2.4: A Close Look at Water Chemistry
1. Hydrogen Bonds, the Polarity and Properties of Water
Hydrogen bonds are a subcategory of electrostatic interaction (i.e., formed by the attraction of oppositely charges). As noted above, water molecules attract one another ( cohere ) because of strong electrostatic interactions that form H-bonds. Because of water’s polar covalent nature, it is able to attract positively and negatively charged groups of solutes, making it a good solvent. Solutes (water-soluble molecules) or molecular surfaces attracted to water are hydrophilic . Lipids like fats and oils are not polar molecules and therefore do not dissolve in water; they are hydrophobic (from hydro, water; phobic, fearing) .
Soluble salts like NaCl dissolve because the Cl- and Na+ ions more strongly attract the partial positive and negative charges (respectively) of water molecules. The result is that the ions separate. We call this separation of salt ionization. The ionization of NaCl dissolving in water is shown below.
Water is also a good solvent for macromolecules (proteins, nucleic acids) with exposed polar chemical groups on their surfaces that attract water molecules, as shown below.
125 Water, Hydrogen & ionic Bonds
In addition to 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!
2. Water Ionization and pH
One last property of water: it ionizes weakly to form H+ and OH- ions, - or more correctly, pairs of water molecules form H3O+ and OH- ions. You can think of this as happening in the following two reactions:
Acid molecules added to water dissociate and release protons. This drives reaction #2, forming more H3O+ 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 OH- (hydroxyl) ions. The increase in OH- ions removes protons from the solution, driving both reaction in reverse and raising the pH of the solution.
To summarize acid-base chemistry:
When dissolved in water
- Acids release H+
- Bases accept H+
Since the pH of a solution is the negative logarithm of the hydrogen ion concentration,
PH and solution
- 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 [H+] in a solution!