Chemistry for Everyone edited by
Chemistry for Kids
John T. Moore Stephen F. Austin State University Nacogdoches, TX 75962
Is Every Transparent Liquid Water?
David Tolar R. C Fisher School Athens, TX 75751
Muhamad Hugerat* and Sobhi Basheer The Arab College for Education in Israel–Haifa, P. O. Box 8349, Haifa 33145, Israel; *
[email protected] Water is one of the most abundant compounds on Earth. Beginning students of chemistry quickly learn that water is the most mobile liquid component of the environment, and it is also an excellent solvent that dissolves many other substances (1). The accepted description for water in schools worldwide is a transparent and colorless liquid. Since students in lower grades (ages 8–13) often see signs warning “Do not drink this liquid—it is not water”, we believe that presenting experiments that demonstrate the inadequacy of the accepted description for water would be beneficial for teachers and their students. One of the most popular experiments in the laboratory for non-science-majors enrolled in a chemistry course is the demonstration of chemical reactions that are accompanied by an observable change such as a change of color, solubility, or electric conductivity. Such demonstrations arouse the interest
A
B
C
and the curiosity of students (1). Chemists may employ these characteristics to illustrate to students that not all materials that have similar external characteristics are the same. We have found that this type of approach is effective in maintaining the attention of students. The characteristics presented here are polarity, electric conductivity, color change due to presence of an acid–base indicator, and electrolysis (1). In these experiments, the term “apparent” refers to the visible similarity of a set of colorless liquids and solutions contained in separate transparent Erlenmeyer flasks. The term “hidden” refers to an invisible property of these apparently similar liquids and solutions that makes them behave differently when they are affected by a specific action. Precautions We suggest that teachers perform these experiments as demonstrations for the students. We recommend that each experiment be performed at a different lab station. Teachers should adopt the following basic safety precautions: 1. Avoid tasting and inhaling the vapors from any of the used materials. 2. Avoid contacting chemicals and keep them in safe place away from combustion site.
Glycerol
Hexane
Ethanol
A
B
C
3. Wear gloves and safety goggles. 4. During conducting the experiments the room should be open for ventilation. 5. Keep all chemicals and solvents in the hood except when they are being directly supervised.
Solubility in “Apparent” and “Hidden” Demonstrations Ethanol
Water
Hexane
Glycerol
Water
A
B
C
Oil
Hexane
Ethanol
+ Water
+ Oil
+ Water
+
Water
Oil Glycerol
Water
Figure 1. Three transparent liquids, glycerol, n-hexane, and ethanol, that have been gently shaken each with a similar volume of water and then with a similar volume of a seed oil.
Students observe that there are three transparent liquids, each in a separate Erlenmeyer flask. These are glycerol (purity of 99%), n-hexane, and ethanol. Since all the flasks look the same externally, students infer that they all contain the same liquid. The teacher gently adds a similar volume of water to each flask (not shaking the flasks and also not leaving the mixture for more than few hours, because glycerol and water are miscible in all proportions). The students will observe different behaviors for the three liquids basically because of their differing miscibility and density. In a second step, the teacher adds a similar volume of seed oil (corn oil or sunflower oil) to each flask (2). The results of the experiment are summarized in Figure 1. The students observe that the three transparent liquids behave differently when water is added in the first step and also when seed oil is added in the second step. If flasks are kept overnight it can be observed that water and glycerol form one phase in flask A (Fig. 1) because they are miscible in all proportions.
JChemEd.chem.wisc.edu • Vol. 78 No. 8 August 2001 • Journal of Chemical Education
1041
Chemistry for Everyone Amp
9-volt battery
−
+ Zn
graphite column
Cu
Figure 2. Galvanic cells for different transparent liquids or solutions.
Hazards and Precautions Hexane and acetone are highly flammable; therefore, contact should be avoided and precautions should be taken to be sure that there are no flames around. Also, hexane and acetone cause gastrointestinal disturbances; therefore, we suggest that the teacher require students to avoid breathing the vapors and to wear gloves and goggles if they are involved in conducting this experiment. Change of Color for Indicators in “Apparent” and “Hidden” Demonstrations In this experiment, the teacher presents 13 flasks, each of which contains one of the following liquids or solutions (1 M or saturated): acetone, distilled water, glycerol, n-hexane, aqueous ammonia, acetic acid solution, ethanol, calcium hydroxide solution, benzoic acid solution, sodium chloride solution, sugar solution, sodium bicarbonate solution, and citric acid solution. Since all solutions are transparent, in order to identify the different liquids the teacher adds red-cabbage juice to each flask and the results are observed. Red cabbage juice is an acid–base indicator that is violet at neutral pH and changes to red at low acidic pH and to blue at high alkaline pH (1). Students observe that although all solutions look similar initially, when they are treated with an acid– base indicator, the change in color indicates that each is different. From the results, the liquids and solutions can be classified as acidic, alkaline, or neutral according to the change of color after the addition of red-cabbage juice.
