OTHER FACTS ABOUT HEAT
20. Boiling. _Heat absorbed in Boiling_. If a kettle of water is
placed above a flame, the temperature of the water gradually
increases, and soon small bubbles form at the bottom of the kettle and
begin to rise through the water. At first the bubbles do not get far
in their ascent, but disappear before they reach the surface; later,
as the water gets hotter and hotter, the bubbles become larger and
more numerous, rise higher and higher, and finally reach the surface
and pass from the water into the air; steam comes from the vessel, and
the water is said to _boil_. The temperature at which a liquid boils
is called the boiling point.
While the water is heating, the temperature steadily rises, but as
soon as the water begins to boil the thermometer reading becomes
stationary and does not change, no matter how hard the water boils and
in spite of the fact that heat from the flame is constantly passing
into the water.
If the flame is removed from the boiling water for but a second, the
boiling ceases; if the flame is replaced, the boiling begins again
immediately. Unless heat is constantly supplied, water at the boiling
point cannot be transformed into steam.
_The number of calories which must be supplied to 1 gram of water at
the boiling point in order to change it into steam at the same
temperature is called the heat of vaporization_; it is the heat
necessary to change 1 gram of water at the boiling point into steam of
the same temperature.
21. The Amount of Heat Absorbed. The amount of heat which must be
constantly supplied to water at the boiling point in order to change
it into steam is far greater than we realize. If we put a beaker of
ice water (water at 0° C.) over a steady flame, and note (1) the time
which elapses before the water begins to boil, and (2) the time which
elapses before the boiling water completely boils away, we shall see
that it takes about 5-1/4 times as long to change water into steam as
it does to change its temperature from 0° C. to 100° C. Since, with a
steady flame, it takes 5-1/4 times as long to change water into steam
as it does to change its temperature from 0° C. to the boiling point,
we conclude that it takes 5-1/4 times as much heat to convert water at
the boiling point into steam as it does to raise it from the
temperature of ice water to that of boiling water.
The amount of heat necessary to raise the temperature of 1 gram of
water 1° C. is equal to 1 calorie, and the amount necessary to raise
the temperature 100° C. is equal to 100 calories; hence the amount of
heat necessary to convert 1 gram of water at the boiling point into
steam at that same temperature is equal to approximately 525 calories.
Very careful experiments show the exact heat of vaporization to be
536.1 calories. (See Laboratory Manual.)
22. General Truths. Statements similar to the above hold for other
liquids and for solutions. If milk is placed upon a stove, the
temperature rises steadily until the boiling point is reached; further
heating produces, not a change in temperature, but a change of the
water of the milk into steam. As soon as the milk, or any other liquid
food, comes to a boil, the gas flame should be lowered until only an
occasional bubble forms, because so long as any bubbles form the
temperature is that of the boiling point, and further heat merely
results in waste of fuel.
We find by experiment that every liquid has its own specific boiling
point; for example, alcohol boils at 78° C. and brine at 103° C. Both
specific heat and the heat of vaporization vary with the liquid used.
23. Condensation. If one holds a cold lid in the steam of boiling
water, drops of water gather on the lid; the steam is cooled by
contact with the cold lid and _condenses_ into water. Bottles of water
brought from a cold cellar into a warm room become covered with a mist
of fine drops of water, because the moisture in the air, chilled by
contact with the cold bottles, immediately condenses into drops of
water. Glasses filled with ice water show a similar mist.
In Section 21, we saw that 536 calories are required to change 1 gram
of water into steam; if, now, the steam in turn condenses into water,
it is natural to expect a release of the heat used in transforming
water into steam. Experiment shows not only that vapor gives out heat
during condensation, but that the amount of heat thus set free is
exactly equal to the amount absorbed during vaporization. (See
Laboratory Manual.)
We learn that the heat of vaporization is the same whether it is
considered as the heat absorbed by 1 gram of water in its change to
steam, or as the heat given out by 1 gram of steam during its
condensation into water.
24. Practical Application. We understand now the value of steam as a
heating agent. Water is heated in a boiler in the cellar, and the
steam passes through pipes which run to the various rooms; there the
steam condenses into water in the radiators, each gram of steam
setting free 536 calories of heat. When we consider the size of the
radiators and the large number of grams of steam which they contain,
and consider further that each gram in condensing sets free 536
calories, we understand the ease with which buildings are heated by
steam.
Most of us have at times profited by the heat of condensation. In cold
weather, when there is a roaring fire in the range, the water
frequently becomes so hot that it "steams" out of open faucets. If, at
such times, the hot water is turned on in a small cold bathroom, and
is allowed to run until the tub is well filled, vapor condenses on
windows, mirrors, and walls, and the cold room becomes perceptibly
warmer. The heat given out by the condensing steam passes into the
surrounding air and warms the room.
