

• Electrolyte solution: Conducts electricity due to the presence of ions.

• Example: Sodium chloride solution lights a bulb in a conductivity apparatus, while sucrose solution does not.

Equations for Dissociation

Strong and weak electrolytes can be represented by chemical equations showing their behavior in water.

• Strong electrolyte (complete dissociation):

$\mathrm{HCl}(aq) \rightarrow \mathrm{H}^+(aq) + \mathrm{Cl}^-(aq)$

• Weak electrolyte (partial dissociation, equilibrium):

$\mathrm{CH}_3\mathrm{COOH}(aq) \rightleftharpoons \mathrm{H}^+(aq) + \mathrm{CH}_3\mathrm{COO}^-(aq)$

Summary Table: Electrolytic Behavior

Type of Compound	Strong Electrolyte	Weak Electrolyte	Nonelectrolyte
Ionic	All soluble ionic compounds	None	None
Molecular	Strong acids, strong bases	Weak acids, weak bases	All other molecular compounds

Additional info: Strong acids include HCl, HNO3, H2SO4, etc. Weak acids include CH3COOH. Strong bases include NaOH, KOH; weak bases include NH3.