Hazards and Precautions Some of the materials (n-hexane, acetone, aqueous ammonia, and acetic acid) are irritants for the skin; therefore, during this experiment, all flasks should be maintained in the hood and contact with their contents should be avoided. Conductivity in “Apparent” and “Hidden” Demonstrations This experiment uses 13 flasks, each of which contains one the 13 transparent liquids used in the preceding demonstration. Visually, all flasks look similar and most beginner student audiences presume they all contain water. This presumption is quickly dispelled when the teacher performs the conductivity
1042
+ −
Figure 3. Conductivity cells for different solutions. The electrodes are two graphite columns, which are connected with a copper wire, a 9-volt battery, and a small lamp.
experiments, because the students observe very different results for the different liquids. In the first of two experiments, the teacher immerses two thin plates, one made of zinc and the other of copper, in each solution (1, 3, 4 ). Next, the two plates are connected with a wire made of copper that passes through an ammeter to measure the generated electric current. Shortly after the two plates are connected, electrons start to migrate from the negative electrode (zinc plate) to the positive electrode (copper plate), provided that the two electrodes are immersed in an ionic solution that allows electrons to migrate between the metals (Fig. 2). The results show that only solutions containing free ions allow electrons to migrate between the two plates, and the current can be measured by the ammeter that is connected to the wire (Table 1). Furthermore, the students observe the production of small bubbles of hydrogen gas in all of the acidic solutions near the positive electrode (copper plate, eqs 1, 2). Zn → Zn2+ + 2e᎑ (on the negative electrode, zinc) (1) 2H+ + 2e᎑ → H2(g) (on the positive electrode, copper) (2) Continuing the experiment, two graphite rods are connected by a copper wire with a 9-volt battery and a small lamp, and immersed in the above liquids or solutions (Fig. 3). The Table 1. Electric Current and Intensity of Light Produced in the Electrolysis Cells Liquid or Solution
Current/mA
Intensity of light a
Distilled water
0
–
Sodium bicarbonate soln, 1 M
5
+++
Glycerol
0
–
Citric acid soln, 1 M
5
+++
Benzoic acid soln, sat.
2
+
Ca(OH)2 soln, sat.
2
+
Acetic acid soln, 1 M
7
+++ ++
Aqueous ammonia, 1 M
3
Sugar soln, 1 M
0
–
Table salt soln, 1 M
4
+++
Ethanol
0
–
Acetone
0
–
n-Hexane
0
–
a Light
intensity: +++ high; ++ medium; + low; – no light.
Journal of Chemical Education • Vol. 78 No. 8 August 2001 • JChemEd.chem.wisc.edu
Chemistry for Everyone
students observe that the intensity of the light varies depending on the solution (4). As electrical current flows, electrolysis occurs, and bubbles of gases will appear on both electrodes (1, 3). Table 1 shows the results of both tests. The intensity of the light produced due to the migration of electrons through the wire varies (1, 3, 4 ). According to the observed results the solutions can be classified into three groups as follows: 1. Solutions that do not show electric conductivity, therefore, neither light nor electric current was observed. 2. Solutions that show lower electric conductivity as evidenced by smaller current and light intensity. Therefore ions in these solutions have chemically changed. 3. Solutions that show more extensive electric conductivity as evidenced by larger currents and light intensity. The bubbles of gas observed among groups 2 and 3 indicate chemical changes as the gas is produced (e.g., hydrogen ions converted to hydrogen gas).
Relative Flammability in “Hidden” and “Apparent” Demonstrations The combustion point of materials may also be tested in “hidden” and “apparent” demonstrations. The teacher presents three flasks, the first containing water, the second containing ethanol, and the third containing equal volumes of water and ethanol. Both of these liquids are polar and transparent. Holding pieces of linen with the help of long forceps, the teacher soaks one piece in each flask for one minute and then places it in the flame of a Bunsen burner. Students can observe that the piece of linen soaked in water does not burn, the one soaked in ethanol does burn, and the one soaked in a solution of 50:50 water–ethanol does not burn although the liquid burns.
Hazards and Precautions This demonstration must be conducted only by the teacher, taking all required safety precautions, such as wearing goggles and gloves. Conclusions There are many chemical and physical methods to demonstrate that not every transparent liquid is water. Some of these methods have been presented in this article. From these experiments the students learn the following. 1. That “apparently” similar materials may have “hidden” differences. The results of the experiments teach the students to avoid contact with unknown materials, as they show that not every transparent liquid is water. 2. That phenomena can be investigated using different chemical and physical approaches. 3. That precautions must be observed to conduct experiments in a safe and reproducible way.
Literature Cited 1. Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry, Vols. 1–4; University of Wisconsin Press: Madison, WI, 1983–1991. 2. Silverstein, T. P. J. Chem. Educ. 1998, 75, 116. 3. Bowden, M. E. Chemistry Is Electric; Chemical Heritage Foundation: Philadelphia, PA, 1997; Chapter 1, pp 6–15. 4. Katz, D. A.; Wills, C. J. Chem. Educ. 1994, 71, 330. Zawacky, S. K. S. J. Chem. Educ. 1995, 72, 728. Guzman, M.; Puga, D. J. Chem. Educ. 1993, 70, 71. Haworth, D. T.; Bartelt, M. R.; Kenney, M. J. J. Chem. Educ. 1999, 76, 625. Ghatee, M. H. J. Chem. Educ. 1993, 70, 944. Berenato, G.; Mynard, D. F. J. Chem. Educ. 1997, 74, 415.
JChemEd.chem.wisc.edu • Vol. 78 No. 8 August 2001 • Journal of Chemical Education
1043
