According
to the kinetic-molecular theory, heat energy
acquired by a body is transformed into increased kinetic energy of the
molecules of the body. We observe this increased kinetic energy whenever a
solid, a liquid, or a gas expands on heating. A further increase in
kinetic energy will eventually cause the particles of a solid or liquid
to become a gas.
Recall that when an ice cube (a solid) is heated, it
melts and becomes liquid water. When the water is heated, it vaporizes and
becomes gaseous water. According to the kinetic-molecular theory, as
increasing amounts of heat are supplied to a piece of ice, the water
molecules move more rapidly until they gain sufficient energy to overcome
the attractive forces holding them together. This permits the ice to liquefy
and become water. Similarly, as still more energy is received, the water
molecules move at even greater speeds. The attractive forces in the liquid
are weakened and the water is converted into gaseous water.
EXPANSION OF SOLIDS
Your laboratory experience with the ball and ring
apparatus indicated the effect of heat on volume. The increase in size is
not due to an increase in the size of the particles that make up the solid
ball, but rather to an increase in the average distance between the
particles. When an object is heated, its particles vibrate faster, collide
more violently, and consequently move farther apart, thereby increasing the
volume of the object.
When the object is cooled, the opposite change occurs
and the volume of the object decreases. This decrease in volume is called contraction.
The expansion of solids by heating may cause serious
practical problems. For example, the expansion of railroad tracks, bridges,
or the concrete in a roadbed can create dangerous situations. Thus,
allowance for the expansion of solids daring hot weather must he made in the
construction of rails, bridges, and roads. For example, when rails arc laid,
gaps between the ends of the rails provide for expansion. If this were not
done, consider what would happen to the railroad tracks on a very hot day.
The metal would expand, making the tracks bend and buckle,
which might cause an oncoming
train to be derailed. In bridge construction, expansion joints allow for
changes in the length of the bridge. Concrete roadbeds are built with spaces
between the sections of concrete to allow for expansion.
The contraction of solids, by cooling, may also
present problems. Thus telephone and electrical wires are strung loosely to
prevent their snapping as contraction takes place during the colder times of
the year.
UNEQUAL
EXPANSION OF SOLIDS
Through extensive studies, scientists have found that
different metals expand at different rates when they are heated. For
example, when a piece of iron and a piece of aluminum of equal size are
heated together, we find that the aluminum expands more than twice as much
as the iron. When two strips of different metals are fastened together, they
form a compound bar, or bimetallic strip, which is employed in, useful
devices such as thermostats and metallic thermometers. In these devices the
two different metals, usually brass and steel, are welded together. When the
bar is heated, it bends because the brass expands more than the iron and
becomes longer than the iron.
Thus, the brass strip will be on the outside of the
bend. As it cools, the bar returns to its original shape.
The thermostat is a device containing a compound liar
that regulates the heating systems of our homes. When the temperature in the
house falls below the setting on the thermostat, the compound bar, which
contracts as it cools, closes the circuit, turning on the heat. As
the room is
warmed, the compound bar in the thermostat expands, bends, and thereby
breaks the circuit, shutting off the heat.
The bimetallic thermometer is often used as an oven
thermometer to indicate the temperature within an oven, or within a piece of
meat that is cooking.
EXPANSION OF LIQUIDS
Liquids, like solids, expand when heated. In the
laboratory experience we demonstrated that when water is heated, it expands.
When the same water is cooled to its original temperature, the water
contracts to its original volume. Many other liquids, such as alcohol and
mercury, behave in the same way. At lower temperatures, however, the
behavior of water is an exception to this rule. As water is cooled from
100° C to 4° C, it contracts-like other liquids do. However, when water is
cooled below 4° C, the water expands-unlike other liquids. Water continues
to expand until it reaches 0° C, its freezing point. It has been found, as
shown in that the spaces between the water molecules in ice are larger than
the spaces between the water molecules in liquid water. Ice is therefore
said to have an open structure. Thus, as ice is formed, the need for
increased space between the molecules causes the volume of the ice to be
greater than that of the water from which it was formed. (This expansion in
volume begins as liquid water
is cooled below 4° C.) Since the volume of ice is greater than the
volume of water from which the ice is formed, the density of ice is less
than the density of water. (Recall that density equals weight divided by
volume.) This is why ice floats on water.
Like solids, different liquids expand at different
rates. As we will see later, the expansion of liquids is used in alcohol and
mercury thermometers.
The expansion of liquids must be considered in
certain heating systems. In a hot-water heating system, allowances
must be made for the expansion of heated water. As the furnace heats the
water in the heating system, the water expands. If the expansion continues,
pressure would build up in the pipes and could damage the entire system. To
avoid this difficulty, an expansion tank is provided. Excess heated water
enters the expansion tank and thereby reduces the pressure in the system.
EXPANSION OF GASES
Cases, like solids and liquids, expand when heated.
Our laboratory experience indicated that, as air is warmed, it expands.
Scientists have made similar observations with other gases which indicate
that gases confined in an elastic container expand when they are heated and
contract when they are cooled.
The expansion of gases by heat must be considered by
automobile tire. manufacturers, since tires may burst if allowed to remain
in the sun indefinitely. A less serious hazard caused by expanding gases is
that bottles of soda may crack or even explode if they are ex posed
to heat for a considerable length of time.
