WEB TUTORIAL 3.1
Chemistry and Water
Text Sections
Section 3.2 Water and Life, p. 32
Introduction
How does the structure of the water molecule correlate with the properties of
water? This tutorial demonstrates these structure-property relationships and
explains the importance of water's unusual characteristics.
Learning Objectives
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Know the four models used to represent molecules.
Understand why water is such a good solvent.
Understand why water has such a high specific heat.
Narration
Water Is Polar
Life is based on water primarily because of water's solvent properties.
Understanding this and the other properties that make water so important means
understanding water's molecular structure.
Any molecule can be represented in several ways, each having particular advantages. Molecular formulas, for example, are compact and indicate the number and
identity of atoms in the molecule. Structural formulas show the bonds between
atoms. Ball-and-stick models take up more space than structural formulas but
include information about bond geometry. Space-filling models are not as easy to
read as ball-and-stick models, but they depict the spatial relationships between
atoms more accurately.
Water is a solvent for many of the molecules in living organisms primarily because
it is a polar molecule. The oxygen atom in the water molecule has a high electronegativity. This means that electrons shared by the hydrogen and oxygen atoms
are more attracted to the oxygen than to the hydrogen. This unequal attraction of
electrons generates partial electric charges—indicated by the Greek symbol delta
(∆)—within the water molecule. The oxygen atom has a partial negative charge,
and the hydrogen atoms have a partial positive charge. Therefore, a water molecule
as a whole has positive and negative ends: The molecule is polar.
When two liquid water molecules approach each other, the partially negative oxygen atom of one water molecule is attracted to the partially positive hydrogen atom
of the other water molecule. Hydrogen bonds form between water molecules, causing them to stick together. These same hydrogen bonds form between water and
other molecules that carry charges.
Here is an example of how water acts as an efficient solvent of table salt, or sodium chloride. Water molecules surround the sodium and chloride ions of salt. The
oxygen atom in each water molecule, with its partial negative charge, bonds to a
positively charged sodium ion. The hydrogen atoms of the water molecule, with
their partial positive charges, bond to the negatively charged chloride ions. Water's
polarity makes it an effective solvent for charged or polar solutes.
Hydrogen Bonds
The hydrogen bonds that form between water molecules are responsible for one of
water's most unusual properties. Although solids are usually denser than liquids,
water responds to a decrease in temperature below 4˚ C by expanding and becoming less dense as it changes from a liquid to a solid. This property allows ice to float
on water.
Let's examine why ice floats on water. In ice, each water molecule forms four
hydrogen bonds with four adjacent water molecules, giving the ice an ordered lat-
tice structure. In this structure, the hydrogen bonds overcome the motion of the
individual water molecules, holding them in a fixed three-dimensional pattern. In
addition, the water molecules are spaced farther apart, making ice less dense than
water.
As ice melts, hydrogen bonds are broken and the water molecules are free to move
closer together, forming liquid water. Liquid water, therefore, is more compact, or
denser, than ice.
Specific Heat
The hydrogen bonds between water molecules are also responsible for another
unusual property of water: A great deal of heat energy is required to change water's
phase (or state) from solid (ice) to liquid or from liquid to gas (steam).
Water has an extraordinarily high specific heat, or capacity to absorb energy.
Specific heat is measured as the amount of energy required to raise the temperature
of 1 gram of a substance by 1ºC.
Water's specific heat is unusually high because it contains many hydrogen bonds.
A large amount of energy is needed to break all of the hydrogen bonds between the
molecules.
Although liquid water is denser than ice, the molecules in liquid water move more
freely. Entropy, which is a measure of the disorder of a system, is higher in liquid
water than in ice. Notice that liquid water continues to absorb energy (it gets very
hot) before it becomes water vapor.
When liquid water has absorbed enough energy to break all of the hydrogen bonds,
it changes from a liquid to a gas-steam. Water vapor has the highest entropy of the
three phases. It's molecules have the greatest motion and the most space between
them.
You should now be able to…
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Describe how the oxygen and hydrogen atoms in a water molecule share
electrons.
Explain what a polar molecule is.
Explain why liquid water is denser than ice.
Discuss the entropy of water in each of its three phases (solid, liquid, gas).