Essential Concepts Chapter 13: Water and its Solutions

Obj 13.1 Demonstrate and explain the uniqueness of water as a chemical substance

Water is a very simple molecule consisting of one oxygen atom and two hydrogen atoms. But its composition, electron structure, and shape give it unique properties that are essential for maintaining life and the stability of earth's climate. Water is the only substance on Earth that exists in large quantities in all three states, gas, liquid and solid. Unlike most substances that are denser as solids than liquids, ice is less dense than water and thus forms on the surface of water.

Water has some other unusual physical characteristics:

  • Other substances of similar size exist as gases at room temperature, but water is a liquid due to the relatively strong attraction of water molecules for each other.
  • Water has an incredibly high specific heat, meaning it takes a lot of energy (heat or heat loss) to change its temperature. Water is slow to heat up or cool down. This characteristic is critical because it results in a fairly stable temperature range on Earth
  • Water has high surface tension, meaning it's hard to break the surface of water, again due to the strong attraction of water particles for one another.
  • Water exhibits capillary action, meaning it will travel up thin tubes, or form a meniscus in larger tubes (like graduated cylinders).

Obj 13.2 Explain and illustrate why water is a polar substance

A water molecule is asymmetric. Although neutral overall, the centers (poles) of the positive and negative charges are in different places. The side of the molecule with hydrogens has a slight positive charge (a positive dipole, +) whereas the opposite side of the molecule has a slight negative charge (a negative dipole, -). A substance made of asymmetric molecules with positive and negative dipoles is termed a polar substance.

Obj 13.3 Illustrate hydrogen bonding and explain how such bonding contribute to water's characteristics

Opposites attract, at least as far as electrical charges are concerned. A slightly positive hydrogen atom of one molecule will have an attraction for a slightly negative oxygen atom of a neighboring molecule. This attraction results in a weak bond called a hydrogen bond. These hydrogen bonds between adjacent molecules are not nearly as strong as the covalent bonds that hold a water molecule together. But hydrogen bonding contributes to the unusual characteristics of water. This is why water is cohesive, why water has a high boiling point (it's hard to get molecules to break away), and why water exhibits surface tension.