WHAT HAPPENS WHEN WATER BOILS? (X-2)

The problem. The use of boiling water and steam in the affairs of everyday life is so extensive that we seldom raise questions about the process of boiling. Does water always boil at the same temperature? Does the air pressure influence the process? Why is the double boiler used in cooking? Does water boil at ordinary temperatures at sea level.

and on a mountain? Does the presence of salt or sugar in the water affect the process? These questions and others are best answered by a careful study of the process of boiling.

What to use. Ring stand, two rings, wire gauze, flask (500 cc.), thermometer, glass tubing, two-hole rubber stopper, Bunsen burner, matches, test tube, beaker, widemouth bottle, salt, ink, sawdust, and rubber tubing.

FIG. 21

What to do. 1. Assemble the apparatus as shown in figure 21. Add a small amount of sawdust to the clear water. Note the temperature of the water, then heat it slowly. Note the movement of the sawdust. When the water is boiling vigorously note the temperature. When boiling has begun does the temperature rise if more heat is applied? Remove the Bunsen burner.

2. Slide the thermometer up out of the water, then boil the water and note the temperature of the steam.

3. Add some salt or sugar; note the temperature of the boiling water and of the steam.

4. Add some ink to the water, connect the long tubes shown in figure 21, and let the steam pass into the cold test tube.

Questions. Why is a wire gauze used under the flask? Does a film appear on the flask? Why? At what temperature do small bubbles appear? Explain the behavior of the sawdust. Is there a difference between the temperature of boiling water and that of steam? between that of salt water and that of steam? Do the salt and ink pass over with the steam? Why?

Suggestions for report. Write a summary of this experiment, giving a complete statement for each important fact.

Reference work. Read sections 132 to 137.

Optional problems. If a strong round-bottom flask is available, the following experiment may safely be performed with interest: Boil the water several minutes, then remove the flame and close at once with a rubber stopper. Invert the flask on a ring stand. Wet a towel with cold water and put it on the flask. Explain. By use of a thermometer determine the temperature at which cooking food simmers.

EFFECT OF EVAPORATION UPON TEMPERATURE (X-3)

The problem. If one stands on the beach after bathing he soon becomes chilled, even though it is a warm day. If the wind is blowing the cooling effect is greater. Alcohol, gasoline, and ether produce a sensation of coldness as they evaporate from the hand. Artificial ice is manufactured by the cooling effect of liquid ammonia which is evaporating rapidly. To what extent does the rapid evaporation of different liquids affect the temperature?

What to use. Ring stand, thermometer, burette clamp, rubber stopper, some clean cotton, rubber band, water, alcohol, and fan.

What to do. 1. Arrange the apparatus as shown in figure 22. Record the temperature of the room. Twist a small piece of cotton around the bulb. Fasten this with a rubber band. Wet the bulb with water at room temperature. Fan the thermometer until no further change is produced. Record the temperature. 2. Repeat the experiment, using alcohol.

FIG. 22

3. The relative humidity of the room can be obtained from the results in 1. The change in temperature on the wet-bulb thermometer depends upon the moisture present in the air. Tables have been devised which will give the relative humidity direct, with no calculation. Find the value for your readings from the graph in figure 23.

Questions. What happens as the water evaporates from the cotton? How does the movement of air affect the temperature ? What results are obtained with alcohol? What is the relative humidity of the room? Account for the temperature changes.

Suggestions for report. Record the observations as follows:

Room temperature (dry bulb)

Room temperature (wet bulb)

Lowest temperature obtained with alcohol

Relative humidity of room

Reference work. Read sections 138 to 144.

Optional problems. Weigh a beaker, add 200 grams of water, and set it aside in a quiet place in the room. Weigh the beaker each day to determine the rate of evaporation. By means of the apparatus shown in figure 61 of the text and the graph used in this. experiment find the relative humidity of several rooms. Write to the New York State College of Agriculture for directions for constructing an iceless refrigerator, and determine the principles and processes involved in this device.

EVAPORATION OF WATER FROM A PLANT (X-4)

The problem. The weight of water evaporated into the air from a large tree in one sunshiny and warm day may be as great as the weight of ten general-science students of average size. Is it possible to demonstrate that plant leaves transpire (that is, lose water to the air) and also to measure the exact amount transpired from some of the

smaller plants?

What to use. Dry bottle with a split cork stopper, three potted plants, sheet of thin rubber (dentist's rubber) large enough to inclose the pot in which one of the plants grows, pair of balances or any accurate scales, string, ring stand, and burette clamp.

FIG. 24

What to do. 1. Insert a few leaves from one of the plants into the bottle and fit the split cork into the bottle about the stem so that the bottle is closed about some of the leaves (see Fig. 64 of the text). Support the bottle so that its weight will not rest upon the plant. Observe from hour to hour to see if drops of water appear in the bottle.

2. Wrap the pot of another plant (Fig. 24) with the sheet rubber and tie it so as to hold the rubber tightly about the stem, making sure not to injure the stem.

3. Weigh the third pot and plant, also the wrapped pot and plant, set in a light place, and weigh regularly. Record the weights.

Questions. What does loss of weight of the wrapped pot indicate? Is rate of loss the same at all periods? Account for any differences. What are the reasons for wrapping the pot with rubber?