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EXPERIMENT NO. 209
THE VAPOR-PRESSURE CURVE FOR ETHER.
References: Stewart, Physics, Sect. 267, 269, 273; Kimball, College Physics, Sect. 431, 433, 436; Duff, College Physics, Sect. 215, 219, 221; Spinney, Text-Book of Physics, Sect. 179, 180, 184, 189.
A J-tube, with one arm closed, is filled with mercury and a small amount of ether is introduced into the closed arm without admitting any air. The mercury in the open tube stands at a lower level than in the closed tube. The pressure on the ether is therefore less than atmospheric pressure by the difference in level between the mercury columns. The pressure due to the weight of the small amount of ether may be neglected. If the ether be heated, its vapor pressure increases and as soon as the temperature becomes high enough to produce a vapor pressure differing from atmospheric pressure by an amount less than that corresponding to the maximum difference in level of the mercury columns, vapor will appear above the ether, and the volume of the vapor will increase as the temperature rises and the vapor-pressure increases. In summer, it is probable that there will be vapor present before any heating is done. The vapor-pressure may be found at any instant by determining the difference in level of the two mercury tubes and subtracting this from the atmospheric pressure. When the mercury in the open tube is higher than that in the closed tube, add the difference in levels to the barometer reading. A section of meter stick is fastened between the two arms of the J-tube, so that readings of mercury levels can be taken.
The J-tube, together with a heating coil, thermometer, and stirrer, is placed in a deep jar filled with cold kerosene and the heating coil connected through a switch to an electric outlet. Unless a low-voltage source is used, a rheostat should also be in the circuit. Before turning on the current, have the instructor inspect the arrangement. Be sure that none of the kerosene gets into the open end of the J-tube. After turning on the heating current, watch the ether column and when a bubble of vapor is seen forming or the pressure of the existing vapor begins to
increase, open the switch, stir the oil thoroughly and read the temperature and the heights of both mercury columns.
Make readings for every two degrees rise in temperature until the mercury in the closed tube is depressed to within two or three centimeters of the bottom of the J, each time taking precautions about stirring as described above.
Tabulate data and plot a curve, using pressures as ordinates and temperatures as abscissae. From this curve determine at what temperature the vapor pressure was equal to that of the atmosphere. What is this temperature called? What is meant by a saturated vapor? Is the vapor in the tube saturated? In what ways can the pressure of a saturated vapor be increased?
EXPERIMENT NO. 210
THE HEAT OF VAPORIZATION OF WATER.
References: Stewart, Physics, Sect. 279; Kimball, College Physics, Sect. 440; Duff, College Physics, Sect. 217; Spinney, Text-Book of Physics, Sect. 176.
The heat of vaporization of water is defined as the quantity of heat per gram needed to vaporize water without change of temperature. If a gram of steam condenses without change of temperature, the heat of vaporization is given out. Using this fact, the heat of vaporization can be found by a method of mixtures.
Steam is generated in a boiler and transmitted to a calorimeter through a tube wrapped with cotton to minimize condensation. A water trap must be inserted in the steam line near the outlet to prevent hot water from passing out with the steam. An asbestos or wooden screen should be placed so as to shield the calorimeter from the heat of the burner, and the burner must be guarded from draughts. If the flame is blown aside by a breeze, water will be forced from the calorimeter back into the steam line as the boiler pressure goes down. Find the water equivalents of the calorimeter and stirrer, and thermometer as previously described. Weigh out enough water to fill the calorimeter about three-fourths full. This water should be from 10 to 15 degrees below room temperature. Read the temperature
carefully, estimating fractions of degrees, then introduce the steam tube below the surface of the water and let steam flow in and condense until the temperature has risen about as high above room temperature as it was below at the start. Remove the steam tube and determine the new temperature. Weigh the calorimeter to find the mass of steam condensed.
Compute the number of calories absorbed by the water, calorimeter, and thermometer as they rose in temperature. Find the number of calories given out by the condensed steam in cooling from the boiling point to the final temperature of the mixture. The difference between these two quantities is the number of calories given out by the known mass of steam in condensing at the boiling point. Divide this difference by the mass of steam to find the heat of vaporization.