There is, however, another reason for the rise in temperature. If a
large pail of hot soup is placed in a larger pail of cold water, the
soup will gradually cool and the cold water will gradually become
warmer. A red-hot iron placed on a stand gradually cools, but warms
the stand. A hot body loses heat so long as a cooler body is near it;
the cold object is heated at the expense of the warmer object, and one
loses heat and the other gains heat until the temperature of both is
the same. Now the hot water in the tub gradually loses heat and the
cold air of the room gradually gains heat by convection, but the
amount given the room by convection is relatively small compared with
the large amount set free by the condensing steam.
25. Distillation. If impure, muddy water is boiled, drops of water
will collect on a cold plate held in the path of the steam, but the
drops will be clear and pure. When impure water is boiled, the steam
from it does not contain any of the impurities because these are left
behind in the vessel. If all the water were allowed to boil away, a
layer of mud or of other impurities would be found at the bottom of
the vessel. Because of this fact, it is possible to purify water in a
very simple way. Place over a fire a large kettle closed except for a
spout which is long enough to reach across the stove and dip into a
bottle. As the liquid boils, steam escapes through the spout, and on
reaching the cold bottle condenses and drops into the bottle as pure
water. The impurities remain behind in the kettle. Water freed from
impurities in this way is called _distilled water_, and the process is
called _distillation_ (Fig. 19). By this method, the salt water of the
ocean may be separated into pure drinking water and salt, and many of
the large ocean liners distill from the briny deep all the drinking
water used on their ocean voyages.
[Illustration: FIG. 19.--In order that the steam which passes through
the coiled tube may be quickly cooled and condensed, cold water is
made to circulate around the coil. The condensed steam escapes at
_w_.]
Commercially, distillation is a very important process. Turpentine,
for example, is made by distilling the sap of pine trees. Incisions
are cut in the bark of the long-leaf pine trees, and these serve as
channels for the escape of crude resin. This crude liquid is collected
in barrels and taken to a distillery, where it is distilled into
turpentine and rosin. The turpentine is the product which passes off
as vapor, and the rosin is the mass left in the boiler after the
distillation of the turpentine.
26. Evaporation. If a stopper is left off a cologne bottle, the
contents of the bottle will slowly evaporate; if a dish of water is
placed out of doors on a hot day, evaporation occurs very rapidly. The
liquids which have disappeared from the bottle and the dish have
passed into the surrounding air in the form of vapor. In Section 20,
we saw that water could not pass into vapor without the addition of
heat; now the heat necessary for the evaporation of the cologne and
water was taken from the air, leaving it slightly cooler. If wet hands
are not dried with a towel, but are left to dry by evaporation, heat
is taken from the hand in the process, leaving a sensation of
coolness. Damp clothing should never be worn, because the moisture in
it tends to evaporate at the expense of the bodily heat, and this
undue loss of heat from the body produces chills. After a bath the
body should be well rubbed, otherwise evaporation occurs at the
expense of heat which the body cannot ordinarily afford to lose.
Evaporation is a slow process occurring at all times; it is hastened
during the summer, because of the large amount of heat present in the
atmosphere. Many large cities make use of the cooling effect of
evaporation to lower the temperature of the air in summer; streets are
sprinkled not only to lay the dust, but in order that the surrounding
air may be cooled by the evaporation of the water.
Some thrifty housewives economize by utilizing the cooling effects of
evaporation. Butter, cheese, and other foods sensitive to heat are
placed in porous vessels wrapped in wet cloths. Rapid evaporation of
the water from the wet cloths keeps the contents of the jars cool, and
that without expense other than the muscular energy needed for wetting
the cloths frequently.
27. Rain, Snow, Frost, Dew. The heat of the sun causes constant
evaporation of the waters of oceans, rivers, streams, and marshes, and
the water vapor set free by evaporation passes into the air, which
becomes charged with vapor or is said to be humid. Constant, unceasing
evaporation of our lakes, streams, and pools would mean a steady
decrease in the supply of water available for daily use, if the
escaped water were all retained by the atmosphere and lost to the
earth. But although the escaped vapor mingles with the atmosphere,
hovering near the earth's surface, or rising far above the level of
the mountains, it does not remain there permanently. When this vapor
meets a cold wind or is chilled in any way, condensation takes place,
and a mass of tiny drops of water or of small particles of snow is
formed. When these drops or particles become large enough, they fall
to the earth as rain or snow, and in this way the earth is compensated
for the great loss of moisture due to evaporation. Fog is formed when
vapor condenses near the surface of the earth, and when the drops are
so small that they do not fall but hover in the air, the fog is said
"not to lift" or "not to clear."
