4.01: General Properties of Aqueous Solutions

Polar Substances

As shown in Figure $4.1.1$, the individual water molecule consists of two hydrogen atoms bonded to an oxygen atom in a bent (V-shaped) structure. As is typical of group 16 elements, the oxygen atom in each O–H covalent bond attracts electrons more strongly than the hydrogen atom does. Consequently, the oxygen and hydrogen nuclei do not equally share electrons. Instead, hydrogen atoms are electron poor compared with a neutral hydrogen atom and have a partial positive charge, which is indicated by δ⁺. The oxygen atom, in contrast, is more electron rich than a neutral oxygen atom, so it has a partial negative charge. This charge must be twice as large as the partial positive charge on each hydrogen for the molecule to have a net charge of zero. Thus its charge is indicated by 2δ⁻. This unequal distribution of charge creates a polar bond in which one portion of the molecule carries a partial negative charge, while the other portion carries a partial positive charge (Figure $4.1.1$). Because of the arrangement of polar bonds in a water molecule, water is described as a polar substance.

Because of the asymmetric charge distribution in the water molecule, adjacent water molecules are held together by attractive electrostatic (δ⁺…δ⁻) interactions between the partially negatively charged oxygen atom of one molecule and the partially positively charged hydrogen atoms of adjacent molecules (Figure $4.1.2$). Energy is needed to overcome these electrostatic attractions. In fact, without them, water would evaporate at a much lower temperature, and neither Earth’s oceans nor we would exist!

As you learned previously,, ionic compounds such as sodium chloride (NaCl) are also held together by electrostatic interactions—in this case, between oppositely charged ions in the highly ordered solid, where each ion is surrounded by ions of the opposite charge in a fixed arrangement. In contrast to an ionic solid, the structure of liquid water is not completely ordered because the interactions between molecules in a liquid are constantly breaking and reforming.

The unequal charge distribution in polar liquids such as water makes them good solvents for ionic compounds. When an ionic solid dissolves in water, the ions dissociate. That is, the partially negatively charged oxygen atoms of the H₂O molecules surround the cations (Na⁺ in the case of NaCl), and the partially positively charged hydrogen atoms in H₂O surround the anions (Cl⁻; Figure $4.1.3$). Individual cations and anions that are each surrounded by their own shell of water molecules are called hydrated ions. We can describe the dissolution of NaCl in water as

$$\begin{array}{}\text{(}\text{4.1.1}\text{)}& \mathrm{NaCl}(\mathrm{s})\stackrel{\mathrm{H}{\phantom{A}}_2\mathrm{O}(\mathrm{l})}{}\mathrm{Na}{\phantom{A}}^{+}(\mathrm{aq})+\mathrm{Cl}{\phantom{A}}^{-}(\mathrm{aq})\end{array}$$

where (aq) indicates that Na⁺ and Cl⁻ are hydrated ions.

Rule of Thumb

Polar liquids are good solvents for ionic compounds.