Water is a wonder!
Water is an important and abundant biological molecule. Our bodies are about 70% water, and plants can be
up to 90% water. We need more water than any other nutrient-- we could only survive a few days without it.
Water is smallest and most simple food molecule: just 3 atoms, but the way those atoms combine gives
water special properties.
When an oxygen atom
combines with 2
hydrogen atoms, they
stabilize into an H2O
molecule, by sharing
electrons in covalent
bonds. However, the way
the electrons are shared
gives the oxygen end of a
water molecule a slightly
negative charge, and the
hydrogen end a slightly
positive charge. It is a
polar molecule.
The polar charges of the molecules
attract water molecules to each other.
These attractions are called hydrogen
bonds.
Although the hydrogen bonds are not strong, there are so many of them in water that they
have a significant effect. POLARITY CAUSES THESE EFFECTS IN WATER:
1. WATER HAS COHESION AND ADHESION—IT STICKS TOGETHER AND STICKS
TO OTHER SUBSTANCES. This is
important for nutrients go travel
through plants and through us. It also
causes a meniscus when measureing
liquids. If you see 2 lines in a liquid
measuring cup, the bottom is the fill
level; the top line is the meniscus
formed by liquid adhering to the cup.
WATER HAS A HIGHER MELTING POINT AND BOILING POINT THAN MANY OTHER
SUBSTANCES: The hydrogen bonds hold molecules together, so it takes more energy to turn a solid into
liquid, or a liquid into solid.
SOLID H2O IS LIGHTER THAN LIQUID H2O. Most substances become more compact as they freeze, but
not water. As water freezes, the hydrogen bonds cause the molecules to arrange themselves into a crystal
structure that actually spreads them farther apart. This makes the ice take up more space than the water did.
Because it is less dense than the liquid water, it floats.
WATER FORMS HYDROGEN BONDS WITH OTHER POLAR SUBSTANCES. Other polar molecules
are attracted to water. This let’s water:
 combine with carbohydrates to swell and break down starch molecules
 help form gluten strands that give breads their structure


attach to minerals
attach to proteins and carbohydrates in dough, then turn to steam during baking, causing doughs to rise
and keeping crusts moister.
 Allow many substances to dissolve in it. (not fat or oils) This is important for 2 reasons:
1. It lets water carry nutrients and other important chemicals.
2. For many reactions to happen, the reactants need to be dissolved. Water accomplishes this.
 Repel non-polar compounds. This is important for cell membranes in your body.
 Use capillary action to pull itself up through plants
CHAPTER 8: Introduction to the Hydrosphere
(a). Physical Properties of Water
We live on a planet that is dominated by water. More than 70% of the Earth's surface is covered with this
simple molecule. Scientists estimate that the hydrosphere contains about 1.36 billion cubic kilometers of
this substance mostly in the form of a liquid (water) that occupies topographic depressions on the Earth. The
second most common form of the water molecule on our planet is ice. If all our planet's ice melted, sea-level
would rise by about 70 meters.
Water is also essential for life. Water is the major constituent of almost all life forms. Most animals and
plants contain more than 60% water by volume. Without water life would probably never have developed
on our planet.
Water has a very simple atomic structure. This structure consists of two hydrogen atoms bonded to one
oxygen atom (Figure 8a-1). The nature of the atomic structure of water causes its molecules to have unique
electrochemical properties. The hydrogen side of the water molecule has a slight positive charge (see Figure
8a-1). On the other side of the molecule a negative charge exists. This molecular polarity causes water to be
a powerful solvent and is responsible for its strong surface tension (for more information on these two
properties see the discussion below).
Figure 8a-1: The atomic structure of a water (or dihydrogen monoxide)
molecule consists of two hydrogen (H) atoms joined to one oxygen (O) atom.
The unique way in which the hydrogen atoms are attached to the oxygen atom
causes one side of the molecule to have a negative charge and the area in the
opposite direction to have a positive charge. The resulting polarity of charge
causes molecules of water to be attracted to each other forming strong
molecular bonds.
When the water molecule makes a physical phase change its molecules arrange themselves in distinctly
different patterns (Figure 8a-2). The molecular arrangement taken by ice (the solid form of the water
molecule) leads to an increase in volume and a decrease in density. Expansion of the water molecule at
freezing allows ice to float on top of liquid water.
molecule at freezing allows ice to float on top of liquid water