TEMPERATURE MODERATION
In addition to the properties already discussed in this chapter, water also has a high heat
capacity. What this means is that water can absorb or release large amounts of energy in the form of
heat while only slightly changing its temperature. Energy must be absorbed to break hydrogen bonds,
and energy is released as heat when hydrogen bonds form. The energy that water initially absorbs
breaks hydrogen bonds between molecules. Only after these hydrogen bonds are broken does the
energy begin to increase the motion of the water molecules, which raises the temperature of the water.
When the temperature of water drops, hydrogen bonds reform, which releases a large amount of
energy in the form of heat. Temperature is a measure of the average kinetic energy (energy of motion)
of particles in a sample of matter. This physical property can determine the rate and extent to which
chemical reactions can occur within living systems. It is commonly measured in degrees Celsius (°C) or
Fahrenheit (°F).
Furthermore, water has a high specific heat, meaning it takes a lot of energy to raise or lower
the temperature of water. Specific heat is a measure of how much energy it takes to raise the
temperature of a substance. Specific heat (measured in cal/(g°C)) is a property that is unique to a given
type of matter. That’s why it’s called specific. Every substance has its own specific heat capacity, with
the specific heat capacity of water being 1 cal/(g°C). The specific heat capacity of water is much higher
than that of other common substances. For the sake of comparison, the specific heat capacity of oil is
about 0.5 cal/(g°C) and the specific heat capacity of aluminum is about 0.2 cal/(g°C). This means it takes
more heat to raise the temperature of water than it would take to raise the temperature of oil or
aluminum.
As a result, water plays a very important role in temperature regulation. Since cells are made up
of water, this property helps to maintain homeostasis. For example, since it takes such a large amount of
energy to change the state of water, sweating is a very effective method of cooling the body. In order to
evaporate, the sweat requires the input of a great deal of heat energy, some of which comes from our
bodies. So, as our sweat evaporates, we begin to feel cooler.
Freezing Point of Water
The freezing point, the temperature at which a liquid changes state to a solid, of water is 0°C
(32°F). Below this temperature, water is a solid (ice). Unlike most chemical substances, water in a solid
state has a lower density than water in a liquid state. This is because water expands when it freezes.
Again, hydrogen bonding is the reason. Hydrogen bonds cause water molecules to line up less efficiently
in ice than in liquid water. As a result, water molecules are spaced farther apart in ice, giving ice a lower
density than liquid water. A substance with lower density floats on a substance with higher density. This
explains why ice floats on liquid water, whereas many other solids sink to the bottom of liquid water.
In a large body of water, such as a lake or the ocean, the water with the greatest density always
sinks to the bottom. Water is most dense at about 4°C (39.2°F). As a result, the water at the bottom of a
lake or the ocean usually has temperature of about 4°C. In climates with cold winters, this layer of 4°C
water insulates the bottom of a lake from freezing temperatures. Lake organisms such as fish can
survive the winter by staying in this cold, but unfrozen, water at the bottom of the lake.
Boiling Point of Water
Boiling point is the temperature at which a liquid changes state into a gas. Hydrogen bonds,
which tend to hold water molecules together cause water to have a relatively high boiling point of 100°C
(212°F). This high boiling point means that liquid water turns into water vapor at a higher temperature
than would be expected due to the size and mass of the molecule. Substances similar to water have a
much lower boiling point because they lack hydrogen bonds. Most water on Earth is in a liquid state
rather than a gaseous state because of its high boiling point. Water in its liquid state is needed by all
Chapter 3: Unique Properties of Water 63