Polarity of Water
A water molecule is made up of one oxygen atom covalently bonded to two
hydrogen atoms. While each hydrogen atom has only one electron to form
a bond with the oxygen atom, the oxygen has eight, so six of them remain
unbonded. These additional electrons create a slightly negative charge
around the oxygen atom, which leaves a slightly positive charge around the
hydrogen atoms. Polarity refers to the uneven distribution of charge
between the molecule’s two ends. Because a water molecule has two
poles—one negative, one positive—scientists classify water as a dipolar
(di- ⫽ “two”) molecule.
When there are many water molecules, the slightly positive and negative
regions behave like magnets: the positive end of one molecule attracts the
negative end of another and so on. Hydrogen bonding is the attraction—the
bond—that forms between a positively charged hydrogen atom of one
molecule and a negatively charged atom of another molecule. This is different
from the ionic bond that holds together two atoms within the same molecule.
25-Minute
Mini
Observing Water’s
Polarity in Action
Materials
• rubber balloon
• piece of silk cloth or fake fur
• water faucet
Procedure
1
Inflate and tie the balloon.
2
Rub the piece of silk or fake
fur on the balloon to give the
balloon a static charge.
3 Turn the faucet on low to
form a thin, steady stream of
water.
4
Oxygen
LAB
Hold the balloon near the
stream of water without
letting it touch the water.
Analysis
Hydrogen
HYDROGEN BONDS Oppositely charged regions on neighboring water molecules (H2O)
are attracted to each other.
As a result of these hydrogen bonds, the attraction among water
molecules is very strong. In order for water to change from liquid to gas,
hydrogen bonds must break. Because the molecules tend to stick together,
water has a high boiling point, meaning it requires a higher temperature
than similarly weighted compounds before it turns to vapor. By contrast,
some molecules of comparable size and mass exist as gases at room
temperature. The strong links among water molecules also make pure
water stable and less likely to change form over a range of temperatures. At
sea level, water remains liquid in temperatures ranging from 0°C to 100°C.
Hydrogen bonds determine the structure of water in its solid state.
As the water molecules freeze, they are rearranged in a rigid pattern with
relatively large spaces between them. This results in an open, lattice-like
crystal structure that is more spacious and less dense than the structure of
liquid water (see photograph and diagrams on page 492).
Describe what you observe. When
the balloon is statically charged,
electrons are transferred from the
piece of silk or fake fur to the
balloon’s surface. If you could
observe the molecules of water in
the stream, which part of the water
molecule, the oxygen atom or the
two hydrogen atoms, would be
attracted to the balloon? Explain
your reasoning.
CLASSZONE.COM
Explore a 3-D model of water
molecules.
Keycode: ES2201
Solutions of Water
Until now, you have learned about the characteristics of pure water; many
properties change when water is combined with other substances, such as
ocean salts. An aqueous solution is a mixture that forms when a salt
dissolves in water. Properties such as density, boiling point, and freezing
point vary depending on the salt concentration in an aqueous solution.
Chapter 22 The Water Planet
493