Products
such as whipped cream, shaving cream, deodorants, and insect repellents are
now supplied in aerosol cans. These cans contain the product itself
and a gas that forces the product
out of the can when the valve, is open. When the product is used up, the can
still contains unused gas. If this can is thrown into incinerator, the gas
becomes heated, expands, and may cause the can to explode. Such aerosol cans
should be discarded in a manner that does not involve heating.
Different solids and liquids expand at
different rates when heated. Gases,
however, generally expand at the same rate when heated to the same
temperature, at a given pressure.
TEMPERATURE
Heat and temperature are two terms that are often confused.
We know that the temperature of a small sample of molten iron is
considerably higher that the temperature of the water in the ocean.
However, the total heat in a sample of molten iron is much less than
the total heat of the water in the ocean.
Scientist now accept Rumford’s theory that heat is related to the
motions of particles in matter. Thus,
heat depends on the total kinetic energy of the particles in a body.
Recall the equation relating kinetic energy with mass and velocity:
K.E. = ½ m v2. Thus,
the total kinetic energy of the particles in a body depends on the number of
particles (mass) and the velocity of these particles.
Because the water in the ocean is colder than the sample of molten
iron, the velocity of the particles in the water is less than the velocity
of the particles in the molten iron. However,
the much larger quantity (mass0 of water compensates for the smaller
velocity of the particles and thus the particles of water in the ocean
possess greater kinetic energy. This means that there is more heat in the water in the ocean
than in a small sample of molten iron.
But why is the temperature of molten iron higher?
Temperature, unlike heat, depends on the average kinetic energy of
the particles, that is, the kinetic energy per particle.
To find this average, we divide the total kinetic energy by the
number of particles. Thus, the
large mass of ocean water has a smaller average kinetic energy per particle
and consequently has a lower temperature than a small sample of molten iron.
MEASURING
TEMPERATURE
Instruments designed
to measure temperature are called thermometers.
Most thermometers are based on the principle that matter, on heating,
expands and, on cooling, contracts. In
general, matter expands and contacts regularly.
This means that the amount of expansion or contraction in length are
generally equal for the same increase or decrease in temperature.
This regular expansion and contraction has made it possible to
construct three different types of thermometers: gas (air), liquid (mercury
and alcohol), and solid (bimetallic) thermometers.
The gas (air) thermometer
In this thermometer, the glass bulb contains air.
When the bulb is warmed, the air in the bulb expands and forces some of the
colored water out of the tube. This changes the level of the liquid in the
tube. By placing a suitable scale alongside the tube, temperature changes
can be measured. Air thermometers of this type, while interesting, are not
very accurate because the volume of a gas is also influenced by the air
pressure around it. (Note that the flask contains a tube open at both ends.
Why?).
LIQUID THERMOMETERS
Thermometers containing liquids such as mercury and
alcohol are useful and accurate because these liquids usually expand and
contract uniformly (regularly).
Mercury thermometers are made by filling a thin glass
tube with mercury at a temperature greater than the maximum to be measured.
The tube is then cut and sealed at the top. When the mercury cools, it
contracts, leaving a partial vacuum above the mercury. (Liquids expand and
contract to a much greater extent than do solids. Thus, in the given
temperature range, the glass tube is scarcely affected by the temperature
change.) The partial vacuum eliminates the effect of air resistance to the
expansion of the mercury. The scale of the. thermometer is generally fixed
by locating the boiling and freezing points of water on the scale. The
distance between the boiling and freezing points is then divided into units
depending on the temperature scale used. This will be discussed in the next
section.
Since mercury freezes at -39° C, it cannot be used
to measure very low temperatures. However, mercury boils at 357° C, which
means that a mercury thermometer can be used to measure temperatures above
the boiling point of water. On the other hand, alcohol freezes at -114° C.
Accordingly, alcohol thermometers are used to measure low temperatures.
However, since alcohol boils at 78° C, alcohol thermometers cannot be used
to measure high temperatures.
SOLID (BIMETALLIC)
Recall that a bimetallic strip behaves as it does
because different metals expand at different
rates. Because most metals melt only at very high temperatures, a
thermometer that uses a bimetallic strip can measure temperatures as high as
1000° C. The dial thermometer, used in most homes as an oven thermometer,
is an example of a bimetallic thermometer. A curved bimetallic strip, with
the faster-expanding metal on the outside of the bend, is attached to a
pointer. Upon heating, the bimetallic strip moves, causing the pointer to
indicate the temperature on a circular scale.
Other metallic thermometers, called resistance
thermometers, use the principle that the resistance of a wire changes with
temperature. Such thermometers also measure high temperatures.
THE
FAHRENHEIT AND CELSIUS
TEMPERATURE SCALES
Temperature markings on thermometers are indicated in
Fahrenheit degrees or Celsius degrees. The Fahrenheit and Celsius scales are
named after their originators, Gabriel
Fahrenheit and Anders
Celsius. (The Celsius scale is also called the centigrade
scale.) Both Fahrenheit and Celsius scales are calibrated by using the
boiling and freezing points of water. The Fahrenheit scale is used in the
English system of measurement and the Celsius scale in the metric system. In
the Fahrenheit scale, the freezing point of water is 32.° F, and the
boiling point of water is 212° F. The remainder of the scale between these
two points is marked off into 180 equal divisions (212 - 32 = 180) .
