Water

Water has many unique properties that make it essential to all life. Most of water's unique properties are a result of the hydrogen bonding between water molecules.

• Water is an excellent solvent. When ionic compounds are placed into water, the ions dissociate or separate. Polar covalent compounds, because they too have charged poles, also dissolve in water. Nonpolar covalent compounds, however, do not dissolve in water. Thus polar covalent compounds are hydrophilic (water loving) while nonpolar covalent compounds are hydrophobic (water fearing).
  
  
• Water has high cohesion. Individual water molecules tend to "stick" with other water molecules due to hydrogen bonding. This leads to two characteristics of water: a high surface tension and strong capillary action. Water's high surface tension is what allows some insects to run across the surface of the water. It is also the reason why belly-flops hurt so much. Water's strong capillary action is what allows the liquid level in a straw to be higher than that in the surrounding drink. Using hydrogen bonding, water molecules attract others up the sides of the straw. This effect is the more noticeable in thinner straws.
  
  
  Image by Chernobyl Frog Dissections
  
  
• Water has a high specific heat capacity. It takes a lot of energy to change the temperature of water. This characteristic makes water a good insulator and a good coolant. When you sweat, your body is using water as a coolant. The evaporating water removes heat with it. At night, oceans are a good insulator. The energy that the sun spent in heating the water all day is slowly released into the night.
  
  
• Finally, water is the most dense at 4 degrees Celsius. Hydrogen bonding arranges water molecules into hollow "cells" when water freezes, making it less dense than liquid water. This characteristic of water is what allows fish in lakes and ponds to survive in the winter. When the water freezes, it becomes less dense and floats to the surface -- leaving the bottom of the lake or pond unfrozen.

Image by Chernobyl Frog Dissections