If ice water is poured into a glass, a mist will form on the outside
of the glass. This is because the water vapor in the air becomes
chilled by contact with the glass and condenses. Often leaves and
grass and sidewalks are so cold that the water vapor in the atmosphere
condenses on them, and we say a heavy dew has formed. If the
temperature of the air falls to the freezing point while the dew is
forming, the vapor is frozen and frost is seen instead of dew.
The daily evaporation of moisture into the atmosphere keeps the
atmosphere more or less full of water vapor; but the atmosphere can
hold only a definite amount of vapor at a given temperature, and as
soon as it contains the maximum amount for that temperature, further
evaporation ceases. If clothes are hung out on a damp, murky day they
do not dry, because the air contains all the moisture it can hold, and
the moisture in the clothes has no chance to evaporate. When the air
contains all the moisture it can hold, it is said to be saturated, and
if a slight fall in temperature occurs when the air is saturated,
condensation immediately begins in the form of rain, snow, or fog. If,
however, the air is not saturated, a fall in temperature may occur
without producing precipitation. The temperature at which air is
saturated and condensation begins is called the _dew point_.
28. How Chills are Caused. The discomfort we feel in an overcrowded
room is partly due to an excess of moisture in the air, resulting from
the breathing and perspiration of many persons. The air soon becomes
saturated with vapor and cannot take away the perspiration from our
bodies, and our clothing becomes moist and our skin tender. When we
leave the crowded "tea" or lecture and pass into the colder, drier,
outside air, clothes and skin give up their load of moisture through
sudden evaporation. But evaporation requires heat, and this heat is
taken from our bodies, and a chill results.
Proper ventilation would eliminate much of the physical danger of
social events; fresh, dry air should be constantly admitted to crowded
rooms in order to replace the air saturated by the breath and
perspiration of the occupants.
29. Weather Forecasts. When the air is near the saturation point,
the weather is oppressive and is said to be very humid. For comfort
and health, the air should be about two thirds saturated. The presence
of some water vapor in the air is absolutely necessary to animal and
plant life. In desert regions where vapor is scarce the air is so dry
that throat trouble accompanied by disagreeable tickling is prevalent;
fallen leaves become so dry that they crumble to dust; plants lose
their freshness and beauty.
The likelihood of rain or frost is often determined by temperature and
humidity. If the air is near saturation and the temperature is
falling, it is safe to predict bad weather, because the fall of
temperature will probably cause rapid condensation, and hence rain.
If, however, the air is not near the saturation point, a fall in
temperature will not necessarily produce bad weather.
The measurement of humidity is of far wider importance than the mere
forecasting of local weather conditions. The close relation between
humidity and health has led many institutions, such as hospitals,
schools, and factories, to regulate the humidity of the atmosphere as
carefully as they do the temperature. Too great humidity is
enervating, and not conducive to either mental or physical exertion;
on the other hand, too dry air is equally harmful. In summer the
humidity conditions cannot be well regulated, but in winter, when
houses are artificially heated, the humidity of a room can be
increased by placing pans of water near the registers or on radiators.
30. Heat Needed to Melt Substances. If a spoon is placed in a vessel
of hot water for a few seconds and then removed, it will be warmer
than before it was placed in the hot water. If a lump of melting ice
is placed in the vessel of hot water and then removed, the ice will
not be warmer than before, but there will be less of it. The heat of
the water has been used in melting the ice, not in changing its
temperature.
If, on a bitter cold day, a pail of snow is brought into a warm room
and a thermometer is placed in the snow, the temperature rises
gradually until 32° F. is reached, when it becomes stationary, and the
snow begins to melt. If the pail is put on the fire, the temperature
still remains 32°F., but the snow melts more rapidly. As soon as all
the snow is completely melted, however, the temperature begins to rise
and rises steadily until the water boils, when it again becomes
stationary and remains so during the passage of water into vapor.
We see that heat must be supplied to ice at 0° C. or 32° F. in order
to change it into water, and further, that the temperature of the
mixture does not rise so long as any ice is present, no matter how
much heat is supplied. The amount of heat necessary to melt 1 gram of
ice is easily calculated. (See Laboratory Manual.)
Heat must be supplied to ice to melt it. On the other hand, water, in
freezing, loses heat, and the amount of heat lost by freezing water is
exactly equal to the amount of heat absorbed by melting ice.
The number of units of heat required to melt a unit mass of ice is
called the _heat of fusion_ of water.
31. Climate. Water, in freezing, loses heat, even though its
temperature remains at 0° C. Because water loses heat when it freezes,
the presence of large streams of water greatly influences the climate
of a region. In winter the heat from the freezing water keeps the
temperature of the surrounding higher than it would naturally be, and
consequently the cold weather is less severe. In summer water
evaporates, heat is taken from the air, and consequently the warm
weather is less intense.