In the Celsius scale, the freezing point of water is 0° C and the boiling
point of water is 100° C. The remainder of the scale between these points
is divided into 100 equal divisions (100 ‑ 0 = 100). Note that there
are 180 divisions between the freezing and boiling points of water in the
Fahrenheit scale and 100 divisions between these points in the Celsius
scale. Thus, each Celsius division ( degree ) is 9/5 as large as
Fahrenheit division. This relationship, together with the fact that
there are 32 Fahrenheit divisions between 0 °F and 32° F, makes it
possible to convert one scale into the other by using the following
formulas:
°
C = 5/9(° F – 32)
°
F = 9/5 °C + 32
THE KELVIN SCALE
Confined gases, like most solids and liquids, expand
and contract uniformly. For this statement to be true, however, a gas must
be heated or cooled in such a way that the pressure remains constant. (
Recall that the air thermometer is inaccurate because it is affected by
surrounding air pressure.) If we start at 0° C, we find that, for every
Celsius degree rise in temperature, the volume of a gas increases 273 of its
original volume ( provided the pressure does not change). Similarly, if we
again start from 0° C, we find that for every Celsius degree drop in
temperature, the volume decreases 273 of its original volume. At -273° C,
the volume of a gas would shrink to zero and all molecular motion would
cease. This, in turn, means that the gas would contain no heat.
(Actually, gases generally liquefy before this
temperature is reached.) Scientists refer to -273°C as absolute zero, a
temperature that has never been attained, although some scientists have come
very close to this point.
Absolute zero, -273°C, is also called 0 Kelvin ( 0 K
). The Kelvin scale, named after its originator, Lord Kelvin, is based on absolute temperatures. Since the Kelvin scale begins with
absolute zero (-273° C), we use the following formula to convert the
Celsius scale to the Kelvin scale:
degrees Kelvin = degrees Celsius
+ 273 degrees
This formula may be written as
K = ° C + 273
Let us find the freezing point of water (0° C) in
the Kelvin scale:
K=0 + 273=273
K
Thus, 0° C is equivalent to 273 K.
Now, let us find the boiling point of water:
K =100 + 273 = 373 K
Thus, 100° C is equivalent to 373 K.
TRANSFER
of HEAT
When
a
metal spoon is placed in a bowl of hot soup, the entire spoon soon becomes
hot because the heat travels from the soup to the bowl-shaped part of the
spoon, and then to the handle. When ice is placed in warm water, the ice
soon melts. Both of these examples show that heat travels from one body to
another. Generally, when objects are at different temperatures, heat is
transferred from the warmer object to the cooler object until both objects
are at the same temperature. Heat transfer can occur through one of three
methods: conduction, convection, or radiation.
CONDUCTION
When one end of a metal rod is held in a flame, the
entire rod will become hot enough to burn the hand. The heat from the
flame reaches the hand by traveling through the rod. Substances that allow
heat to travel through them are called conductors. In general, as we
learned before, metals are good conductors. However, some metals conduct
heat more readily than others. This can be demonstrated by inserting rods
of aluminum, copper, iron, nickel, and brass into a brass sphere or disk
and then attaching a small ball of wax to the end of each rod. When the
center of the brass disk is heated, the wax at the tip of each metal melts
in the order in which the different metals conduct heat. The wax at the
tip of the copper melts first and the wax at the tip of the iron melts
last.
Conduction in most materials can be explained by the
kinetic-molecular theory. When one end of a rod is heated, the molecules
in that end of the rod vibrate faster and strike other nearby molecules,
causing them to vibrate faster. In this manner, the increased molecular
motion is transferred from one end of the rod to the other, permitting the
heat to travel through the rod.
Substances that do not readily allow heat to pass
through them are called insulators. Gases and liquids are generally poor
conductors of heat because their molecules are farther apart than are the
molecules in solids. Therefore, neighboring molecules in a gas or in a
liquid are less affected by the increased motions of heated molecules, and
consequently heat is not conducted rapidly.
Substances like wood or plastic are poor conductors
of heat, so they are used to make handles for metallic objects that are to
be heated. The clothing we wear is also a poor conductor of heat, enabling
us to retain body warmth. Porous material is generally non-conducting
because it contains layers of trapped air which do not permit heat
transfer.
CONVECTION
Although gases and liquids are poor conductors of
heat, heat is transferred through them by the process of convection.
Convection is the transfer of heat due to the motion of the liquid or gas
itself. For example, when a beaker of water is heated the water layer closest to the heat source is warmed slowly by
conduction. As the water becomes warmer, it expands, becomes less dense,
and rises. This brings heat to the upper layer. At the same time, cooler water
from the upper portion
of the
beaker moves down, takes the place of the rising water, and becomes heated
itself. When warm enough, this water rises and carries heat upward. As
these processes continue, heat that enters the bottom of the beaker is
distributed throughout the beaker until all the water is at the same
temperature. The moving water in such a case is said to have set up a
convection current.
Heat is also transferred through gases by convection.
It is by this means, in part, that a stove or a radiator heats a room.
Heat from the radiator warms the air above it, causing the air to expand,
become less dense, and rise. The cooler air that moves in to take the
place of the warmed air is also soon warmed. As this air rises, a
convection current is established. The convection current continues to
distribute heat throughout the room until the entire room is warmed.
