water

=__Chapter 3 - Water and the Fitness of the Environment__= //by Connie Chuensumran//

=Objectives -=

**Effects on Water's Polarity**

 * The polarity of water molecules results in hydrogen bonding
 * Organisms depend on the cohesion of water molecules
 * Water moderates temperatures on Earth
 * Oceans and lakes don't freeze solid because ice floats
 * Water is the solvent of life

=Vocabulary -=
 * **cohesion** - the act or state of cohering, uniting, or sticking together.
 * **polar molecule** - a molecule in which the centroid of the positive charges is different from the centroid of the negative charges.
 * **adhesion** - the molecular force of attraction in the area of contact between unlike bodies that acts to hold them together.
 * **surface tension** - A property of liquids arising from unbalanced molecular cohesive forces at or near the surface, as a result of which the surface tends to contract and has properties resembling those of a stretched elastic membrane.
 * **kinetic energy** - the energy of a body or a system with respect to the motion of the body or of the particles in the system.
 * **solution** - A homogenous mixture of one or more solutes dissolved in a solvent.
 * **solvent** - a substance that dissolves another to form a solution.
 * **solute** - the part that is being dissolved by a solvent in a solution.
 * **aqueous solution** - a solution in which water is the solvent.
 * **hydrophilic** - substances that have an affinity for water.
 * **hydrophobic** - compounds that do not dissolve easily in water.

=Water comes in 3 forms -=


 * **Solid** - Below freezing temperature, 32 degrees Fahrenheit.
 * **Liquid** - Between freezing and boiling.
 * **Gas** - Above the boiling point, 212 degrees Fahrenheit.



=Properties of Water -=

Water molecules are polar, one end has a negative charge and the other end has a positive charge. The hydrogen end has a positive charge, while the oxygen end has a negative charge. The hydrogen of other water molecules are attracted to the oxygen end of water molecules, this gives water it's cohesive and adhesive properties. These bonds form, break and reform with great frequency.
 * Cohesion & Adhesion -**
 * Video -** [|Water: To Stick Or Not To Stick]

The interface between water and air is an ordered arrangement or water molecules hydrogen-bonded to one another and to the water below. An example of surface tension is when a drop of water is placed on a penny and it creates a round shape because the molecules are attracted to each other and not to the penny. Surface tension also allows water striders to 'walk on water'.
 * Surface Tension** -
 * Video** - [|Surface Tension Demo]



Water stabilizes air temperatures on Earth by absorbing heat from the air that is warmer and releasing heat to the air that is cooler. Ice cubes also absorb heat as they melt. As water boils, the molecules are moving so fast that they are able to overcome the attraction to each other and can depart from the liquid phase and enter the gas phase.
 * Ability to stabilize temperature -**
 * Learn more** - [|Evaporation Vs. Boiling]

Most liquids contract when they reach a freezing point, but water expands as it freezes. Water beings to freeze when the molecules are no longer moving fast enough to break it's hydrogen bonds. When the temperature reaches about 0 degrees Celcius, water locks into a crystal lattice structure. Ice floats because it's less dense than liquid water.
 * Expansion upon freezing -**
 * Video** - [|Time lapse of water freezing]

Capillary action is when water is attracted to the walls of an object, like a straw, and each molecule moves up the surface pulling other molecules up with it too. The surface tension of water holds the surface together, thus pulling up the whole liquid surface and not just the edges. Capillary action can only go up so far before the pull of gravity weighs it down.
 * Capillary Action** -