|How does water and air act like a heat reservoir?
|Question Date: 2014-12-17|
Let's start with what a heat reservoir is. A heat
reservoir is basically some system that in thermal
contact with some other system, such that the
exchange of heat does not (significantly) change
the temperature of the reservoir. In other
words, a heat reservoir is a sink and source of
thermal energy, but essentially stays at the
same temperature. A perfect heat reservoir is an
idealistic construct (i.e., there isn't a material
that would not change temperature upon exchange of
heat), but you can get pretty close to one!
What makes a good heat reservoir?
The basic property in any good heat reservoir is
the ability to absorb heat without heating up too
much (or conversely, to expel heat without cooling
down too much). This property is known as heat
capacity, and has units of Joules/Kelvin; it may
be interpreted as the the amount of energy
required to change the temperature of your
material by 1 Kelvin (or whatever units of
temperature you use). Heat capacity depends on how
much stuff you have (e.g., it take more energy to
heat up more water), so it is more useful to
define a specific heat capacity, which is has
units of Joules/Kelvin/Kilogram. Specific heat
capacity may be interpreted as the amount of
energy required to the change the temperature of 1
kg of your material by 1 Kelvin (or whatever units
of energy, mass, and temperature you use). Compare
this definition to that of heat capacity to notice
An important thing to note is that there is an
implicit assumption that we are only dealing with
specific heat capacity. Heat capacity deals with
the heating up of materials. However, you probably
know that water boils off once the temperature
hits 100 C. At this point, you are no longer
heating up the water, you are cause a change in
phase from liquid to gas. Measuring how much
energy it takes to make a change in phase is
latent heat (in this case, of
Why are water and air used as heat reservoirs?
Here is a table of specific heat capacities of
several materials, including air and water.
Why is water a particularly good heat
It is because it has such a high specific heat
capacity. That is, it takes a lot of energy to
change the temperature of water. Perhaps, this
is why a watched pot never boils...
Although it has a much higher specific heat
capacity, a reason hydrogen is not used is
because it can be quite explosive.
Basically, you can build a large enough
reservoir that surrounds the actual system you are
interested in. Once in thermal contact (via
conduction), your water or air can put in or take
out heat from the system. With enough water or air
(and possibly some convection to spread the heat
around the reservoir), you can maintain the
temperature of the reservoir. As you can see from
the above table, air is not the most ideal heat
reservoir, so water is often the material choice.
In fact, it is this same property that water is
used as a cooling agent for a number of
applications, like in nuclear reactors.
And if you are curious, where does the
specific heat capacity come from?
Or perhaps differently worded, what about water
makes it have such a high specific heat capacity?
There are two factors: 1) small molecular mass and
2) more ways to store energy.
The first one is easier to grasp. A smaller
mass means there are more molecules per unit of
mass (remember, specific heat capacity is measured
with respect to mass!), so there are more
molecules to absorb heat per kilogram for
The second is trickier. The first thing to
recognize is that a molecule is not stationary.
It moves around, rotates, and vibrates. This
ability to move around, rotate, and vibrate is
encapsulated in an idea known as a degree of
freedom. You can imagine not all molecules have
the same amount of freedom to move around as other
do. The more degrees of freedom, the higher the
heat capacity. That is, the more degrees of
freedom, the more ways you can store energy.
Water is not only a relatively light molecule,
but it has a
bent structure -
The angle between the hydrogens is not 180
degrees. This bent structure gives water more
degrees of freedom than for instance nitrogen gas
(N2), where the angle between the
nitrogens is 180 degrees. In fact, it's this bent
structure that gives water many of its peculiar
properties (like why ice floats in water, even
though it's a solid).
For something to be called a heat reservoir, it
must be able to give or take a lot of energy from
another object without changing temperature
itself. Usually this is accomplished by a heat
reservoir having a lot of mass which means it has
a lot of molecules. An object’s temperature
will rise as more energy is added which means that
on average the molecules in the object are moving
faster. If the object has more molecules in
it, it will take more energy for all of the
molecules to move faster and will therefore not
raise in temperature as much as a smaller object.
Water and air are probably the two easiest and
cheapest substances to get a lot of so it is easy
to make a heat reservoir out of them.
Additionally, water has the ability to absorb a
lot of energy before rising in temperature which
makes it a good heat reservoir. However, if
you have enough of any substance it could
potentially act as a heat reservoir though air and
water are probably some of the best choices.
This is a good question! Water, air, and other
materials all exhibit a property called
"specific heat." Different materials have
different specific heats. The specific heat of a
material is the amount of thermal energy (i.e.
"heat") it takes to raise the temperature of a
unit mass of the material by 1 degree. Since
different materials can have different masses, and
we want to be able to accurately compare specific
heats of materials, we say "unit mass," which
could be a gram or a mole, for example. If we
want to know how much thermal energy it takes to
raise the temperature of a specific example of
material (for example, the swimming pool at a
school gym, or a pond in someone's garden), then
we can look at heat capacity, which is an
extensive quantity. Extensive means that the it
depends on how much of the material there is.