The formation of a convection current in air is
demonstrated with a convection box apparatus. First the candle is lighted,
then smoking touch paper is placed over the
chimney, opposite the candle. The smoke, coloring the air, can be
seen to move down this chimney, across the box, and out through the other
chimney. This occurs because the air over the candle is heated, becomes
less dense, and rises, leaving a partial vacuum. Cooler, more dense air
from the first chimney moves in to fill the partial vacuum. This cycle
continues as long as heat is given off by the burning candle.
RADIATION
We know that light energy and heat energy travel from
the sun to the earth through space, which is an almost perfect
vacuum. These forms of energy, traveling without the aid of molecular
collisions, are transferred from the sun to the earth by radiation, that
is, by means of rays, or waves. You can understand this method of heat
transfer by standing a short distance from an open fire or by placing your
hand a little to one side of, but not touching, a hot radiator. Since
neither source of heat is being touched, you cannot receive heat by
conduction. Since warm air rises vertically from the heat source, the heat
cannot reach you by convection. The heat that is transferred to you from
the fire or radiator reaches you by radiation.
The heat radiated by one body ( the sun, for example)
is most rapidly absorbed by other bodies that are black in color and rough
in texture. In warm climates, white clothing
which reflects the radiant heat of the sun is cooler
than dark clothing which quickly absorbs the radiant heat. Similarly,
bodies that are rough and dark tend to radiate heat better than shiny
smooth bodies. This is why steam radiators are often dark and have a
roughened surface. It is for the same reason that coal burning stoves are
black.
Bodies that are shiny and smooth do not absorb heat
readily. Instead, these bodies reflect heat. Thus, aluminum used for
roofing keeps homes cool in the summer and warm in the winter. This
principle is utilized in the thermos (vacuum) bottle, which is so
constructed as to permit liquids to retain their temperatures for a long
time. A thermos bottle is double walled, with a partial vacuum between the
walls to prevent heat transfer by conduction or convection. A cork stopper
also prevents heat transfer by conduction. The inner glass walls are
silvered to reflect radiant heat back into the liquid, thereby minimizing
heat loss by radiation. Thus, a hot liquid remains hot because heat is
lost very slowly. A cold liquid remains cold in thermos bottles because
outside heat enters very slowly by conduction, convection, or radiation.
MEASURING
HEAT
We learned that temperature is a measure of the average kinetic
energy of the molecules of a substance. This is the same as saying that
temperature represents the average intensity of the motion of the
molecules, or the degree of hotness of a substance. Average kinetic energy
means the total kinetic energy divided by the total number of particles.
Recall that the ocean contains much more heat
than does a
small amount of molten iron. However, since the ocean contains many more
particles than the molten iron, the temperature
of the ocean
(that is, the total kinetic energy divided by the total number of
particles) is lower than that of the molten iron.
Temperature, therefore, does not tell us the quantity
of heat present. The quantity of heat represents the total kinetic energy
contained by all of
the particles of the substance.
In the metric system, we measure the quantity of heat
by a unit called the calorie. The calorie is the amount of heat needed to
raise the temperature of 1 gram of water 1 Celsius degree. In the English
system, heat is measured by a unit called the British Thermal Unit (BTU).
A BTU is the amount of heat needed to raise the temperature of 1 pound of
water 1 Fahrenheit degree. The amount of heat energy present in a
substance cannot be measured directly with simple measuring devices.
Instead, it is measured by observing its effect on a given quantity of
water in a device called a calorimeter.
One type of calorimeter, consists of two polished metal cups
surrounded by air, a poor conductor of heat.
An insulating cover, holding a thermometer, makes up the top of the
calorimeter. The polished cups reflect heat, thus maintaining the
temperature of the liquid in the container.
To determine the amount of heat energy absorbed (or
lost) by a given quantity of water, we multiply the weight of the water in
grams by the change in temperature of the water in Celsius degrees. Thus:
amount of heat = weight of water X change in temperature
In a calorimeter, when 20 grams of water at 20°C are
heated to a temperature of 30°C, how much heat is absorbed?
The temperature change = the final temperature - the
initial temperature = 30°C - 20°C = 10 Celsius degrees. Substituting,
amount of
heat = 20 grams X 10 C° = 200 calories
We conclude that 200 calories have been absorbed.
(We assume that no heat has escaped from the calorimeter.)
HEAT
EXCHANGE IN WATER
In a calorimeter, when a quantity of water at a given temperature is
mixed with a quantity of water at a different temperature, the amount of
heat lost by the "hot" water is equal to the amount of heat
gained by the "cold" water.
Suppose
we mix 100 grams of water at 90° C with 100 grams of water at 40° C and
find that the final temperature of the mixture is 65° C. Let us calculate
the number of calories lost. by the hot water and gained by the cold
water.
The
temperature of the hot water dropped from 90° C to 65° C, a decrease of
25 Celsius degrees. Since we began with 100 grams of hot water that
underwent a temperature change of 25° C, we determine the amount of heat
lost:
amount of heat =
weight of water X change in temperature
(calories)
(grams) (Celsius
degrees)
amount of heat = 100 grams X 25° C amount of heat = 2500 calories
The minus sign in the answer indicates that 2500
calories of heat have been lost.
