Discuss how water is bonded together and the unique properties that results from the bonds.
Water molecules are held together by polar covalent bonds. Since oxygen is more electronegative than hydrogen, it pulls the shared electrons closer resulting in a negative charge, while the two hydrogen atoms have a slight positive charge.
The polar nature of water molecules allows it to bond with four other water molecules by hydrogen bonding. This property is called cohesion and it explains why water molecules can travel to the highest branches of trees against the downward pull of gravity. As water moves up the conducting tissue of plants called xylem tubes, water molecules below are being pulled up due to cohesion until the entire column of water moves up the tree. As some escapes the leaves as water vapor during the process of transpiration, it leaves space for more water molecules to move into the roots.
Water will sometimes stick to the surface of other substances. For example, as water moves up the xylem tubes in a plant, some of the water will stick to the sides of the conducting tubes in a process called adhesion.
Because water is polar, it is excellent as a solvent for other polar substances. Many of life's chemical reactions occur in a solution containing water.
Another property of water is that it has a high specific heat of 1 calorie/gm/degree Celsius. This is higher than most other substances. Compared to other materials, it takes water a long time to absorb enough heat energy for its temperature to increase. This is one reason that life can exist in the water and why organism's bodies which are primarily water, have relatively stable temperatures. Water can lose heat to the surrounding environment and bodies of water can provide heat during the cooler months to coastal areas, thus moderating temperature.
Water is unique because when it freezes, ice is less dense than liquid water and can float. If ice were heavier, life on Earth couldn't exist. Organisms that lived in the ocean would die if ice froze from the bottom of the sea to the top. Instead, the top of the body of water freezes and this ice floats on top of liquid water which is insulated and warm enough to allow life to exist beneath the icy covering.
To summarize, the polar covalent bonds found in water allow it to have unusual properties that allow life to exist on Earth.