Why is this Important? Example: Water – A Polar Covalent Substance
What we have learned about the water molecule in this unit:
The bonds in a water molecule (H2O) are covalent because the electrons are shared between the oxygen atom and the hydrogen atoms (as opposed to an ionic bond where electrons are transferred from one atom to another). Nevertheless, the electrons are more attracted by the oxygen atom than by the hydrogen atoms, because oxygen is more electronegative than hydrogen. Consequently, there is an unequal charge distribution within the molecule. The oxygen atom holds a negative partial charge (δ-) while the hydrogen atoms hold positive partial charges (δ+). These partial charges in combination with the bent shape of the water molecule (which results from the two lone electron pairs) make water a dipole, i.e. a polar molecule (fig. 1).
Figure 1: Dipole character of water
Why this is interesting:
The polar character of water has far reaching consequences for its properties. The partial charges of one water molecule attract the respective opposite partial charges of other water molecules (fig. 2). Due to the relatively strong attraction between the water molecules (+/- attraction), water has a relatively high melting and boiling point, as compared to less polar substances.
Figure 2: Structure and bonding of water molecules
[Qwerter, M. Maňas, Magasjukur2, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license, https://commons.wikimedia.org/wiki/File:3D_model_hydrogen_bonds_in_water.svg, modified]
Because of the attraction of + and – charges, many other polar substances as well as ionic substances such as table salt, NaCl, are soluble in water (fig. 3).
Figure 3: Schematic representation of the dissolution of NaCl in water. The polar nature of the water molecules relaxes the strong ionic attractions between ions in the crystal lattice and leads to dissolution of NaCl. [http://www.biology.arizona.edu/biochemistry/tutorials/chemistry/page3.html]
Nonpolar substances, on the other hand, are not soluble in water. This is why oil, a nonpolar substance, and water don’t mix (fig. 4).
Figure 4: Oil spill [http://www.greenpeace.org/usa/90000-gallon-oil-spill-looks-like/]
- J.C. Crittenden et al., 2012. MWH's Water Treatment: Principles and Design, p. 234 and pp. 20. John Wiley & Sons, Inc., Hoboken (NJ).
- S.E. Manahan, 2005. Environmental Chemistry, 8th edition, p. 51. CRC Press LLC, Boca Raton.