The temperature of the cold water increased from 40° C to 65° C,
an increase of 25 Celsius degrees. Since we began with 100 grams of cold
water that underwent a temperature
change of 25° C, we determine the amount of heat gained:
amount
of heat = weight of water X change in temperature
(calories)
(grams)
(Celsius degrees)
amount of heat = 100 grams X 25° C amount of heat = 2500 calories
Note that the amount of heat lost by the hot water
(2500 calories) is the same as the amount of heat gained by the cold water
(2500 calories). We assume that the heat exchange was "perfect"
and that no heat escaped from the calorimeter.
The
quantity of heat needed to raise the temperature of 1 gram of a substance
1 Celsius degree is called the specific heat of the substance. For
water, the specific heat is 1. This means that 1 calorie of heat will
raise the temperature of 1 gram of water 1° C. Water is the only
substance for which this is true. Other substances vary in the quantity of
heat needed to raise 1 gram of the substance 1 Celsius degree.
Consequently the formula
amount of heat =weight of water X change in
temperature applies only to
water.
CALORIES
AND FOOD
Your body requires energy in order to perform its daily tasks.
Most of this energy comes from energy‑rich foods such as
carbohydrates and fats. This energy is released when the body utilizes
these foods. Using special calorimeters, scientists have measured the
energy content, or the number of calories present, in fixed quantities of
certain foods. For example, a slice of white bread contains about 60 000
calories; a typical chocolate bar may contain about 300 000 calories.
Nutritionists
use a special kind of notation when discussing the calorie content of
foods. They define a food Calorie ( written with a capital letter) as 1000
calories. On a calorie table, therefore, we would read that a slice of
white bread contains about 60 Calories and that a chocolate bar contains
about 300 Calories.
This
information comes from "Physical Science Workbook", 1961
The History of Heat
Aristotle and the Greeks had their idea of fire as one of the 4 Primal
Elements. Even the ancients realized that heat and light were not alike as aspects of fire,
though. After the fire had gone out and the light gone, the heat of the
kettle and its contents remained.
First modern chemist to study heat was
Joseph Black (1728 - 1799). Black
tried to explain heat in terms of a fluid.
He explained how a kettle of water placed over a fire increased in
temperature but a kettle filled with water and ice placed over a fire did
not change in temperature till all the ice was melted.
He said that until the ice was saturated with the heat-fluid and thus
became melted could its temperature rise.
Lavoisier accepted this theory and gave the name for this heat-fluid
“caloric” from the Latin word for heat.
Another idea competed with the caloric theory.
Scientist knew that kinetic energy of motion plus the stored energy
called potential energy was given the name mechanical energy and that
friction was a part of the conservation of these energies. They knew friction could warm up an object so maybe the
invisible motion of invisible particles was what we call heat. Summed up;
friction was converting mechanical energy into heat. The problem was this
idea of really small particles of matter (i.e., atoms and molecules).
Count Rumford (really Benjamin Thompson - a spy for the British
authorities during the Revolutionary War) was to supervise the boring of
cannon for Bavarian army. Using
a horse to work a treadmill he realized that the solid block of brass grew
hot as the borer cut its way in. Rumford
calculated that if the caloric theory were correct the heat released during
the boring would have melted the entire block of metal first.
He pointed out that heat was produced without fire, without light,
without chemical combustion. It
came just out of motion.
John Dalton comes along with his ideas of atoms and the kinetic
energy idea of heat began to gain favor.
An English physicist, James Prescott Joule (1818-1889) was attempting
to find the mechanical equivalent of heat.
In the end he found that a given amount of energy of whatever form
always yielded that same amount of heat (at 4.18 joules per calorie). The relationship of the motion of atoms to temperature and
heat was placed on firm theoretical basis about 1860 by the Scottish
physicist James Clerk Maxwell.
Specific Heat
The ability of water to stabilize temperature depends on its relatively
high specific heat. The specific heat of a substance
is defined at the amount of heat that must be absorbed or lost for 1 g of that
substance to change its temperature by 1º C.
The specific heat of water is 1.00 cal/g ºC.
Compared with most other substances, water has an unusually high
specific heat. For example, ethyl
alcohol, the type in alcoholic beverages, has a specific heat of 0.6 cal/g
ºC.
Because of the high specific heat of water relative to other materials,
water will change its temperature less when it absorbs or loses a given amount
of heat. The reason you can burn
your finger by touching the metal handle of a pot on the stove when the water
in the pot is still lukewarm is that the specific heat of water is ten times
greater than that of iron. In
other words, it will take only 0.1 cal to raise the temperature of 1 g of iron
1ºC. Specific heat can be
thought of as a measure of how well a substance resists changing its
temperature when it absorbs or releases heat.
Water resists changing its temperature; when it does change its
temperature, it absorbs or loses a relatively large quantity of heat for each
degree of change.
We can trace water’s high specific heat, like many of its other
properties, to hydrogen bonding. Heat
must be absorbed in order to break hydrogen bonds, and heat is released when
hydrogen bonds form. A calorie of
heat causes a relatively small change in the temperature because must of the
heat energy is used to disrupt hydrogen bonds before the water molecules can
begin moving faster. And when the
temperature of water drops slightly, many additional hydrogen bonds form,
releasing a considerable amount of energy in the form of heat.
What is the relevance of water’s high specific heat to life on Earth?
By warming up only a few degrees, a large body of water can absorb and
store a huge amount of heat from the sun in the daytime and during summer.
At night and during winter, the gradual cooling water can warm the air.
