2.4: Water Chemistry

Soluble salts like NaCl dissolve because the \(\rm Cl^{-}\) and \(\rm Na^{+}\) ions attract the partial positive and negative charges (respectively) of water molecules more strongly than other water molecules do. As a result, the salt ionizes; the ions separate from each other. The ionization of NaCl dissolving in water is shown below (Figure 2.8).

Figure 2.8: Water’s solvent properties result from its polar covalent structure, allowing electrostatic interactions between water molecules at the right and NaCl at the upper left, disrupting ionic bonds, drawing the \(\rm Na^{+}\) and \(\rm Cl^{-}\) ions into solution.

Water is also a good solvent for macromolecules (e.g., proteins and nucleic acids) with exposed polar chemical groups on their surface that attract water molecules (Figure2.9).

Figure 2.9: In hydrophilic interactions, charged groups on a macromolecule (e.g., a protein) attract the partial charges on water molecules, hydrating the molecule.

125-2 Water: Hydrogen & Ionic Bonds

CHALLENGE

What are some examples on insoluble salts and why don’t they dissolve in water? See if you can reason this out... or look it up!

In addition to its being a good solvent, we recognize the following properties of water (all of which result from its polar nature and H-bonding abilities):

• Cohesion: the ability of water molecules to stick together via H-bonds.
• High Surface tension: water’s high cohesion means that it can be hard to break the surface; think the water strider, an insect that ‘walks’ on water.
• Adhesion: this results from water’s electrostatic interactions with ions and the partial charges on polar covalent molecules or functional groups. Adhesion explains water’s solvent properties and (at least in part) capillary action where water molecules ‘crawl’ along hydrophilic surfaces, often against the force of gravity.
• High specific heat: The cohesion of water molecules is so strong that it takes a lot of energy to separate the molecules and make them move faster, i.e., to heat water; specifically it takes 1 Kcal, (1 Calorie, with a capital C) to heat a gram of water 1oC. Incidentally, high specific heat also explains why water “holds its heat” (i.e., stays hotter longer that the pot that it’s in!).
• High heat of vaporization: It takes even more energy per gram of water to turn it into water vapor!

One last property of water: it ionizes weakly to form \(\rm H^{+}\) and \(\rm OH^{-}\) ions—or more correctly, \(\rm H_3O^{+}\) and \(\rm OH^{-}\) ions. You can think of this as happening in the following two reactions:

Acid molecules added to water will dissociate and release protons. This drives reaction 2 to form more \(\rm H_3O^{+}\) ions in the solution, in turn driving reaction 1 forward. A pH meter measures the relative acidity or concentration of protons in a solution. Acidic solutions have a pH below 7.0 (neutrality). Bases ionizing in water release hydroxyl (\(\rm OH^{-}\)) ions. The increase in \(\rm OH^{-}\) ions removes protons from the solution, driving both reactions in reverse and raising the pH of the solution. To summarize acid-base chemistry: when dissolved in water, acids release \(\rm H^{+}\), while bases accept \(\rm H^{+}\). Since the pH of a solution is the negative logarithm of the hydrogen ion concentration, the following are true:

• at pH 7.0, a solution is neutral
• below a pH of 7.0, a solution is acidic
• above a pH of 7.0, a solution is basic

Check a basic chemistry book to remind yourself of the relationship between pH and the [\(\rm H^{+}\)] in a solution!