This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
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Water standing in an open vessel gradually evaporates, so that eventually all the water disappears in the form of vapor. We know that evaporation takes place more rapidly on warm days than on cool ones, and more rapidly from a wide shallow vessel than from a narrow deep one. As the temperature is increased, the rapidity of the motion of the molecules is increased. Therefore a larger number escape from the liquid as the temperature increases, most of them being carried away by air currents.
Saturated vapor. If we cover a vessel of water, leaving an air space between the surface of the liquid and the cover, evaporation takes place for a time just as from an uncovered vessel. But when the vapor cannot be carried away, the air above the liquid soon becomes filled with vapor molecules. Of course some of them reenter the liquid. When they are entering the surface of the liquid as rapidly as they are leaving it, the air is said to be saturated and is in equilibrium with the liquid, i.e., the saturation point is reached when the air holds all the vapor possible at that temperature. If the temperature is increased the velocity of the molecules is increased, and they leave the surface of the liquid faster than they enter until equilibrium is again established. If the temperature is reduced, a part of the vapor condenses into the liquid and forms drops of liquid on the sides and cover of the vessel or on the surface of the liquid.
The boiling point. When water is heated slowly enough, air bubbles are noticed forming on the sides and bottom of the pan. They come from the air that has been held in solution by the water. A similar thing may be noticed on a warm day when a glass or pitcher of cool water is left in a warm room. The air bubbles collect on the sides of the glass or pitcher, and if the vessel is jarred many of them will rise to the top of the water and break. If heating of the water is continued the vapor begins to form. Many of the first vapor bubbles collapse before they reach the surface.
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 constant, 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 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.
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 preventing 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.
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 of 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 combined atomic weights. For example, sucrose is composed of carbon, hydrogen, and oxygen with the formula C12H12O11. The atomic weight of carbon is 12; that of hydrogen is 1; of oxygen, 16. Thus 12 carbons, 22 hydrogens, and 11 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 substances 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.
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 according 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 quantity of salt that the food becomes too salty. It may be used for potatoes that are not peeled.
Sugar solutions behave abnormally in regard to the boiling point. In Experiment 4 it is found that the sugar solutions do not behave like the salt solution. They do not reach a constant boiling point. A mole of sucrose (342 grams) measures about 1 3/4 cups. It can be readily seen by consulting the solubility table of sucrose that its solubility will account for only a partial elevation of the boiling point. The boiling point of the sugar solution increases with its concentration until the melting point of the sugar is reached. Occasionally, in cooking a sucrose solution (Experiment 4A), some of the sucrose crystallizes on the edge of the pan, thermometer, and top of the sirup, similar to the salt solution, but this is not the usual result.
When the melting point of the sucrose is reached these crystals melt. Temperatures far above the melting point of the sugars can be obtained. However, with the very high temperatures, caramelization or decomposition of the sucrose occurs quite rapidly.
There is no very satisfactory explanation for the abnormal behavior of the sugars. Chemists tell us that one explanation may be that the sugar and water combine chemically giving a new compound with a new boiling point, or the combination of the sugar with the water may give a very concentrated solution, thus elevating the boiling point.
Boiling point of sucrose solutions. Browne in his "Handbook on Sugar Analysis" lists the boiling point of sucrose solutions as follows.
Table 10 Boiling Point of Sucrose Solutions (Browne)
Per cent sucrose.......
A 10 per cent solution of sugar is one that contains 10 grams of sugar and 90 grams of water or one having these proportions.
Heat of solution. Some substances that are soluble may liberate heat when they go into solution. The example of mixing water and sulfuric acid (H1SO4) is a well-known one. Other substances, instead of giving off heat, cause the temperature to drop when they go into solution. They are said to have a negative heat of solution, and heat is absorbed.
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If sugar and water of the same temperature are mixed, the temperature of the solution drops as the sugar is dissolving. Salt and many other sub-stances also absorb heat as they go into solution. When the substances that absorb heat as they go into solution are crystallized from solution, heat is liberated and the temperature is elevated slightly. This is often notice-able in making fondant or fudge. Frequently the sirup softens so that it is not so viscous and is easier to stir when crystallization starts. As the first crystals formed are not visible, one may think that the sirup is not going to crystallize because of this softening. It is more noticeable with larger amounts of fondant and fudge than with very small ones.