This is the reason coastal areas generally have milder climates than
inland regions. The high specific
heat of water also makes ocean temperatures quite stable, creating a favorable
environment for marine life. Thus,
because of its high specific heat, the water that covers most of planet Earth
keeps temperature fluctuations within limits that permit life.
Also, because organisms are made primarily of water, they are more able
to resist changes in their own temperatures than if they were made of a liquid
with a lower specific heat.
Water is one of the few substances that are less dense as
a solid than as a liquid. While
other materials contract when they solidify, water expands. The cause of this exotic behavior is, once again, hydrogen
bonding. At temperatures above
4º C, water behaves like other liquids, expanding as it warms and contracting
as it cools. Water begins to
freeze when its molecules are no longer moving vigorously enough to break
their hydrogen bonds. As the
temperature reaches 0º C, the water becomes locked into a crystalline
lattice, each water molecule bonded to the maximum of four partners.
The hydrogen bonds keep the molecules far enough apart to make ice
about 10% less dense than liquid water at 4º C.
When ice absorbs enough heat for its temperature to increase to above
0º C, hydrogen bonds between molecules are disrupted.
As the crystal collapses, the ice melts, and molecules are free to slip
closer together. Water reaches it
greatest density at 4º C and then begins to expand as the molecules move
faster.
The ability of ice to float because of the expansion of water as it
solidifies is an important factor in the fitness of the environment. If ice sank, then eventually all ponds, lakes, and even the
oceans would freeze solid, making life as we know it impossible on Earth.
During summer, only the upper few inches of the ocean would thaw.
Instead, when a deep body of water cools, the floating ice insulates
the liquid water below, preventing it from freezing and allowing life to exist
under the frozen surface.
| Metal |
Specific Heat |
Thermal Conductivity |
Density |
Electrical Conductivity |
| |
cp
cal/g° C |
k watt/cm K |
g/cm3 |
1E6/Ωm |
| Brass |
0.09 |
1.09 |
8.5 |
|
| Iron |
0.11 |
0.803 |
7.87 |
11.2 |
| Nickel |
0.106 |
0.905 |
8.9 |
14.6 |
| Copper |
0.093 |
3.98 |
8.95 |
60.7 |
| Aluminum |
0.217 |
2.37 |
2.7 |
37.7 |
| Lead |
0.0305 |
0.352 |
11.2 |
|
Heat
and Temperature Teaching Notes
1)
Heat flows from hot to cold areas
due to a temperature difference only.
example: A small hot block
of a material is placed next to a larger, cooler block. Heat flows from the
small hot block to the larger block till equilibrium is reached.
2) Note the difference between the heat content and temperature. A
lake may be cooler in temperature than a liter flask of water but the lake has
a much greater heat content due to the vast number of particles and their
associated motion.
3) Human perception of heat and temperature is not adequate for
scientific work. So we must investigate tools that provide the accuracy and
repeatability we need.
4) For temperature we will be using thermometers. There are other
devices that allow you to measure temperature.
5) For heat we will be working with simple calorimeters. We will
look at these devices when we look at the transfer of heat.
Thermometers
and Temperature Scales see
comparisons charts
 |
thermometers = instruments to measure temperature
|
see drawing: gas (air) thermometers
see drawing: liquid (Hg and alcohol)
see drawing: solid (bimetallic)
Mercury
thermometer --
fill thin glass tube with Hg at a temp. greater than the maximum to be
measured; tube is cut and sealed; the Hg cools and contracts leaving a partial
vacuum above the Hg (eliminates effects of air resistance on expansion of Hg);
then calibrate thermometer
Hg freezes at -39° C so it cannot be used for low temperatures (use
alcohol which freezes at -114° C)
can use Hg for high temperature boils at 357° C (alcohol cannot
be used for high temperature work due to its low boiling point - 78° C)
use alcohol thermometers in schools unless extreme accuracy is needed
due to safety factor. The alcohol may be inaccurate by 1 - 2 degrees but this
may not be a problem if you are looking at changes in temp.
Calibration
Both
Celsius (Anders Celsius) and Fahrenheit (Gabriel
Fahrenheit) scales are established by using the
boiling
and freezing point of water at 1 atmosphere
of pressure.
step 1 -- establish a mixture of ice and water in equilibrium (0° C)
mark
point of liquid in thermometer at 0° C
establish a mixture of steam and water at equilibrium (both at a
pressure of 1 atm.) steam condensing
and water vaporizing)
label this point of liquid as 100 °C
step
2 -- divide the interval between 0° C and 100° C into 100 equal parts, each
representing a change in temperature of 1° C.
using this scale you can extend your marks below 0 °C and above 100 °C
as far as ,you wish.
The Fahrenheit scale labels the freezing point at 32°
(his
label for the temperature he could achieve with an ice and water mixture and
labeled the boiling point temperature of water at 212 ° which was a
number chosen for convenience apparently creating 180 divisions.
You might ask your self about the amount of heat energy need to
cause a 1 degree change in temperature on a Celsius scale compared to that
needed on a Fahrenheit scale. (more heat needed to cause change of 1 degree on
Celsius scale.)
KELVIN scale
We
know that gases decrease in volume 1/273 of its
original volume for each degree drop in temperature.
Thus
at -273° C the volume of gas would shrink to zero
and the gas would have no molecular
motion. We
know this is impossible (particles have zero-point energy).
