Station 1 – Surface Tension & Adhesion
Water has a simple molecular structure: H2O. Each molecule of water is made up of two atoms
of hydrogen connected to one atom of oxygen. The way that these atoms are made, and the
way that they join into a molecule of water, make the water molecule into a miniature magnet.
Water is thus called "polar." Have you ever noticed that magnets like to stick together? So do
water molecules. This is called cohesion. These little magnets also like to stick together when
they are on a liquid's surface, which is called surface tension, and is the reason that a raindrop
tends to be round. This also explains why water beads up when poured on a smooth surface like
glass.
Atoms are most stable when they have a particular configuration of their outer shells, a concept
which will be discussed in chemistry. These configurations explain why hydrogen in water takes
on a partial positive charge and why oxygen takes on a partial negative charge. These partial
charges cause water molecules to “stick” to each other like magnets. The “stickiness” (known as
cohesion) in this particular case is due to hydrogen bonding. In this case, hydrogen bonding
involves the attraction between the positively charged hydrogen atom of one water molecule and
the negatively charged oxygen atom of another water molecule. (As no electrons are actually
shared however, hydrogen bonds are much weaker than covalent bonds - they easily break and
easily form again).
Hydrogen bonds hold water molecules together so
tightly that the water’s surface acts like a membrane.
A water strider, like the one pictured on the left, can
walk on the water surface without breaking through.
Station 2 – The Climbing Property of Water
Ever wonder how a tree as tall as a redwood can get water all the way from its roots to its top leaves?
Water is pretty heavy, yet the redwood tree moves thousands of gallons of water (that's 8,000
pounds, or 4 tons) up into its canopy every day, and it does it without doing any work. That's pretty
amazing! Here is how it works.
Water molecules love some other
molecules and hate other
molecules.
Have you ever noticed that magnets like to stick to
other metals? So do water molecules. When water
molecules stick to other molecules that are also little
magnets, it is called adhesion. This explains why it is
easy to clean up spilled water with a paper towel: the
water molecule's little magnets like to stick to the
cellulose molecules of the paper, which are also like little
magnets. Water molecules will stick to any other
molecules that are like little magnets (polar), but do not
like to get involved with any molecules that hate little
magnets (nonpolar), like oil. Oil and water don't mix,
right? That is why you have to shake the salad dressing
real hard before you pour it: the oil molecules hate the
water little magnets.
Paper towels are made out of trees, and trees are made
out of cellulose. The leaves make molecules of sugar out
of sunlight, water, and carbon dioxide, then combine the
sugars into huge sugar chains. These sugar chain
molecules are called cellulose. This name comes from
the fact that the plant material is made of cells, and the
"-ose" word ending means "sugar". Cellulose is also a
great magnet (polar molecule), so water sticks to
cellulose just like a magnet to the refrigerator door!
Now that we know a bit about water molecules, let's
look at how water acts in a little tube.
Water Molecules
bonded to paper towel
Water loves to rise in a tube
Meniscus shown in blue
water molecules want to stick together and stick to side
Have you ever noticed that water in a glass tends to hug the sides and even stick up above the
water's surface? The edge of the water that sticks up above the water's surface is called a meniscus.
When you put a small tube into water, the water likes to stick to each side, with a meniscus on each
side. If the tube is so skinny that the meniscus on one side can touch the meniscus on the other side,
the water will rise up the tube (each meniscus wants to go up the side, and they chase each other).
This is called capillary action.
The redwood tree's trunk is made up of millions of little bitty tubes (xylem), and these tubes are
made of cellulose. The water molecules like to stick together and like to stick to the walls of the tubes
of cellulose, so they rise up the tubes by capillary action. All the way to the top!
The water pressure decreases as it rises up the
tree. This is because the capillary action is fighting
the weight of the water. Although the xylem tube
is very thin, and therefore the weight of the water
is very low, it is not zero. Eventually, the effects of
gravity on the water starts to equal the effects of
capillary action. Scientists have found that the
pressure inside the xylem decreases with the
height of the tree, and similarly, the size of the
redwood leaves decreases with the decrease in
pressure. (See an excellent article in the San
Francisco Chronicle, the source of the
illustration to the right.)