It turns out that water has a very high
specific heat and heat capacity, and therefore can
absorb a lot of thermal energy as its temperature
increases. That means that at any given
temperature, a body of water has a certain amount
of thermal energy stored. This thermal energy can
then be released slowly as the temperature of that
body of water decreases.
The specific heat of air is about 1/4 that of
water. This means that when you raise the
temperature of a body of water by 1 degree, and
you raise the temperature of a body of air that
has the same mass as the water by 1 degree, the
air's thermal energy only increased by 1/4 the
amount the water's thermal energy increased.
However, it still absorbs thermal energy when its
temperature increases and therefore can still act
like a "heat reservoir!"
Water and air, like everything else on earth is
made of atoms. Atoms bond together to form
molecules. For example, chemically pure water
is made of two Hydrogens (H) and one Oxygen (O)
that are bound together forming the H2O
molecule. You can imagine the H2O
molecule as two tiny balls of H attached to O by
springs in a V-shaped geometry.
Each of these springs can compress and
decompress, a process by which they can store
"vibrational energy". In addition the
molecule can rotate about an axis, which stores
rotational energy. It is also free to move
in any direction, which store kinetic
energy. These different ways of storing energy
are called different degrees of freedom.
It is the energy that is stored within these
degrees of freedom, that we call as "heat".
More the heat stored, more the atoms
vibrates, and faster the molecules move and
rotate. Molecules often bump into each other, and
when they do, they exchange heat energy.
This is how water and any other substance
stores heat and act as "heat reservoir". Air is a
mixture of many gases, such as water vapor,
Oxygen, Nitrogen, etc.. Each of these molecules
can be imagined as little vibrating and moving
balls that store energy.
When water is cooled, you are essentially
removing the stored energy. This cause the
molecules to move and vibrate slower and slower.
On cooling, you reach a point where the molecules
cannot move at all and are stuck to a specific
position. This is when you form ice.
When water is heated, you are adding energy. This
causes the molecule to vibrate and move
vigorously. At this point (called the boiling
point), some molecules start jumping out of the
vessel to form water vapor, which is the gas
Water is composed of water molecules, which
when heated up, either on your stovetop in the
case of a pot of water or by the sun in the case
of a lake, start moving faster. They can translate
faster, rotate faster and vibrate a little faster
too. This extra ability for motion that they have
means that they have more energy: more kinetic and
rotational energy. We can measure this energy by
measuring the temperature of the water. Higher
temperature means more energy exists.
Have you noticed how it easier to heat up
oil in a pan than water? To increase the
temperature of the water by 1 degree, it takes
much more energy compared to increasing the
temperature of the oil. This means that we can add
and remove big chunks of energy from water without
changing its temperature much! So, water is a
great heat reservoir! ...and oil is not,
because its temperature fluctuates a lot as we add
and remove energy. Oil is really good at
transferring heat, so we use it in car engines to
take the energy away so that we don't melt our
engine. In scientific language, how much energy a
substance can absorb before it changes its
temperature by one degree is called "heat
capacity". Water has a large heat capacity,
larger than oil. If you think of the sea and the
lakes, their temperature does not change very much
between day and night or winter and summer,
exactly because water has a large heat capacity.
This is why the changing of the temperature of the
oceans is such a big deal.
What we call humidity is the water contained
in the air. Death Valley today has humidity 64%,
and Santa Barbara 82%. Since Santa Barbara has
more water in the air, it should be able to keep
the temperature more constant than Death Valley
throughout the day. That's why desert temperature
fluctuates a lot during the day. It is not the
only factor, of course. Air can act as a heat
reservoir too. But because it doesn't have many
molecules in space, it is very dilute, there are
not many molecules to take up the energy by
translating and rotating faster. Therefore, if we
want a heat reservoir, it's not a good idea to use
air, and we should prefer water.
If you want to read more, a similar question was
answered previously in ScienceLine
click here to read
This is actually due to the nature of heat.
Heat is actually the energy stored in the
vibrations of the atoms of a material. The
temperature of a material that we measure is
directly related to the speed of the vibrations of
the atoms that make up that material. Heat can
travel in a variety of ways, one of which is where
vibrating atoms bump into their neighbors and
cause them in turn to vibrate. This method of
transferring heat is called conduction. Another
important concept to understanding heat is a
concept called specific heat. This means that
different materials respond differently when heat
energy is added to them. The difference between
water and air is a good example of this. If you
were to add exactly the same amount of heat energy
to an equal weight of both water and air, their
temperatures would not go up the exact same
amount. The air's temperature would go up
significantly more than the water's temperature.
This means that the air molecules would be
vibrating more quickly than the water molecules.
In short, it takes more energy to get the water
molecules vibrating at the same speed. This means
that water can act as a heat reservoir, because it
can absorb more heat than other materials, while
having its temperature change by a small amount.
This is why blacksmiths plunge red hot metal into
water, and it is also related to why the climate
near oceans is much more moderate than it is
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