1.
1 Structure of Water and
Hydrogen Bonding
Water Molecules
Water is a polar molecule, meaning the ends have opposite partial
charges or unequal distribution of charge. The chemical formula of water
H2O tells us that a water molecule is made up of two hydrogens and
one oxygen. Water is polar because the hydrogens carry partial positive
charges, and oxygen has a partial negative charge, which we can
examine further when we discuss hydrogen bonding.
The simple phrase, "like attracts like," helps us to remember that polar
substances are attracted to other polar substances, just as nonpolar
substances are attracted to other nonpolar substances. Ever tried
mixing oil and water? Oil molecules are nonpolar and have equally
balanced charges rather than positive and negative poles like water.
Because of this, oil and water are immiscible or do not combine. Polar
and nonpolar do not attract because they are not alike.
In regard to water, there is speci c terminology to describe attractive
interactions. A hydrophilic substance has an af nity for water, which
means it attracts water. Conversely, a hydrophobic substance is one
that avoids water or does not have an af nity for water. For example,
lipids are hydrophobic because they have relatively nonpolar bonds,
while water is polar. Water is the most common molecule in living
organisms, so it is important that we understand its interactions.
fi
fi
fi
Hydrogen Bonding
Hydrogen bonding is a type of chemical intermolecular bond that
occurs when a hydrogen atom bonds to a highly electronegative atom,
including oxygen, nitrogen, and uorine. This creates a dipole moment in
the molecule, where the hydrogen carries a partial positive charge, and
the electronegative atom has a partial negative charge—the type of
structure we observed that gives water its polarity, as water molecules
bond together through hydrogen bonding. Hydrogen bonds are generally
weaker than covalent bonds, but they are still relatively strong and play
an important role in many chemical and biological processes.
In biology, hydrogen bonding is critical, as hydrogen bonding occurs in
many different types of molecules, including water, DNA, and proteins.
Hydrogen bonding is responsible for many molecular properties, such as
the shape and function of proteins, the stability of many chemical
compounds, and the formation of intermolecular interactions in
crystalline solids. Hydrogen bonds between water molecules give water
the properties of cohesion, adhesion, surface tension, speci c heat, and
evaporative cooling.
Properties of Water
1. Cohesion: the attraction of water molecules. Strong cohesive forces
are present because they form hydrogen bonds with each other.
• Example: Cohesion due to hydrogen bonding contributes to the
transport of water and nutrients against gravity in plants.
Transpiration is the loss of water from a plant in the form of water
vapor.
fl
fi
2. Adhesion: when one substance is attracted to another. Water
adheres to other molecules and surfaces.
• Example: water on a glass surface, like a window or windshield.
3. Surface Tension: dif culty to break the surface of the water because
of cohesive forces.
• Example: water strider insects rely on surface tension to stay a oat
and walk on the surface of bodies of freshwater.
4. Speci c Heat: the amount of heat energy it takes to raise or lower
the temperature of one gram of a substance by one degree Celsius.
Water has a high speci c heat, so it can absorb or release a large
amount of heat with only a slight change in its own temperature so large
bodies of water take a while to evaporate.
5. Evaporative Cooling: water has a high heat of vaporization, so the
water can absorb a lot of heat and leave the surface cooler.
• Example: excess body heat is used to convert beads of sweat/
perspiration into vapor, which cools down the body
6. Dissociation of Water: hydrogen shifts from one water molecule to
another.
• When a molecule is increasing hydronium concentration by
releasing hydrogen ions into solutions, it is an acid. (Ex. HCl ➡
H+ + Cl-)
fi
fi
fi
fl
• When a molecule is increasing hydroxide concentration by
absorbing or accepting hydrogen ions, it is a base. (Ex: NaOH ➡
Na+ + OH-)
The counterbalance of hydronium and hydroxide results in water having
a neutral pH of 7.0, meaning it is neither acidic nor basic.
• pH scale range (0-14): acidic (< 7), neutral (7), alkaline/basic (> 7).
Each level on the pH scale is a ten-fold change because the pH
scale is logarithmic. Most biological uids are in the pH range of
6-8. pH = −log [H+]
fl
