In the Classroom edited by
tested demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
Experimental Methods To Demonstrate Physicochemical Behavior of Gases with a Resin Tube Submitted by:
Chieko Suzuki,* Sumiyo Ihda, Mayumi Suzuki, and Seiji Kurumi Department of Education, Shiga University, 2-5-1 Hiratsu, Ohtsu, Shiga 520, Japan
Checked by:
Cole McWherter and George Gilbert Department of Chemistry, Denison University, Granville, OH 43023
Elementary and secondary school students would be attracted by simple experiments in which they can observe the results through their own five senses. We will present here simple methods that demonstrate two kinds of physicochemical behavior of gases using a vinyl resin tube. One is for recognizing the presence of air, and the other is for enhancing the understanding of adiabatic compression and expansion.
In states 2 and 3, the lower end of the tube can be stopped with a plastic sheet and rubber band. In 3 and 4, the upper end can be stopped with your palm as soon as the ball is released from your fingers.
Demonstration of the Presence of Air The presence of air is too natural to be easily recognized by children. In elementary school, it is usually shown either by feeling the elasticity when touching a plastic bag filled with air, or by making bubbles in water in which something like an empty bottle or a dry sponge is submerged. The method described here should be accessible to secondary school students. They will be able “notice” the presence of air by observing the sharp difference in velocities of a ball falling through a resin tube when one or both ends are closed, and when both ends are open.
Materials • •
• • • •
Pu
S
wooden ball
Fc
Pl
inθ
mgs
b
mg
be
tu resin
00 ←1
open or closed
4 cm cm
30 cm
→
a
θ
Figure 1. Presentation 1: The case in which the tube is held inclined. a: The general setup. b: The forces on the ball and pressures in the upper or lower space within the tube.
wooden ball with diameter of 3–4 cm colorless transparent vinyl resin tube, > 100 cm long, with inner diameter slightly larger than ball’s diameter sheet of plastic, 80 mm × 160 mm rubber band stand with a clamp stopwatch
wooden ball ←
open or closed
Pu
Procedure Preparation Carefully check that the wooden ball can smoothly move inside the tube, which should fit around it closely, with a minimum gap. Presentation 1 Position the clamp on the stand at a 30-cm height and hold one end of the tube in the clamp as illustrated in Figure 1a. Put the center of the ball at the end of the tube, let the ball start from rest, and measure the time: Tm (m = 1, 2, 3, 4) taken for the ball to run down through the tube in the following states:
m F c |
S mg table Pl
b
Figure 2. Presentation 2: The case in which the tube is held perpendicularly. a: The general setup. b: The forces on the ball and pressures in the upper or lower space within the tube.
1. both ends of tube open 2. lower end closed and upper end open 3. both ends of tube closed 4. lower end open and upper end closed *Corresponding author.
←
open or closed box
a
Vol. 74 No. 9 September 1997 • Journal of Chemical Education
1071
In the Classroom Presentation 2 Hold the tube perpendicularly as illustrated in Figure 2a, and measure the time taken in order for the ball to run down through the tube under the same conditions as in Presentation 1, except that the angle between the tube and the table is a right angle. The lower end should be stopped with a plastic sheet and a rubber band because it cannot be stopped completely by simply standing the tube directly on the table.
Results It can be easily recognized without using a stopwatch that T2, T3, and T4 are far longer than T1 for both presentations. But the differences between T2, T3, and T4 cannot be ascertained just by eye estimation. Estimating with a stopwatch is rather difficult, and it takes practice to push the button at just the moment when the ball reaches the bottom of the tube. The measurement error of Tn becomes bigger with shorter tubes. The Tn’s estimated with stopwatches are shown in Tables 1 and 2. The results show that if the tube is held either perpendicularly or on an incline, stopping one or both ends brings the time to about 3 times as long as in the case where both ends are open.
Table 1. Time Taken by Ball To Pass through Inclined Tube upper end: lower end:
open open
open closed
closed closed
closed open
T 1 (s) 1.0 1.1 0.9 1.0 0.9 0.9 1.0 1.0 1.0 1.0 average 1.0
T 2 (s) 3.1 2.9 2.9 3.3 3.1 3.0 2.8 3.1 3.1 3.1 3.0
T 3 (s) 3.0 2.9 3.1 2.9 3.1 3.0 3.1 3.1 3.1 3.1 3.0
T 4 (s) 3.0 2.9 2.8 2.7 2.8 2.8 2.9 2.8 2.7 2.9 2.8
Trial No. 1 2 3 4 5 6 7 8 9
Discussion The fact that T2, T3, and T4 are all longer that T1 should be one indication that the tube is filled with something: air. The force (see Figs. 1b and 2b) directly or obliquely downward along the tube in the ball in each case is: Fn = m g sinθ – (Pl – Pu ) n ? S – (Fc ) n ; n = 1, 2, 3, 4 where m is the mass of the ball, g is the acceleration due to gravity, θ is the angle between the tube and the table, S is the cross section of the ball, and P u and P l are the pressures at the upper and the lower side of the ball in the tube. (F c ) n is the force induced by the air current that flows through the gap between the tube and the ball from the lower-side space of the ball to the upper-side space. F c is always positive for any n. If the lower end of the tube is stopped, the air in the tube below the ball is compressed as the ball goes down, though the air gradually leaks away through the slight gap between the tube and the ball. On the other hand, if the upper end of the tube is stopped, the air in the tube above the ball is decompressed as the ball goes down. This compression and decompression makes Pl and P u higher or lower than the atmospheric pressure. In case 1, while the ball goes through the tube, both Pl and P u remain at atmospheric pressure; (Pl – Pu) = 0, (Fc ) 1 6 0. Therefore, F 1 = m g sinθ. In case 2, as the ball goes down, Pl increases above atmospheric pressure, but Pu does not change and keeps to the atmospheric pressure. (Pl – P u) > 0 , and (Fc ) 2 > 0. Therefore, F2 < F 1 and T2 > T1. In case 3, as the ball goes down, P l rises higher than atmospheric pressure and P u decreases to lower than atmospheric pressure. (Pl – Pu)3 > 0 and (Fc ) 3 > 0. Therefore, F3 < F1 , and T3 > T1. In case 4, as the ball goes down, Pl stays at atmospheric pressure, but Pu falls below atmospheric pressure. (P l – P u)4 > 0 and (F c ) 4 > 0. Therefore, F 4 < F1, and T4 > T1. Thus, any one of times T2, T3, or T4 is longer than T1. Demonstration of Effects of Adiabatic Compression and Expansion
open open
open closed
closed closed
closed open
Trial No.
