Water
A.
B.
C.
D.
E.
Structure of water
Ionization of water
Solvent properties of water
Thermal properties of water
Monomers and polymers
A.
Structure of Water
A water molecule is composed of two hydrogen
atoms covalently bonded to an oxygen atom
The hydrogen atoms form an angle of about
110°, so the molecule is “bent”
A.
Structure of Water
The oxygen nucleus exerts a greater “pull” on
the electrons in the covalent bonds
– Therefore, the oxygen atom has a partial negative
charge
– And the hydrogen atoms have partial positive
charges
A.
Structure of Water
Because opposite charges attract each other,
water molecules are attracted to each other and
to other charged molecules or ions
(This is an example
of a hydrogen bond.)
A.
Structure of Water
The bent geometry of water and the attraction
between water molecules gives rise to unique
properties that are essential for its role in living
organisms and the environment
B.
Ionization of Water
Consider a glass of pure water:
– In a tiny fraction of the water molecules
(1 out of 10 million), one of the hydrogen nuclei is
completely pulled off the molecule
– This forms two ions:
• A hydrogen ion (H+)
• And a hydroxyl ion (OH–)
– This is caused by the attraction of the water
molecules for each other
B.
Ionization of Water
Water molecules are continuously splitting into
ions and rejoining to form water molecules
B.
Ionization of Water
In chemically pure water, the number of H+ and
OH– ions are the same
Certain chemical substances, when dissolved in
water, can change the amounts of H+ or OH–
B.
Ionization of Water
Acid
– A substance that increases the amount of H+
(and decreases the amount of OH–)
Base (Alkaline)
– A substance that increases the amount of OH–
(and decreases the amount of H+)
Neutral substance
– A substance that does not change the amounts of
H+ and OH–
(so H+ remains equal to OH–)
B.
Ionization of Water
Acidity and alkalinity are represented by a value
called “pH”
–
–
–
–
–
pH = -log[H+]
Acids: pH value is less than 7
Bases: pH value is greater than 7
Neutral substances: pH value is equal to 7
Each pH value represents a 10-fold change in the
amount of H+ in the solution
– So a substance with pH = 5 has a 10 times greater
amount of H+ than a substance with pH = 6
C.
Solvent Properties of Water
Solution
– A mixture of two (or more) different substances in
which the particles of one substance are completely
interspersed with the particles of the other
substance(s)
– Solvent: The substance that is present in the largest
amount
– Solute: The substance(s) that are present in smaller
amounts
C.
Solvent Properties of Water
Hydrophilic substances
– Substances that can be dissolved in water
– Water molecules are attracted to ions or to other
molecules that have partial positive and negative
charges
– Examples of hydrophilic substances:
• Sodium chloride (table salt): This substance consists of
sodium ions and chloride ions
• Sucrose (table sugar): This substance is a compound with
many -OH groups in its structure,
with many partial positive and negative charges
C.
Solvent Properties of Water
Hydrophobic substances
– Substances that cannot be dissolved in water
– Water molecules have difficulty interacting with
uncharged molecules. These substances tend to
separate from water.
– Example of a hydrophobic substance:
• Cooking oil: The molecules of cooking oil have long
chains of carbon atoms bonded to hydrogen.
The atoms do not have the “bent” geometry of water, so
there are no partial charges to attract the water.
Therefore, oil and water don’t mix!
C.
Solvent Properties of Water
Amphipathic substances
– Substances in which part of the molecule is
hydrophobic, and part of the molecule is hydrophilic
– When amphipathic substances are mixed in water,
its molecules form into clusters called “micelles”
• with the hydrophilic part on the outside of the micelle in
contact with water
• and the hydrophobic part on the inside of the micelle,
away from the water .
C.
Solvent Properties of Water
Amphipathic substances (cont.)
– Example of an amphipathic substance:
• Soap: Soap molecules have an ionic group attached to
one end, and an oily hydrocarbon chain attached to the
other end. When soap is mixed with water, it forms
micelles that trap oily dirt molecules.
D.
Thermal Properties of Water
Molecules are in constant motion due to the heat
energy (kinetic energy) they contain
Phases of matter:
– Solid
• Limited movement of molecules; non-fluid
– Liquid
• Molecules can move freely around each other; fluid
– Gas
• Molecules have greatest freedom of movement;
substance can expand to fill the available space
D.
Thermal Properties of Water
Water has unusual thermal properties because
of the attraction of water molecules for each
other
– Water has relatively high melting and boiling points
– Water remains in a liquid state over a wide
temperature range
– Water has a high heat capacity: it can absorb a large
amount of heat with a small change in temperature
– The solid form of water (ice) is less dense than the
liquid, so ice floats on water
E. Monomers and Polymers
Monomer
– An organic molecule that serves as a “building block”
to build larger organic molecules
Polymer
– An organic molecule composed of two or more
monomer units linked together by covalent bonds
E. Monomers and Polymers
Condensation reaction
– Polymers are often formed by the process of
condensation
– In this process, two hydrogen atoms and an oxygen
atom are removed from two monomer units
– And a covalent bond forms between the monomers
E. Monomers and Polymers
E. Monomers and Polymers
Hydrolysis reaction
– Polymers are often broken down by the process of
hydrolysis
– In this process, a water molecule is inserted between
the monomer units of a polymer
– To split the polymer into its monomer units
E. Monomers and Polymers