Unit 1: Physical and Chemical Properties of Water Notes
I. Structure of the Water Molecule
A molecule of water (H2O) consists of two hydrogen (H) atoms bound to one oxygen atom (O).
The electrons of hydrogen are shared when bonded to Oxygen. The arrangement, however,
makes the H2O molecule chemically stable, but gives it an unusual shape. It also creates a
small imbalance in the distribution of the electrical charge within the molecule. Thus, water is
a polar molecule, one side has a slightly positive side, and the other side a slightly
Hydrogen Bonding
negative charge.
How Do Water Molecules Bond With Each Other?
As a result of the difference in charges, the neighboring water molecules are drawn to
each other by forces called hydrogen bonds. A hydrogen bond is an attractive force between
areas of slight positive and negative charges on neighboring water molecules. The positively
charged hydrogen atom of one water molecule and the negatively charged oxygen atom of another water molecule attract
and “stick” together. Hydrogen bonds are much weaker than covalent bonds because they do not actually share electrons.
This allows them to easily break and easily form again. Hydrogen bonds are responsible for most of the unique
characteristics of water.
II. Physical Properties and States of Matter
A. Water is colorless, tasteless, odorless, and transparent in its pure liquid state. Pure water (distilled) has a neutral pH
of 7.0 (it is neither acidic nor basic). Most natural water has a pH around 6. Acid rain can have a pH as low as 4.
Seawater has a pH of around 8.
B. Water boils at 212oF or 100oC. Water freezes at 32oF or 0oC.
C. Water is the only naturally occurring substance that can exist in all three physical states. Water can be a solid (ice),
a liquid (water), or a gas (water vapor).
Three States of Water
The following phase changes are possible:
Liquid
1)
2)
3)
4)
5)
6)
Freezing (liquid to solid)
Ex.
Melting (solid to liquid)
Ex.
Condensation (gas to liquid)
Ex.
Evaporation (liquid to gas)
Ex.
Frost Formation (gas to solid)
Ex.
Sublimation (solid to gas)
Ex.
small clumps
of bonded
molecules
Ice
Hexagonal
crystal lattice
III.
Water Vapor
Widely spread unbonded molecules
The Density of Water
Temperature and Density in Water
Like most substances, water increases in density as
As temperature of water decreases, density of water
its temperature decreases. The density of water
increases.
continues to increase until it reaches 4oC. At this point,
At 4oC, density begins to decrease until it freezes solid at
water is unique because its density begins to decrease
0oC.
as the temperature decreases to 0oC (the freezing point of water).
As the freezing point is approached, hydrogen bonds between water molecules relax and form a crystal lattice, keeping
molecules farther apart than when in liquid form. This is why ice floats and ponds don’t freeze solid. Bodies of water freeze
from the top down, not the bottom up. The fact that ice floats on liquid water is important because it allows the existence of
large areas of polar sea ice. The sea ice forms an insulating layer, allowing organisms to live below. The sea ice also
affects heat flow between the ocean and the atmosphere, helping to stabilize ocean temperatures and Earth’s climate.
(We’ll learn more later )
IV. Surface Tension
Water has high surface tension, a special property of liquid water that occurs because of two important things:
1. The hydrogen bonding between its molecules
2. The attraction of water molecules to each other (which is called cohesion).
In any grouping of water molecules, the molecules on the surface tend to be drawn together and inward toward the center
of the group, forming a type of “skin” that is resistant to being broken apart. Surface tension can be thought of as the
force that has to be exerted to break through the top layer, or “skin”, of the water. Surface tension causes water to bead up
on hard surfaces. It also allows us to skip rocks on water, and for insects to walk on the water’s surface and plays a part in
the formation of ocean waves.
Gravity has an affect on surface tension as water droplets fall. Air pushes up on the bottom of the water drop, causing the
droplet to flatten out a bit. Also, surface tension decreases when the water becomes warmer. Soaps and detergents also
decrease surface tension.
Water molecules below surface
are pulled toward each other on
all sides
Water molecule at the surface – the
only forces act inward, or sideways,
toward other surface molecules
V. Capillary Action and Adhesion
The same forces that cause the cohesive properties of water (hydrogen bonds) also cause adhesion, or the
attraction of water molecules to stick to other substances. This allows water to flow in tubes, against gravity.
Water can creep up the tubes of plants, blood vessels, etc. This is called capillary action. Water does not
adhese to non-polar compounds (lipids), however.
V. High Specific Heat Capacity (Resistant to temperature changes)
Because of hydrogen bonding, liquid water has an unusually high heat capacity, compared to all other known liquids. In
fact, in terms of specific heat, water is very high (4.19) when compared to other liquids. (Specific heat capacity is the
energy (in joules) needed to raise the temperature of 1 gram of a substance by 1o C.) When heat is added to water, most
of that heat is used to break the hydrogen bonds that link the water molecules together. Only a small amount of the heat
energy is left to increase the vibrations of those water molecules. (The vibrations of the molecules are detected as a rise in
temperature.)
This means that areas of water (lakes, oceans, etc.) can absorb and release huge amounts of heat energy without much
change in the overall temperature of the water. In fact, the sun warms the sea more slowly than it does the land. It also
means that movements of water, such as ocean currents, transfer extremely large amounts of heat energy around the
planet (an example of thermal conductivity). This role of ocean currents is vital to Earth’s climate. (We will learn more
about this later)
VI. High Heat of Vaporization
Heat of vaporization is the amount of heat required for 1 gram of a substance to be converted from a liquid to a gas.
Hydrogen bonds make it difficult for water molecules to escape the liquid state (evaporate) and are responsible for water’s
high heat of vaporization. The hydrogen bonds must be broken before water can evaporate and this requires considerable
energy.
This property of water helps our bodies (through sweating) and our planet (surface water evaporation) to maintain a
temperature within a specific range… (remember homeostasis?)
VI. Water is the Universal Solvent
Because of water’s polarity, it makes an excellent solvent. This is because other molecules that have regions of partial
positive and negative charges attract water molecules and our dissolved. About ½ of all the natural elements on earth can
dissolve in water. Water is an inert solvent, meaning that it is not altered by any substance it dissolves, and in turn those
substances being dissolved are not altered by the water. Most substances are easily dissolved in water. In fact, most of
the nutrients and minerals required by plants and animals are dissolved by and carried by water.
Water will not form solutions, however, with substances that are non-polar. Lipids, such as fats, oils, and waxes, are
examples of non-polar compounds that will not dissolve in water.
VII. Water and Homeostasis
Water is important for maintaining a balance within not only the bodies and cells of organisms, but also within the many
land and water ecosystems on the planet. Water insulates, conducts and transports, resists temperature changes, makes
a good coolant, is a universal solvent, and controls the planet’s climate. Ain’t water great!!!!!