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THE SPECIAL CASE OF WATER
Water is the single most significant inorganic molecule in all life forms. It promotes complexity because of its tendency to dissolve a broad
spectrum of both inorganic and organic molecules. Because of its polar quality, it promotes the dissociation of many molecules into ions,
which play a role in regulating such biological properties as muscle contraction, permeability, and nerve impulse transmission.
Why must both sides of a chemical equation balance? Balance the equation for the production of water from elemental hydrogen (H,)
and oxygen (O,)."
Because the law of conservation of matter tells us that matter can be neither created nor destroyed, all equations must be balanced; that is, the
number and kinds of atoms appearing on one side of the equation cannot be destroyed and so must appear in the same number and kind on the
other side. In representing the formation of water by the simple addition of hydrogen and oxygen we might select the equation
H2 + O2 = H2O
However, this equation is not balanced, because there are different numbers of atoms on each side of the equation. Balance is achieved by manipulating
the coefficients, which indicate how many of each of the molecules are involved in the equation:
Now the equation is balanced.
2H2 + O2 = 2H2O
Difusion is the tendency of molecules to disperse throughout a medium or container in which they are found. How does diffusion
differ from osmosis; how is it similar?
Diffusion involves movement of solute particles in the absence of a semipermeable membrane. Osmosis is a special case of diffusion involving
movement of solvent molecules through a semipermeable membrane. The two processes are similar in that movement of the molecules in each
is driven by their collisions and rebounds with their own kind and proceeds toward areas in which collisions are less likely, namely, areas with
fewer molecules of their kind (from crowded to less crowded regions).
We will examine four emergent properties ofwater that contribute to Earth's suitability as an environment for life: 1. cohesive behavior, 2.  ability to moderate temperature, 3. expansion upon freezing, and 4. versatility as a solvent.

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