• Aqueous solutions: water is the dissolving medium or solvent.

4.1 Water, the Common Solvent

• polar molecule: molecules with unequal charge distribution

  • $\delta$ indicates a partial charge (less than one unit of charge).

  It is this polarity that gives water its greater ability to dissolve compounds.

• Hydration is the process in which the “positive end” of the water molecules are attracted to the negatively charged anions and the “negative ends” are attracted to the positively charged cations.

• It is important to recognize that when ionic substances (salts) dissolve in water, they break into individual cations and anions.

  $\ce{NH4NO3(s) \xrightarrow{\ce{H2O(l)}} NH4+(aq) + NO3-(aq)}$

• The solubility of ionic substances in water varies greatly.

  • The differences in the solubilities of ionic compounds in water typically depend on the relative attractions of the ions for each other and the attractions of the ions for water molecules.

• Water also dissolves many nonionic substances.

  Ethanol ($\ce{C2H5OH}$), for example, is very soluble in water. The molecule contains a polar $\ce{O-H}$ bond like those in water, which makes it very compatible with water.

• Many substances do not dissolve in water.

  Pure water will not, for example, dissolve animal fat because fat molecules are nonpolar and do not interact effectively with polar water molecules.

4.2 The Nature of Aqueous Solutions: Strong and Weak Electrolytes

• Solute dissolves in solvent.
• Electrical conductivity: the ability to conduct an electric current.
  • Some aqueous solutions conduct current very efficiently; these solutions contain strong electrolytes.
  • Other solutions conduct only a small current; these solutions contain weak electrolytes.
  • Some solutions permit no current to flow; these solutions contain nonelectrolytes.

Strong Electrolytes

• Strong electrolytes are substances that are completely ionized when they are dissolved in water.

  $\ce{HCl \xrightarrow{\ce{H2O}} H+(aq) + Cl-(aq)}$ $\ce{HNO3 \xrightarrow{\ce{H2O}} H+(aq) + NO3-(aq)}$ $\ce{H2SO4 \xrightarrow{\ce{H2O}} H+(aq) + HSO4-(aq)}$

• Arrhenius proposed that an acid is a substance that produces $\ce{H+}$ ions (protons) when it is dissolved in water.

  • $\ce{HA(aq) + H2O(l) → H3O+(aq) + A-(aq)}$

  • Strong acids completely ionize when placed in water.

    If 100 molecules of $\ce{HCl}$ are dissolved in water, 100 $\ce{H+}$ ions and 100 $\ce{Cl-}$ ions are produced. Virtually no $\ce{HCl}$ molecules exist in aqueous solutions.

  • Strong bases: soluble ionic compounds containing the hydroxide ion ($\ce{OH-}$).

    $\ce{NaOH(s) \xrightarrow{\ce{H2O}} Na+(aq) + OH-(aq)}$ $\ce{KOH(s) \xrightarrow{\ce{H2O}} K+(aq) + OH-(aq)}$

Weak Electrolytes

• Weak acid: Any acid that dissociates only to a slight extent in aqueous solutions is called a weak acid.