To
label these very low temperatures a scale called the
absolute or Kelvin scale is often used. It designates
-273°
C at the zero point, and is called called absolute zero.
Extrapolation
to absolute zero:
A good research project for students or a quick review of graphing can be done
by using the method to calculate absolute zero.
Use a capillary tube with a trapped bubble of air between light machine oil.
Measure the length of the bubble after placing it in different temperature
mixtures. Plot the length versus temperature.
Since it is not easy to obtain very cold temperatures, the linear series of
points that you did obtain should allow you to extrapolate, (extend a curve
beyond the known data points following the apparent pattern of the curve)
until it intersects the temperature axis.
See actual plot!
Transfer
of Heat by Conduction, Convection, Radiation
Conduction is
a consequence of the kinetic behavior of matter. Faster vibrating particles
collide with less energetic neighbors and transfer some of their kinetic
energy to the slower moving particle.
Through successive molecular collisions energy travels through a
material without the average position of the particles being changed. There
must be a temperature differential (one end of some object at a higher
temperature than the other) for heat to be conducted.
Gases are poor conductors of heat (compared to liquids and solids)
because the molecules are relatively far apart and collisions are infrequent.
Metals have the greatest ability to conduct heat (for the same reason as
their high electrical conductivity). This is due to a significant number of
electrons being able to move about freely instead of being bound permanently
to particular atoms.
Thermal conductivity
of a material is a measure
of its ability to conduct heat.
Example: wood and metal
(see class discussion)
Convection
involves the actual motion
of a hot fluid from one place to another, displacing a colder fluid in its
path and setting up a convection current. Convection is the chief mechanism of
heat transfer in fluids.
Natural convection occurs when the buoyancy of heated fluids leads to
motion. Heated fluids (gas or liquid) expand and becomes less dense than
surrounding cooler fluids. It then rises.
Radiation is
defined as the energy that is transmitted by electromagnetic waves and
requires no material medium for passage.
All objects radiate electromagnetic waves with the higher the
temperature of an object the shorter the predominating wavelength of its
radiation.
Example: see glass lined thermos bottle in heat packet in class.
Transfer of Heat
|
Conduction
-- place iron rod in fire -- the
end you are holding becomes warm due to conduction |
|
Convection
-- stove heats room by
convection
|
|
Radiation
-- heat the earth receives
from the sun is radiation
|
Natural direction of heat flow is from hot bodies to cold ones.
Conduction -- conduction is a consequence of kinetic behavior
of matter
|
faster vibrating particle
collide with less energetic neighbor and transfer some of their kinetic
energy the slower moving particle
|
example:
place hand on wood and metal sample at same temperature. The metal will seem
cooler because it conducts the heat away much faster than the wood
Convection
-- actual motion of
hot fluid from one place to another, displacing cold fluid in its path
setting up a convection current = chief mechanism of heat transfer in fluids
in most instances
|
natural convection - the
buoyancy of heated fluids leads to motion ‑ heated fluid (gas or
liquid) expands and becomes less dense than surrounding cooler fluids
and rises
|
Radiation --
energy that is transmitted by electromagnetic waves and requires no
material medium for passage
|
all objects radiate
electromagnetic waves but the higher the temperature of an object the
shorter the predominate wavelength of its radiation |
There
are many examples you can use to demonstrate these three ideas but
discussing
a glass lined thermos bottle will allow you discuss them as well as
High
specific heat capacity material demonstrates relatively small change in
temperature for a given change in internal energy content
add 1 calorie of heat to 1 gram
of water, helium, ice, gold
temperature rises: water 1 °C
helium 1.3 °C
ice
2 °C
gold
33 °C
Calorimetry:
1)
Use 2 polystyrene cups (one within the other) -- the polystyrene will
not absorb very much heat.
2) You may find if
difficult for the students to be patient when working with the calorimeters.
Your step-by-step procedure must be very simple and clear. This type lab is
also a very dangerous time for your thermometers.
3) Several labs have been included in the packet involving the use of
the calorimeters and thermometers: a) the heat of reactions and heat of
solutions labs deal with endothermic and exothermic reactions (and you can
incorporate the use of the mole as review) b) the specific heat capacity lab
is an excellent demonstration of the principle of heat exchange as well as
specific heat capacity of different metals and liquids. 1) for better
results with this lab try to use as much metal as possible and as little
fluid as possible and still cover the metal in the cups. Drain the metal
samples quickly after removing them from the boiling water. This is a good
time to have the students watch the boiling process which will be one of the
final topics in this unit. 2) the math may be a bit difficult for you at
first.
Thermal Expansion of water
1) From 0° C to 4°
C the volume of water in a sample decreases (the greatest density is at 4°
C)
2) We know that ice
floats (less dense) so that a body of water in winter freezes from the top
down. The ice is a poor conductor of heat so that the initial layer of ice
that freezes impedes further freezing allowing fish and plant life to live
through the winter.
3)
The spaces between molecules in ice are greater than the same spaces
in liquids.
4) Ice has what is
called an Open Structure --> each water molecule can participate
in 4 bonds with other water molecules, while other solid molecules can have
as many as a dozen bonds with surrounding molecules resulting in a more
compact substance.
5) As stated the density of the water increases from 0 °C
to 4 °C. Large
clusters
of water molecules break into smaller clusters that occupy less
space in the aggregate as the temperature rises to 4 °C. Only above
4 °C does the normal thermal expansion show a decreasing density with
increasing temperature.
