Water

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    Water has many interesting propertiesHere is a schematic drawing (by me) of a water molecule:  water.jpg (2689 bytes).  In this drawing, the atomic nuclei are indicated by the letters (either H for hydrogen or O for oxygen).  The covalent bonds between the atoms are indicated by the lines; these are actually polar covalent bonds, since oxygen draws the electrons closer to it.   The blue area represents the area where the electrons are spinning.  Because the electrons are drawn more toward the oxygen nucleus, that end of the molecule seems a bit more negative, that is why the partial negative charges (d-) are there.  Similarly, the hydrogen nuclei don't get the electrons as close, so they end up seeming partially positive.  Because of this, a partial positive charge (d+) is on each hydrogen.

    The partial charges, due to the polar covalent bonds that build this molecule, cause water molecules to orient toward each other so that the more negative end of one water molecule ends up next to the more positive end of another water molecule.  This is a HYDROGEN BOND.  This orientation, shown in the animated picture below, and the ability to form hydrogen bonds with other molecules or ions, gives water its numerous important properties.

water.gif (40786 bytes)

Some of the properties of water are listed below.  The first one is the most important for you to understand, so that you can handle the A&P material this year.

bulletWater is an excellent solvent
bulletWater is highly cohesive (allowing for surface tension)
bulletWater freezes into a solid that is less dense than the liquid
bulletWater doesn't change temperature too quickly

Water is an excellent solvent:
    When any of many different types of molecules are thrown into water, they dissolve.  For example, salt dissolves in water.  And sugar dissolves in water.  And food coloring dissolves in water.  But some other molecules do not dissolve in water, like oil.  You are learning in this unit that oil is mainly a nonpolar molecule (a triglyceride) and is thus hydrophobic; because of its hydrophobicity it does not know how to associate with water, but instead must run off to be with more hydrophobic molecules (that's when it separates out).
        All water mixtures with dissolved material in it is called a solution.  The water, the part that does the dissolving and is the liquid, is called the solvent.  The stuff you put into the water, like the salt or the sugar, is called the solute. How is it that water can dissolve so many solute molecules so well?  That's because water's polarity allows it to associate with other polar molecules (or regions on molecules).  As it does that, it dissolves them.
    Let's take table salt as an example.  When the table salt molecules go into water, they go in as Na+Cl- molecules.  These molecules are strongly polar, since one half is very positive and the other is very negative.  The O side of the water molecules (which are partially negative) orient themselves toward the positive sodium ions.  At the same time, the H sides of the water molecules begin to orient themselves toward the negative chloride ions.  As the water begins to surround the ionic halves of the salt molecule, the charges on each ion are now being accomodated by the partial charges of the water molecules, and the two ions are less and less attracted to each other.  This culminates in a breaking apart of the salt molecule, and a complete surrounding of each ion by water.  This surrounding layer of water is called a sphere of hydration.  You can see this entire thing happen in a little movie that I downloaded from the internet.  In this movie, the larger circles in the square of salt are the chloride ions.
    Throughout the year you will be seeing the importance of ions in our bodies.  Please keep in mind that for every positive ion that exists in our body, there is a negative ion, too.  They just don't stay attached to each other because each gets surrounded by a sphere of hydration and can float anywhere in our body.
    As long as a molecule is polar or at least has some polar regions in it, it will be able to dissolve in water.  Dissolving only means breaking apart when it is a salt.  Sugars dissolve, but they don't change their molecular make-up.   All that happens there, is that water molecules orient toward the partial charges on the sugar molecule.  Since water can then surround each sugar molecule, it "disappears" from view, or dissolves.
    I hope this information on water helped you understand it better!

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2011 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.