Elevation of the boiling point. The boiling point of a liquid can
be elevated by increasing the pressure upon it. This may be done by pre-
venting the vapor above the liquid from escaping. If a soluble substance
is added to a liquid the resulting solution has a higher boiling point than
the pure liquid.
The pressure of a gas increases as the temperature increases. When
vapor is confined, as in a pressure cooker, the boiling point of a liquid
in the cooker is elevated. As the temperature is increased, the pressure
of the confined vapor on the surface of the liquid is increased. Therefore
a higher temperature is required to form great enough pressure in the
vapor bubbles within the liquid for them to reach the surface of the liquid.
48 SUGAR COOKERY
Thus the boiling point is elevated. The higher the temperature of the
confined vapor the greater the pressure on the liquid. The greater the
pressure on the liquid the higher the boiling point.
The boiling point o£ solutions. Each gram-molecular weight (mole)
of a non-ionized substance in a liter of water elevates the boiling point
0.52°C. A gram-molecular weight of a substance is its atomic or com-
bined atomic weights. For example, sucrose is composed of carbon, hydro-
gen, and oxygen with the formula C12H22O11. The atomic weight of
carbon is 12; that of hydrogen is 1; of oxygen, 16. Thus 12 carbons, 22
hydrogens, and 1 1 oxygens give a molecular weight of 342 grams. The
boiling point of a liter of water containing 342 grams of sucrose is elevated
0.52°C. Two moles of sucrose would elevate the boiling point 1.04°C.
The boiling point can be elevated as long as the substance added is soluble.
When the solution becomes saturated, or the point is reached at which no
more can be dissolved, the boiling point is constant for substances that
behave normally in solution. If boiling is continued so that the solvent
is vaporized, the excess solute beyond saturation is crystallized. This is
illustrated in Experiment 3 in the laboratory outline.
Effect of ionized substances on the boiling point. Some sub-
stances when dissolved in water are ionized. In a solution of sodium
chloride, for example, not only sodium chloride molecules are found but
also sodium ions and chlorine ions.
NaCl :^^ ^ Na+H-Cl-
The atomic weight of sodium is 23 and that of chlorine is 35. If 58
grams of sodium chloride (a mole) are added to a liter of water, and
the sodium chloride is completely ionized, the boiling point will be elevated
1.04°C. The mole of sodium will elevate it 0.52° and the mole of chlorine
will elevate it 0.52°. Sometimes a substance that ionizes in solution is not
completely ionized. When this happens, the boiling point is elevated accord-
ing to the degree of ionization.
At high altitudes it is possible to cook foods more rapidly by adding salt
to the cooking water. To be very effective this requires such a large quan-
tity of salt that the food becomes too salty. It may be used for potatoes
that are not peeled.
Page 42
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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Elevation of the boiling point. The boiling point of a liquid can
be elevated by increasing the pressure upon it. This may be done by pre-
venting the vapor above the liquid from escaping. If a soluble substance
is added to a liquid the resulting solution has a higher boiling point than
the pure liquid.
The pressure of a gas increases as the temperature increases. When
vapor is confined, as in a pressure cooker, the boiling point of a liquid
in the cooker is elevated. As the temperature is increased, the pressure
of the confined vapor on the surface of the liquid is increased. Therefore
a higher temperature is required to form great enough pressure in the
vapor bubbles within the liquid for them to reach the surface of the liquid.
48 SUGAR COOKERY
Thus the boiling point is elevated. The higher the temperature of the
confined vapor the greater the pressure on the liquid. The greater the
pressure on the liquid the higher the boiling point.
The boiling point o£ solutions. Each gram-molecular weight (mole)
of a non-ionized substance in a liter of water elevates the boiling point
0.52°C. A gram-molecular weight of a substance is its atomic or com-
bined atomic weights. For example, sucrose is composed of carbon, hydro-
gen, and oxygen with the formula C12H22O11. The atomic weight of
carbon is 12; that of hydrogen is 1; of oxygen, 16. Thus 12 carbons, 22
hydrogens, and 1 1 oxygens give a molecular weight of 342 grams. The
boiling point of a liter of water containing 342 grams of sucrose is elevated
0.52°C. Two moles of sucrose would elevate the boiling point 1.04°C.
The boiling point can be elevated as long as the substance added is soluble.
When the solution becomes saturated, or the point is reached at which no
more can be dissolved, the boiling point is constant for substances that
behave normally in solution. If boiling is continued so that the solvent
is vaporized, the excess solute beyond saturation is crystallized. This is
illustrated in Experiment 3 in the laboratory outline.
Effect of ionized substances on the boiling point. Some sub-
stances when dissolved in water are ionized. In a solution of sodium
chloride, for example, not only sodium chloride molecules are found but
also sodium ions and chlorine ions.
NaCl :^^ ^ Na+H-Cl-
The atomic weight of sodium is 23 and that of chlorine is 35. If 58
grams of sodium chloride (a mole) are added to a liter of water, and
the sodium chloride is completely ionized, the boiling point will be elevated
1.04°C. The mole of sodium will elevate it 0.52° and the mole of chlorine
will elevate it 0.52°. Sometimes a substance that ionizes in solution is not
completely ionized. When this happens, the boiling point is elevated accord-
ing to the degree of ionization.
At high altitudes it is possible to cook foods more rapidly by adding salt
to the cooking water. To be very effective this requires such a large quan-
tity of salt that the food becomes too salty. It may be used for potatoes
that are not peeled.