Above 4° C
, the normal thermal expansion of materials is seen. Here as the temperature
rises the density decreases.
Heat
and Temperature
HEAT Heat is a
form of internal energy which is transferred from one object to another due to
a difference in temperature between the objects. Heat is the total energy of
motion of all particles (the total kinetic energies of all the particles.)
TEMPERATURE The temperature of a body of matter is a measure of the
average kinetic energy of the random motion of its particles. Temperature is
the kinetic energy divided by the number of particles. Temperature is that
property of a substance which determines whether it is in thermal equilibrium
with another object.
Thermal Conductivity - a measure of the ability of a substance to
conduct heat
THERMAL EQUILIBRIUM
This is the situation
in which no heat moves from one object to another.
CALORIE A
15° Calorie is the amount of heat energy needed to change the temperature of
of 1 gram of water by 1° C (from 14.5° C to 15.5° C at 1 atmosphere of
pressure). 1 calorie = 4.185 Joules and 1 kilocalorie = 1000 calories.
SPECIFIC
HEAT CAPACITY This
is the amount of heat (in calories or Joules) that must be added or removed
from a unit mass of that substance to change its temperature by one degree.
Different substances have different capacities because they absorb and release
heat at different rates.
WATER Water has a specific heat capacity of 1.00 cal/g °C or 4.185
Joules/g °C. The SI unit would be 4185 J/kg °C.
PRINCIPLE OF HEAT
EXCHANGE The heat lost by an object must equal the heat gained by
the object to which the heat is transferred. There must be a temperature
difference for heat to be transferred.
Q (heat energy) = m (mass) x At (temp.) x cp (specific heat
capacity)
(cal/Joules)
(g)
(°C)
(cal/g °C) or (Joule/g °C)
Problems:
1) How much heat energy is needed to raise the temperature of 100 grams
of water from 0 degrees to 30°
C?
2)
A calorimeter contains 300 grams of water at 10° C. After a food sample is
burned in the calorimeter the water temperature changes to 15° C. How much
heat was given off by the food sample?
LATENT
HEAT Latent
heat is the heat required to bring about a change in state.
HEAT
OF FUSION The
heat of fusion is the amount of heat that must be supplied to change a unit
mass of the substance at its melting point from solid to liquid. The heat of
fusion of water is 80 calories per gram (80 kcal/kg).
HEAT
OF VAPORIZATION
The heat of vaporization is
the amount of heat that must be supplied to change a unit mass of the
substance at its boiling point from liquid to gas or vapor state. For water
this is 540 cal/g or 540 kcal/kg.
HEAT
OF SUBLIMATION The
heat of sublimation is the heat needed to change a solid to a gas.
HEAT
OF CONDENSATION The heat of condensation is the reverse of
the heat of vaporization, it is the heat given off when a gas condenses to a
liquid.
Four
States of Matter
Matter is
defined as any material that has mass, occupies volume, and exhibits inertia
(resistance to movement).
Solids
definite shape and volume, resist deformation
|
very
close spacing of particles that make up the solid
|
|
these
particles appear to vibrate about fixed points
|
|
particles
vibrate faster at higher temperatures, slower at lower temperatures
|
Crystalline solids
particles
are arranged in regular, repeated patterns - said to have "long-range order" to
their structure -example would be NaCl (table salt)
Amorphous solids
solids that lack the definite arrangement in crystals are
`amorphous' which means `without form'. Can be thought of as liquids
whose stiffness is due to exaggerated viscosity. These solids are said
to have "short range order". Examples are pitch, glass, plastics.
Polymers are flexible and some will change their structure when undergoing a
physical change (examples are rubber bands, Saran Warp, Lucite, DNA, fats,
cellulose, glycogen. Jello is a natural glucose polymer.
Liquids
definite volume, resist compression, will flow, takes the shape of its
container
|
greater
spacing between molecules, liquid particles appear to travel in straight
line paths between collisions but appear to rotate or vibrate about moving
points
|
Gases
Have no definite shape or
volume, takes the shape and volume of its container
|
can
be compressed or dispersed, the particles vibrate very rapidly, are
relatively far far apart, and there are no forces holding them together
|
Plasma
very high
temperature ionized gas (as high as 100 million degrees in some fusion
reactors). These plasmas have no fixed volume or shape, most are
mixtures that are not easily containable. They all have particles that
are electrically charged and of low density. The Milky Way is a huge
plasma.
Energy
Definitions
Energy: having
the ability to do work (move matter)
Work: a push or pull over some distance
(force x distance)
Force: a push or a pull
Potential Energy: the
energy a body possesses by virtue of its position, composition, and or
condition
stored energy or energy of position
P.E. = mass x gravity x height
examples: water behind a dam, stretched/compressed spring, explosives
Kinetic Energy: the
energy of motion (conserved in
every elastic collision)
K.E. = 1/2 mass x velocity2
heat energy flows from hot objects to cooler ones through transfer of K.E.
when particles collide
Momentum: mass
time velocity (momentum is
conserved in every collision where there is no friction)
Linear momentum of a moving body is a measure of its tendency to
continue in motion at a constant velocity.
The conservation of linear momentum states that in the absence of
forces from outside the system the total momentum of colliding particles
cannot change but the distribution of the total momentum may change.
Momentum is redistributed in a collision.
Intermolecular Forces:
