Transcript Document

Water – did you know..
The diagram above shows the orientation of the 2 hydrogen atoms and the 2 other pairs of electrons around the central
oxygen atom within a water molecule. Each gray spoke radiating out from the oxygen represents a pair of electrons, so
oxygen has filled its outer shell by sharing electrons with the 2 hydrogens.
The important factor shown by this diagram is the angle between the 2 hydrogen atoms. This angle is also the same
between the hydrogen atom and the pairs of electrons shown at lower right. The significance of having 2 hydrogen
atoms on one side of a water molecule is that the oxygen, being a stronger electron grabber than hydrogen, is able to
pull the electrons shared with each hydrogen towards it. The result is an unequal sharing of the electrons.
This makes a water molecule polar, in that the oxygen end of the molecule has more electrons (a negative charge), while
the hydrogen end has a slightly more positive end (as the electrons are found there less frequently). Having both a
positive and negative end, water thus acts like an electromagnet. The positive end is able to attract negative ions or the
negative end of other polar molecules. The negative end is able to attract postive ions or the positive end of other polar
molecules. Because water does this very well, it is able to dissolve many substances, and it is thus called a universal
solvent.
Really, the angle is always 104.45?
A covalent bond? In water?
Another bond? James? No, Hydrogen…
Tension at the surface?
Within the water, at least a few molecules away from the surface, every molecule
is engaged in a tug of war with its neighbors on every side. For every "up" pull
there is a "down" pull, and for every "left" pull there is a "right" pull, and so on, so
that any given molecule feels no net force at all. At the surface things are
different. There is no up pull for every down pull, since of course there is no liquid
above the surface; thus the surface molecules tend to be pulled back into the
liquid. It takes work to pull a molecule up to the surface. If the surface is
stretched - as when you blow up a bubble - it becomes larger in area, and more
molecules are dragged from within the liquid to become part of this increased
area. This "stretchy skin" effect is called surface tension. Surface tension plays
an important role in the way water behaves.
Remember this guy?
The water strider is an insect that hunts
its prey on the surface of still water; it has
widely spaced feet rather like the pads of
a lunar lander. The skin-like surface of
the water is depressed under the water
strider's feet.
Be impressed:
Surface tension goes cohesion:
It turns out that this surface tension is the result of the
tendency of water molecules to attract one another
(called cohesion). The lowest energy state for this drop
occurs when the maximum number of water molecules
are surrounded on all sides by other water molecules -meaning that the drop should have the minimum
possible surface area, which is a sphere.
Water goes adhesion:
In addition, the water’s surface does not react to being poked with
a stick as a skin would. In fact, the water not only sticks to itself, it
sticks to the wood as you can see by looking at the arrows in the
picture. This is called adhesion because the attraction is to a
different substance.
Am all wet, or are adhesions and cohesion always both present?
In all systems within which water interacts with another surface, both adhesion
and cohesion are factors. When cohesion is more of a factor, the water forms
spherical droplets; when adhesion is more of a factor, we get sheets of water.
water drop on oiled plastic wrap
water drop on untreated plastic
wrap (cohesion of water molecules
to one another as well as strong
adhesion of water to plastic)
Wait, what a drip, you think water can move up, counter to gravity?
As we said, adhesion is the attraction between molecules of water and
molecules of another substance. Adhesive forces when combined with
surface tension can result in a phenomenon known as capillary action. This
action results in the movement of water up a narrow tube, against the force
of gravity for example in the xylem vessels of plants.
Capillary action occurs because water molecules adhere to the inside wall
of the tube. The molecules are pulled up the tube by surface tension which
tends to try to straighten the water’s surface. Additional water molecules
are pulled up the tube as a result of cohesion. This action is very important
in plants and is aided by transpiration pull.
•trees have specialized structures to transport water: xylem and
phloem “plumbing” water molecules are “dragged” from the roots
to the top of the tree by capillary action and cohesion. Hydrogen
bonds help water molecules cling to each other.
Good for a laugh:
From: http://www.s-anand.net/blog/calvin-and-hobbes-dad-explainsscience/
Wow, its cold in here:
Why does this happen? Does it matter?
Cuz….
Solids are almost always the most dense form of a substance, then
liquids and then gases. As temperature increases, the density
generally decreases. Pure water is an exception to this and is the
only substance which has its highest density as a liquid. Water is at
its most dense at about 4 oC. This is because hydrogen bonds
between water molecules give ice a very stable open ordered
structure. At low temperatures, water has a higher density than ice
and this means that ice floats
At 4°C, water molecules are as tightly packed as they can get. As
water cools below 4°C, each molecule rearranges itself into a more
stable crystalline structure and forms a stable solid, ice, at 0°C. It is
during this process of rearrangement, when the volume of the water
increases for its molecules to move around. Its mickymouse shaped
molecules neatly stack on top of each other, and trap air bubbles in
the process. This creates more space in between the water
molecules in its solid form than in its liquid state. The round, bent
structural shape of the water molecule also begin repelling each
other once they get too close.
Remember this?
So that means water can…..
Universal Solvent
 A liquid that is a completely homogeneous mixture of two or more
substances is called a solution.
 A sugar cube in a glass of water will eventually dissolve to form a
uniform mixture of sugar and water.
 The dissolving agent is the solvent and the substance that is dissolved is
the solute.
 In our example, water is the solvent and sugar the solute.
 In an aqueous solution, water is the solvent.
 Water is not really a universal solvent, but it is very versatile because of
the _____________ of water molecules.
 Water is an effective solvent as
it can form hydrogen bonds.
 Water clings to polar
molecules causing them to
be soluble in water.
 Hydrophilic - attracted to
water
 Water tends to exclude
nonpolar molecules.
 Hydrophobic - repelled by
water