Since the heat is applied at the bottom of the pan the vapor forms at
the bottom of the liquid. With the increased speed of the molecules, due
to the increased temperature, greater pressure is obtained, so that the
formation of vapor is more rapid until a point is reached at which the
rate of loss of heat from the water in the escaping vapor is equal to the
heat received by the liquid. If the rate at which the heat is applied is con-
stant, the bubbles are uniform in size. If a thermometer is held in the liquid
it is found that when this point is reached the temperature is constant.
This is the boiling point. A child might say that when a liquid is bubbling
it is boiling, and it would be a fairly good definition. However, the chemist
or physicist would word his definition differently. With vapor formation,
pressure is exerted. Since the bubble is less dense than the liquid it comes
ELEVATION OF THE BOILING POINT 47
to the surface. But the bubble cannot reach the surface until the pressure
within it is just a little greater than the pressure of the liquid on the
bubble. The pressure on the bubble in an open pan comes from the weight
of the column of liquid above it and the atmospheric pressure on the
surface of the liquid. Another way to define the boiling point is to say that
it is the temperature at which the pressure of the saturated vapor within
the liquid is just greater than the outside pressure on the surface of the
liquid.
If you live at sea level the boiling point of water is 100°C. The Bureau
of Standards defines the boiling point of water as the point at which
ebullition is violent. Slow-bubbling water does not register quite as high
a temperature as rapidly bubbling water, but in cooking food in water
there is no great advantage in having the water boiling violently. The food
will cook nearly as rapidly in the slower bubbling water. With gas or
electricity it is an economy of fuel to lower the heat when the water begins
to boil, unless it is desirable to evaporate the liquid quickly.
The conversion of water from a liquid to a gaseous state requires a
certain amount of energy. This energy is expressed in terms of heat. To
change a gram of water at 100°C. to vapor at 100° requires about 540
calories of heat. If the heat applied to boiling water is increased, the
quantity of water changed to vapor in a given time is increased. The vapor
escapes from the surface of the liquid, but in a pan the free surface is
limited. However, in boiling water it escapes from the free surface and
from the surface of the bubbles. The temperature of the water cannot be
increased because the heat lost by evaporation is equal to the heat received.
If the heat is increased, the heat lost by evaporation is increased and the
surface of the bubbles is increased enormously beyond the free surface of
the liquid to aid evaporation.
Lowering the boiling point. The boiling point of a liquid may be
lowered by reducing the pressure on the liquid. This may be done by
boiling the liquid in a partial vacuum. The boiling point is also lowered
with increased elevation above sea level. The atmospheric pressure is not
so great at high altitudes because of the lessened column or depth of air.
For each 960 feet above sea level the boiling point is decreased 1°C.
Page 41
Presented as published in 1932. Historical recipes may not meet modern food-safety standards. Cook from the modern interpretation, not the original instructions.
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Since the heat is applied at the bottom of the pan the vapor forms at
the bottom of the liquid. With the increased speed of the molecules, due
to the increased temperature, greater pressure is obtained, so that the
formation of vapor is more rapid until a point is reached at which the
rate of loss of heat from the water in the escaping vapor is equal to the
heat received by the liquid. If the rate at which the heat is applied is con-
stant, the bubbles are uniform in size. If a thermometer is held in the liquid
it is found that when this point is reached the temperature is constant.
This is the boiling point. A child might say that when a liquid is bubbling
it is boiling, and it would be a fairly good definition. However, the chemist
or physicist would word his definition differently. With vapor formation,
pressure is exerted. Since the bubble is less dense than the liquid it comes
ELEVATION OF THE BOILING POINT 47
to the surface. But the bubble cannot reach the surface until the pressure
within it is just a little greater than the pressure of the liquid on the
bubble. The pressure on the bubble in an open pan comes from the weight
of the column of liquid above it and the atmospheric pressure on the
surface of the liquid. Another way to define the boiling point is to say that
it is the temperature at which the pressure of the saturated vapor within
the liquid is just greater than the outside pressure on the surface of the
liquid.
If you live at sea level the boiling point of water is 100°C. The Bureau
of Standards defines the boiling point of water as the point at which
ebullition is violent. Slow-bubbling water does not register quite as high
a temperature as rapidly bubbling water, but in cooking food in water
there is no great advantage in having the water boiling violently. The food
will cook nearly as rapidly in the slower bubbling water. With gas or
electricity it is an economy of fuel to lower the heat when the water begins
to boil, unless it is desirable to evaporate the liquid quickly.
The conversion of water from a liquid to a gaseous state requires a
certain amount of energy. This energy is expressed in terms of heat. To
change a gram of water at 100°C. to vapor at 100° requires about 540
calories of heat. If the heat applied to boiling water is increased, the
quantity of water changed to vapor in a given time is increased. The vapor
escapes from the surface of the liquid, but in a pan the free surface is
limited. However, in boiling water it escapes from the free surface and
from the surface of the bubbles. The temperature of the water cannot be
increased because the heat lost by evaporation is equal to the heat received.
If the heat is increased, the heat lost by evaporation is increased and the
surface of the bubbles is increased enormously beyond the free surface of
the liquid to aid evaporation.
Lowering the boiling point. The boiling point of a liquid may be
lowered by reducing the pressure on the liquid. This may be done by
boiling the liquid in a partial vacuum. The boiling point is also lowered
with increased elevation above sea level. The atmospheric pressure is not
so great at high altitudes because of the lessened column or depth of air.
For each 960 feet above sea level the boiling point is decreased 1°C.