The chief difficulty is that hot water is carried into the calorimeter and counted as steam. Therefore condensation in the delivery tube must be avoided as far as possible. With ordinary apparatus the values of the heat of vaporization obtained may differ as much as 10% from the accepted value, but with care and repeated trials some good results can usually be obtained in a laboratory period. Care must be taken to avoid all spilling of water or the carrying of water out of the calorimeter on the stirrer or thermometer.
EXPERIMENT NO. 211
BOILING AND FREEZING POINTS OF SOLUTIONS.
References: Stewart, Physics, Sect. 265, 278; Kimball, College Physics, Sect. 429, 439; Duff, College Physics, Sect. 210, 211, 221.
(a) Fill a metal cup about three-fourths full of water and heat it over a Bunsen flame till boiling occurs. Immerse a thermometer in the water and find the temperature of the water as it boils.
Dissolve a tablespoonful of common salt in the water and again note the boiling temperature. Dissolve a second, third, and fourth tablespoonful in the water, noting the boiling point after each addition, and being sure that each quantity of salt is
dissolved before taking the temperature. Be careful to make the quantities of salt as nearly equal as possible. Record the observed temperatures.
(b) Prepare a quantity of freezing mixture, consisting of cracked ice and salt. In this freezing mixture place a small vessel about half-full of water, and stir this water gently with a thermometer as its freezing point is approached. Record the reading as soon as ice first begins to form in the cup. In this manner the freezing point is determined.
Determine the effect on the freezing point of dissolving, first one, then two and three large tablespoonfuls of salt in the water. The ice which is formed should be all melted before the addition of each new quantity of salt. Record the temperatures observed.
EXPERIMENT NO. 212
EFFECT OF PRESSURE CHANGES ON THE BOILING POINT.
References: Stewart, Physics, Sect. 271; Kimball, College Physics, Sect. 437; Duff, College Physics, Sect. 219; Spinney, Text-Book of Physics, Sect. 180.
The apparatus consists of an air-tight brass cylinder, in which the water is boiled, connected through a condenser to a large air-chamber in which the pressure can be made greater than that of the atmosphere by means of a bicycle pump, or less than that of the atmosphere by means of an aspirator fastened to the water-tap. The cylinder has a thermometer projecting through the lid to indicate the temperature of the steam, and the airchamber is connected to a U-tube which contains mercury and is used as a pressure-gauge. The true pressure is found by adding or subtracting the indication of this gauge to the reading of the barometer. The condenser is sloped so that all the steam condensed runs back into the cylinder. Consequently the pressure in the apparatus does not rise appreciably during the boiling, but remains constant. The lower feed-tube of the condenser must be fastened to a water-tap so that a constant stream of cold water flows up through the outer space of the condenser and back into the sink.
In order that the water may boil freely, it is necessary that some pieces of broken glass or crockery be put into the boiling cylinder. Otherwise the water may "bump."
Close everything air-tight, having the cylinder about half full of water. Then pump air into the apparatus till the gauge indicates about 10 cm. above atmospheric pressure. Then close the stop-cock leading to the pump, light the burner, and, when the water is boiling freely, read thermometer and pressure-gauge. Increase the pressure about 10 cm. and proceed as before. In this way carry it up to as high pressures as the gauge will indicate.
Then connect the aspirator to the apparatus, and proceed to take a series of readings below atmospheric pressure.
Finally, having read the barometer, plot a curve with pressures as abscissæ and the corresponding boiling temperatures as ordinates.
EXPERIMENT NO. 213
References: Stewart, Physics, Sect. 288-291; Kimball, College Physics, Sect. 445, 446; Duff, College Physics, Sect. 218; Spinney, Text-Book of Physics, Sect. 195-198.
The absolute humidity is the number of grams of water vapor contained in a cubic meter of air at any time. The relative humidity is the ratio between the absolute humidity and the number of grams of vapor per cubic meter necessary to produce saturation at the existing temperature. This ratio is commonly expressed as a percentage. The mass per cubic meter necessary for saturation increases as the temperature rises, and conversely; so, if air is cooled sufficiently, the existing water vapor will become saturated. The temperature at which this takes place is called the dew-point.
(a) Dew-point method. This method of finding the relative humidity depends on determination of the dew-point.
Fill a polished, nickel-plated cup about half-full of water. Add ice a little at a time and stir the water. As soon as moisture begins to condense on the outside of the cup (it may be detected