32. Molding of Glass and Forging of Iron. The fire which is hot
enough to melt a lump of ice may not be hot enough to melt an iron
poker; on the other hand, it may be sufficiently hot to melt a tin
spoon. Different substances melt, or liquefy, at different
temperatures; for example, ice melts at 0° C., and tin at 233° C.,
while iron requires the relatively high temperature of 1200° C. Most
substances have a definite melting or freezing point which never
changes so long as the surrounding conditions remain the same.
But while most substances have a definite melting point, some
substances do not. If a glass rod is held in a Bunsen burner, it will
gradually grow softer and softer, and finally a drop of molten glass
will fall from the end of the rod into the fire. The glass did not
suddenly become a liquid at a definite temperature; instead it
softened gradually, and then melted. While glass is in the soft,
yielding, pliable state, it is molded into dishes, bottles, and other
useful objects, such as lamp shades, globes, etc. (Fig. 20). If glass
melted at a definite temperature, it could not be molded in this way.
Iron acts in a similar manner, and because of this property the
blacksmith can shape his horseshoes, and the workman can make his
engines and other articles of daily service to man.
[Illustration: FIG. 20.--Molten glass being rolled into a form
suitable for window panes.]
33. Strange Behavior of Water. One has but to remember that bottles
of water burst when they freeze, and that ice floats on water like
wood, to know that water expands on freezing or on solidifying. A
quantity of water which occupies 100 cubic feet of space will, on
becoming ice, need 109 cubic feet of space. On a cold winter night the
water sometimes freezes in the water pipes, and the pipes burst. Water
is very peculiar in expanding on solidification, because most
substances contract on solidifying; gelatin and jelly, for example,
contract so much that they shrink from the sides of the dish which
contains them.
If water contracted in freezing, ice would be heavier than water and
would sink in ponds and lakes as fast as it formed, and our streams
and ponds would become masses of solid ice, killing all animal and
plant life. But the ice is lighter than water and floats on top, and
animals in the water beneath are as free to live and swim as they were
in the warm sunny days of summer. The most severe winter cannot freeze
a deep lake solid, and in the coldest weather a hole made in the ice
will show water beneath the surface. Our ice boats cut and break the
ice of the river, and through the water beneath our boats daily ply
their way to and fro, independent of winter and its blighting blasts.
While most of us are familiar with the bursting of water pipes on a
cold night, few of us realize the influence which freezing water
exerts on the character of the land around us.
Water sinks into the ground and, on the approach of winter, freezes,
expanding about one tenth of its volume; the expanding ice pushes the
earth aside, the force in some cases being sufficient to dislodge even
huge rocks. In the early days in New England it was said by the
farmers that "rocks grew," because fields cleared of stones in the
fall became rock covered with the approach of spring; the rocks and
stones hidden underground and unseen in the fall were forced to the
surface by the winter's expansion. We have all seen fence posts and
bricks pushed out of place because of the heaving of the soil beneath
them. Often householders must relay their pavements and walks because
of the damage done by freezing water.
The most conspicuous effect of the expansive power of freezing water
is seen in rocky or mountainous regions (Fig. 21). Water easily finds
entrance into the cracks and crevices of the rocks, where it lodges
until frozen; then it expands and acts like a wedge, widening cracks,
chiseling off edges, and even breaking rocks asunder. In regions where
frequent frosts occur, the destructive action of water works constant
changes in the appearance of the land; small cracks and crevices are
enlarged, massive rocks are pried up out of position, huge slabs are
split off, and particles large and small are forced from the parent
rock. The greater part of the debris and rubbish brought down from the
mountain slopes by the spring rains owes its origin to the fact that
water expands when it freezes.
[Illustration: FIG. 21.--The destruction caused by freezing water.]
34. Heat Necessary to Dissolve a Substance. It requires heat to
dissolve any substance, just as it requires heat to change ice to
water. If a handful of common salt is placed in a small cup of water
and stirred with a thermometer, the temperature of the mixture falls
several degrees. This is just what one would expect, because the heat
needed to liquefy the salt must come from somewhere, and naturally it
comes from the water, thereby lowering the temperature of the water.
We know very well that potatoes cease boiling if a pinch of salt is
put in the water; this is because the temperature of the water has
been lowered by the amount of heat necessary to dissolve the salt.
Let some snow or chopped ice be placed in a vessel and mixed with one
third its weight of coarse salt; if then a small tube of cold water is
placed in this mixture, the water in the test tube will soon freeze
solid. As soon as the snow and salt are mixed they melt. The heat
necessary for this comes in part from the air and in part from the
water in the test tube, and the water in the tube becomes in
consequence cold enough to freeze. But the salt mixture does not
freeze because its freezing point is far below that of pure water. The
use of salt and ice in ice-cream freezers is a practical application
of this principle. The heat necessary for melting the mixture of salt
and ice is taken from the cream which thus becomes cold enough to
freeze.