Scientists Dr. George W. Koch of Northern Arizona
University, Dr. Gregory M. Jennings of Humboldt
State in Arcata, California, and Dr. Stephen D.
Davis of Pepperdine University in Malibu,
California, have studied the water pressure inside
a coast redwood. They studied the correlations
among the tree's height, its internal water
pressure, leaf size, photosynthesis and other
factors. Dr. Koch and his colleagues have
concluded that no existing species of tree can
grow higher than 130 meters (427 feet).
Redwood Tree and Leave Size
The leaves are smaller at the top of the tree
(graphic modified from the SF Chronicle, see reference in text to left)
Station 3 – Polarity of Water
Oil is a hydrophobic or “water hating” molecule, so called because its chemical structure does
not allow the formation of hydrogen bonds. Therefore, oil does NOT dissolve in water. When
mixed, the two substances form separate layers, and because oil is less dense, it sits on top of
water.
This characteristic behavior of water and oil is crucial,
among other things, in the structure of the cell membrane.
Let’s look at the cell membrane and see how that
membrane keeps all of the pieces inside. When you think
about a membrane, imagine it is like a big plastic bag with
some tiny holes. That bag holds all of the cell pieces and
fluids inside the cell and keeps any nasty things outside the
cell. The holes are there to let some things move in and out
of the cell.
The cell membrane is NOT one solid piece. Compounds called proteins and phospholipids
make up most of the cell membrane. The phospholipids make the basic bag. The phospholipids
are in a shape like a head and a tail. The heads like water (hydrophilic) and the tails do not like
water (hydrophobic). The tails bump up against each other and the heads are out facing the
watery area surrounding the cell. The two layers of cells are called the bilayer.
Station 4 – Water as Universal Solvent
A solvent is a substance that dissolves, or breaks apart,
another substance (known as a solute). Whether a
substance will dissolve in a solvent depends upon its
polarity. Polar solvents dissolve polar solutes and
nonpolar solvents dissolve nonpolar solutes.
Water is not only a good solvent, it is the best. Because
of its high polarity, water is called the universal
solvent. It dissolves more different substances than any
other solvent known. This is because so many other
molecules are ionic or polar, and their electrical charges
make them attracted to the water molecules, causing
them to stay in solution. Thus we find that water
dissolves many kinds of salts and sugars, many proteins,
and a variety of hormones that dissolve in our blood (since blood is mostly water) and regulate
various life processes. Even nonpolar molecules dissolve in water to some extent if they are
small. Thus enough oxygen dissolves to allow fish and other aquatic animals to survive, and
enough carbon dioxide dissolves to enable algae and many plants to live underwater.
Station 5 – Water’s High Specific Heat and Heat of Vaporization
Water is also unusual in being able to absorb a lot of heat energy without having its temperature
increase by very much. It is said to have high specific heat. An amount of heat that will raise the
temperature of a container of water by 10 degrees will raise the temperature of an equal weight
of alcohol by 20 degrees and an equal weight of iron by 94 degrees. Water molecules are held
together so strongly by their hydrogen bonds that an amount of heat that will get other
molecules moving much faster will not speed up water molecules much at all. This property of
water helps to reduce temperature fluctuations in the animal of plant body (homeostasis), and it
also makes for mild climates in the vicinity of large bodies of water.
Heat of vaporization is the amount of heat energy required to evaporate a given weight of a
liquid. Water has a very high heat of vaporization, which means that it takes a lot of heat to
evaporate just a little water. This keeps water in many more lakes and ponds during the summer
than would be the case if water had a lower heat of vaporization. Heat of fusion is the heat
energy that must be removed from a given weight of water in order to freeze it. Water’s
relatively high heat of fusion means that it takes much longer for lakes and streams to freeze in
the winter, allowing living things more time to adjust to the change.