After all the liquid is frozen, the temperature
of the ice may drop below 0°C., just as the temperature of the ground or
a stone may assume the temperature of the surroundings.
83
84 FREEZING
If ice and water that are not at the same temperature are mixed, they
are not in equilibrium, since ice and water are in equilibrium only at the
freezing point or 0°C. If the temperature of the ice is 0°C. and the
temperature of the water is 20°C. some of the ice melts. In melting, each
gram of ice absorbs 80 calories of heat, and the temperature of the water
is lowered. If sufficient ice is added to the water, and the temperature
of the surroundings does not influence the mixture, ice melts until the
temperature of the water reaches 0°C. and equilibrium is established. If
the quantity of ice added to the water is not sufficient to lower the
temperature of the water to 0°C., the temperature of the water is lowered
as long as the ice melts and provided the surroundings do not influence
the temperature. After the ice is all melted the temperature of the water
cannot be lowered except from the surroundings.
Lowering of the Freezing Point
When a soluble substance is dissolved in a liquid the freezing point is
lower than that of the pure solvent. A gram-molecular weight (mole)
of a non-ionized substance in a liter of water lowers the freezing point
1.86°C. The lowering of the freezing point can be continued as long as
the substance is soluble. If no more than 3 moles of a non-ionized substance
are soluble in a liter of water, the freezing point cannot be depressed
below — 5.58°C. Substances that are not very soluble do not affect the
freezing point particularly, because such a small portion of a mole of the
substance is dissolved. Thus substances in suspension, such as fat in milk
or cream, do not affect the freezing point of an ice-cream mix in the same
way that sugar does, for the fat is not soluble.
Ionized substances and the freezing point. Sugar does not ionize
or dissociate in the solution to an extent great enough to influence the
freezing point. Salt belongs to the groups of electrolytes* which dissociate
into ions in the solution. The amount of ionization depends upon the
concentration of the solution : the less concentrated the solution the greater
the dissociation. In a very dilute solution, complete dissociation into ions
may occur. If 58 grams of salt or sodium chloride in 1 liter of water
are completely ionized into sodium and chlorine ions, there will be 1 mole
of sodium ions and 1 of chlorine ions. Thus the two lower the freezing
point (2 X 1.86) or — 3.72°C. The molecular weight of sucrose is 342
grams, that of levulose is 180, and that of dextrose is 180 grams. None of
the sugars ionize to an extent that affects the freezing point. Compare
the freezing points, if ^ cup, or 150' grams, each, of sucrose, levulose, or
dextrose is added to a quart of cream.
Calcium chloride ionizes into 3 ions. Therefore, a mole of calcium
chloride depresses the freezing point more than a mole of sodium chloride.
FREEZING MIXTURES 85
The calcium chloride is also more soluble than sodium chloride, so that a
greater number of moles can be dissolved in a liter of water.
CaCla < ^ Ca++2C1-
The Freezing Point of Ice Cream and Ices
Page 67
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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After all the liquid is frozen, the temperature
of the ice may drop below 0°C., just as the temperature of the ground or
a stone may assume the temperature of the surroundings.
83
84 FREEZING
If ice and water that are not at the same temperature are mixed, they
are not in equilibrium, since ice and water are in equilibrium only at the
freezing point or 0°C. If the temperature of the ice is 0°C. and the
temperature of the water is 20°C. some of the ice melts. In melting, each
gram of ice absorbs 80 calories of heat, and the temperature of the water
is lowered. If sufficient ice is added to the water, and the temperature
of the surroundings does not influence the mixture, ice melts until the
temperature of the water reaches 0°C. and equilibrium is established. If
the quantity of ice added to the water is not sufficient to lower the
temperature of the water to 0°C., the temperature of the water is lowered
as long as the ice melts and provided the surroundings do not influence
the temperature. After the ice is all melted the temperature of the water
cannot be lowered except from the surroundings.
Lowering of the Freezing Point
When a soluble substance is dissolved in a liquid the freezing point is
lower than that of the pure solvent. A gram-molecular weight (mole)
of a non-ionized substance in a liter of water lowers the freezing point
1.86°C. The lowering of the freezing point can be continued as long as
the substance is soluble. If no more than 3 moles of a non-ionized substance
are soluble in a liter of water, the freezing point cannot be depressed
below — 5.58°C. Substances that are not very soluble do not affect the
freezing point particularly, because such a small portion of a mole of the
substance is dissolved. Thus substances in suspension, such as fat in milk
or cream, do not affect the freezing point of an ice-cream mix in the same
way that sugar does, for the fat is not soluble.
Ionized substances and the freezing point. Sugar does not ionize
or dissociate in the solution to an extent great enough to influence the
freezing point. Salt belongs to the groups of electrolytes* which dissociate
into ions in the solution. The amount of ionization depends upon the
concentration of the solution : the less concentrated the solution the greater
the dissociation. In a very dilute solution, complete dissociation into ions
may occur. If 58 grams of salt or sodium chloride in 1 liter of water
are completely ionized into sodium and chlorine ions, there will be 1 mole
of sodium ions and 1 of chlorine ions. Thus the two lower the freezing
point (2 X 1.86) or — 3.72°C. The molecular weight of sucrose is 342
grams, that of levulose is 180, and that of dextrose is 180 grams. None of
the sugars ionize to an extent that affects the freezing point. Compare
the freezing points, if ^ cup, or 150' grams, each, of sucrose, levulose, or
dextrose is added to a quart of cream.
Calcium chloride ionizes into 3 ions. Therefore, a mole of calcium
chloride depresses the freezing point more than a mole of sodium chloride.
FREEZING MIXTURES 85
The calcium chloride is also more soluble than sodium chloride, so that a
greater number of moles can be dissolved in a liter of water.
CaCla < ^ Ca++2C1-
The Freezing Point of Ice Cream and Ices