T 1 (s)
T 2 (s)
T 3 (s)
T 4 (s)
1
0.4
1.3
1.3
1.1
Students must understand the adiabatic process before they are taught why the air temperature is lower at higher altitudes within the troposphere and how clouds are formed in the sky. Almost all science textbooks for secondary schools in Japan describe the experimental method illustrated in Figure 3 (1). In this experiment, however, you should use a 1500-mL PET bottle1 instead of the glass bottle, and an air pump for a bicycle instead of the injector, to ascertain the change in bottle temperature by touch and by sight without using a thermometer. Here we propose another method to demonstrate the effects of adiabatic compression and expansion by the use of a vinyl resin tube.
2
0.4
1.2
1.1
1.1
Materials
3
0.5
1.2
1.3
1.1
•
the vinyl resin tube used in the first demonstration
4
0.4
1.2
1.1
1.1
•
5
0.4
1.4
1.3
1.0
a wooden bar longer than the tube length (e.g., a mop handle)
6
0.4
1.3
1.2
1.1
•
some old newspapers
•
some water
10
Table 2. Time Taken by Ball To Pass through Tube Held Perpendicularly upper end: lower end:
7
0.4
1.2
1.3
1.1
8
0.4
1.2
1.1
1.1
9
0.5
1.2
1.1
1.1
10
average
1072
0.4
1.3
1.2
1.1
0.4
1.3
1.2
1.1
Procedure Preparation First of all, two stoppers should be made. Wet one page of a newspaper with water, fold it into a shape like a belt of about 5 cm width, and then roll it up as shown in Figure 4.
Journal of Chemical Education • Vol. 74 No. 9 September 1997
In the Classroom
Figure 3. The demonstration for adiabatic compression and expansion in the textbooks for junior high school in Japan.
5 cm
(a)
(b)
(c)
(d)
(e)
(f)
Figure 5. A method for the demonstration of adiabatic compression and expansion with a vinyl resin tube. (a) Beating a wet roll with a wooden bar makes a stopper. (b) Inserting stoppers into the ends. (c) The 1st adiabatic compression. (d) The 1st adiabatic expansion. (e) The 2nd adiabatic compression. (f) The 2nd adiabatic expansion.
Figure 4. How to roll a page from an old newspaper.
Put the roll into the end of the tube, and stand the tube on the floor with the roll at the bottom. Then, strike the roll with the bar many times as shown in Figure 5a until the roll becomes compact enough to be a stopper. Make 2 stoppers. Presentation Insert these stoppers into both ends of the tube as shown in Figure 5b. Then, push the lower stopper with the bar, while holding the upper stopper with both hands as shown in Figure 5c, until the space between the stoppers is reduced to about 1/4 of the total length of the tube. You can feel the compressed air column’s temperature increase by touching your hand to this portion of the tube. Then, remove your hand from the upper stopper while continuing to push the lower stopper. The upper stopper should blast off and fog should appear around the upper end of the tube as shown in Figure 5d. If the end of the tube is covered with the palm of the hand and the lower stopper is pushed more, you can feel the air column’s temperature increase and see the fog disappear in the tube, as shown in Figure 5e. Then, if the end of the tube is reopened as the lower stopper is pushed, you can see the fog reappear around the end of the tube because of the lowering of the air temperature around the end of the tube, as shown in Figure 5f.
Discussion This method can show not only that air can be easily compressed, but also that adiabatic compression of air causes heat generation and adiabatic expansion accompanies heat absorption. Water can vaporize from the wet stopper until the vapor pressure reaches the saturation point of the higher temperature. On the other hand, as adiabatic expansion brings about consumption of heat, the air cools down enough to convert oversaturated vapor into a fog. When the air is compressed again and the temperature increases, the fog in the tube vaporizes until the saturated vapor pressure is reached and the tube clears up. The change of temperature can be felt through touch, and the appearance and disappearance of the fog can be seen by the eye. Note 1. A bottle made of polyethylene terphthalate, which is used for containers of drink such as juice.
Literature Cited 1. Kurita, K.; Hosoya, H.; Miyawaki, A. Science for Junior High School 2nd Division; Kyoiku-shuppan: Tokyo, 1992; Vol. 2, p 13.
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