Properties of Water
Water as a Polar Molecule
Water is made of a single oxygen atom that is covalently bonded to two hydrogen atoms. As oxygen is highly electronegative, it attracts the shared electrons more than the hydrogens do. Even though the electrons are being shared, they spend more time near the oxygen, giving oxygen a partial negative charge and the hydrogens a partial positive charge; this makes water a polar molecule.
This polarity gives water the ability to form hydrogen bonds. These hydrogen bonds are responsible for a variety of properties that are essential to life.
Cohesion
Water molecules will regularly form hydrogen bonds with each other. This is known as cohesion. It is this cohesive property of water that is responsible for the formation of water droplets.
Cohesive forces are also responsible for surface tension in water. A water molecule in the middle of a body of water is being pulled in all directions by neighboring water molecules it is bound to. However, a molecule in the surface of a body of water is not; it is not being pulled above itself and is therefore pulled inward, resulting in the molecules contracting to the smallest surface area possible. this results in the surface of water resisting external forces. It is this property that allows for things that are denser than water to float on top of it.
Adhesion
Water is not limited to only hydrogen bonding with itself. Water can hydrogen bond with other polar molecules as well, through a property known as adhesion.
Excellent Solvent
Water is often called the "universal solvent." A solvent is something that dissolves other things (the solutes) inside of it. While it's obviously not true that water is a "universal" solvent - there are a large list of things that cannot be dissolved in water, it is an excellent solvent: it dissolves more things - and more of them - than any other solvent.
Water's polarity makes it very good at dissolving polar and ionic materials. These molecules are known as hydrophilic ("water-loving") as they attract water. Charged molecules (whether it be a full or partial charge) will be surrounded by water molecules, which will hydrogen bond with them and separate them from each other, which is what results in the dissolving. In ionic molecules, such as NaCl, water holds the ions apart by surrounding the molecules much as they would for a polar molecule, thus weakening the attractive electrostatic forces between the ions.
As stated earlier, not everything can be dissolved in water. Nonpolar molecules, which cannot be dissolved by water and repel it, are known as hydrophobic ("water-fearing"). Some molecules have regions that are hydrophilic and other regions that are hydrophobic; these are known as amphipathic.
High Specific Heat
The individual hydrogen bonds between water molecules are pretty weak, but their sheer number allows for them to have a large effect. The cohesion within water leads to water having a higher melting point, boiling point, and heat of vaporization than most common solutes. This high specific heat (the amount of energy required to raise the temperature of 1g of water by 1° Celsius) results from breaking hydrogen bonds as heat is absorbed, resulting in water molecules moving around randomly, and hydrogen bonds forming (which releases small amounts of energy) as water molecules slow down due to a decreasing temperature.
Water also has a high heat of vaporization, meaning it takes a lot of energy to transition from a liquid to a gas. This results in evaporative cooling, which is why sweat helps to cool your body down and allows you to maintain your body temperature.
Less Dense as a Solid
As the temperature of water decreases, the molecules slow down and form more hydrogen bonds. In ice, water molecules will tetrahedrally hydrogen bond with four other water molecules. This regular, crystalline structure is more open than liquid water (which has constantly moving molecules that are regularly breaking and forming new hydrogen bonds), resulting in an expansion and therefore lower density.
Ice being less dense than liquid water is extremely important for life as it results in ice floating in bodies of water rather than sinking. This is very important for the aquatic organisms that live in the water. In addition, if ice sank, it would be isolated from the sun and would be permanently frozen in all but the shallowest parts, resulting in oceans of ice.
Capillary Action
Water can "climb" through thin tubes against gravity through the use of both cohesion and adhesion. When the tube molecules are more polar than water molecules, water will be more strongly attracted to it, causing water to extend upward as it contacts more and more of the tube. Due to cohesion, water will "pull along" other water molecules with it.
It is this property that helps with things such as water moving upwards in the xylem of a tree.