TOPIC 1:
Water
Common Definition of
Water
1) A chemical substance, with chemical formula H2O, that is a
clear, colorless, odorless, and tasteless liquid that may also occur
in various forms such as gas (water vapor) and solid (ice)
2) An aqueous solution of a substance, for example, ammonia
water, wastewater
3) A body of water, like seas, rivers, and lakes, and naturallyoccurring water like mineral water
4) Amniotic fluid, as in the pregnant woman’s water breaks
In BioChem:
Water is a chemical substance, with
chemical formula H2O, that is a clear,
colorless, odorless, and tasteless liquid
that may also occur in various forms such
as gas (water vapor) and solid (ice).
In BioChem:
Water, a substance composed of the
chemical elements hydrogen and oxygen
and existing in gaseous, liquid, and solid
states.
In BioChem:
It is one of the most plentiful and
essential of compounds. A tasteless and
odourless liquid at room temperature, it
has the important ability to dissolve many
other substances.
Molecular Structure of
Water
What is the structure of a water molecule?
A water molecule consists of two
hydrogen atoms and one oxygen
atom. The three atoms make an angle;
the H-O-H angle is approximately
104.5 degrees. The center of each
hydrogen atom is approximately
0.0957 nm from the center of the
oxygen atom. The structure of a single
water molecule is shown on the left:
What is the structure of a water molecule?
Because oxygen is more electronegative than hydrogen (in other words,
electrons tend to be in the neighborhood of the oxygen), the hydrogen
atoms end up with a partial positive charge and the oxygen atom with a
partial negative charge. This separation of charge produces a net dipole
moment on the molecule; for the isolated water molecule this dipole
moment is approximately 1.85 Debye units.
This molecular structure leads to hydrogen bonding, which is a stabilized
structure in which a hydrogen atom is in a line between the oxygen atom
on its own molecule and the oxygen on another molecule.
What is the structure of a water molecule?
These pictures show a hydrogen-bonded
structure between two water molecules:
Properties of
Water
1. Water is Polar.
Water molecules are polar, with partial positive charges on the
hydrogens, a partial negative charge on the oxygen, and a bent overall
structure.
This is because oxygen is more electronegative, meaning that it is
better than hydrogen at attracting electrons.
2. Water is an Excellent Solvent.
Water has the unique ability to
dissolve many polar and ionic
substances.
This is important to all living things
because, as water travels through
the water cycle, it takes many
valuable nutrients along with it.
3. Water has High Heat Capacity.
It takes a lot of energy to raise the
temperature of a certain amount of
water by a degree, so water helps with
regulating
temperature
in
the
environment.
For example, this property allows the
temperature of water in a pond to stay
relatively constant from day to night,
regardless of the changing atmospheric
temperature.
4. Water has High Heat of Vaporization.
Humans (and other animals that sweat) use
water’s high heat of vaporization to cool off.
Water is converted from its liquid form to
steam when the heat of vaporization is
reached.
Since sweat is made mostly of water, the
evaporating water absorbs excess body heat,
which is released into the atmosphere. This is
known as evaporative cooling.
5. Water has cohesive and adhesive
properties.
Water molecules have strong cohesive forces due to their
ability to form hydrogen bonds with one another.
Cohesive forces are responsible for surface tension, the
tendency of a liquid’s surface to resist rupture when placed
under tension or stress.
Water also has adhesive properties that allow it to stick to
substances other than itself.
5. Water has cohesive and adhesive
properties.
These cohesive and adhesive properties
are essential for fluid transport in many
forms of life.
For example, they allow nutrients to be
transported to the top of a tree against
the force of gravity.
6. Water is Less Dense as a Solid than as
a Liquid.
As water freezes, the molecules form a
crystalline structure that spaces the
molecules further apart than in liquid
water.
This means that ice is less dense than
liquid water, which is why it floats.
This property is important, as it keeps
ponds, lakes, and oceans from freezing
solid and allows life to continue to thrive
under the icy surface.
Functions of
Water
Water is the "Universal Solvent"
Water’s extensive capability to dissolve a
variety of molecules has earned it the
designation of “universal solvent,” and it is this
ability that makes water such an invaluable
life-sustaining force.
On a biological level, water’s role as a solvent
helps cells transport and use substances like
oxygen or nutrients.
Water-based solutions like blood help carry
molecules to the necessary locations.
Water Supports Cellular Structure
Water also contributes to the formation of membranes surrounding cells.
Without water, cell membranes would lack structure, and without proper
membrane structure, cells would be unable to keep important molecules inside
the cell and harmful molecules outside the cell.
Water Supports Cellular Structure
Water also contributes to the formation of membranes surrounding cells.
Without water, cell membranes would lack structure, and without proper
membrane structure, cells would be unable to keep important molecules inside
the cell and harmful molecules outside the cell.
Water Supports Cellular Structure
Chemical Reactions of Water
Water is directly involved in many chemical reactions to build and break
down important components of the cell. Photosynthesis, the process in
plants that creates sugars for all life forms, requires water.
Water also participates in building larger molecules in cells. Molecules like
DNA and proteins are made of repetitive units of smaller molecules.
Putting these small molecules together occurs through a reaction that
produces water.
Water is required for the reverse reaction that breaks down these
molecules, allowing cells to obtain nutrients or repurpose pieces of big
molecules.
Chemical Reactions of Water
Water buffers cells from the
dangerous effects of acids and
bases.
Importance of
Water