Johnstone’s approach to understanding gas laws
S. Athavan Alias Anand
Once when I was teaching gas laws, I asked my students, “Why does the volume of the balloon increase when we put it in hot water?” One student said, “The volume increases due to the increase in temperature.” Then I asked, “What is really happening to the gas molecules when there is an increase in temperature?” Nobody answered. When I explained the concept at the molecular level, the students become more excited and were motivated to learn the gaseous properties in different conditions. This self-engagement among students and the learning outcomes made me realize the importance of Johnstone’s triangle in chemistry education.
Professor Alex H Johnstone was a professor in chemistry and is celebrated in the field of education for his exemplary research in chemistry education. He emphasized that a learner should understand a chemistry concept at three levels1 if he/she is to truly imbibe what is taught. The macroscopic (what we see, hear and smell), the microscopic (the invisible world of atoms and molecules) and the symbolic (the written and mathematical representation of molecular behaviour at the microscopic level). In our schools, most of the time students only observe chemical reactions in the classroom/laboratory (macroscopic understanding) and memorize equations and symbols (symbolic understanding) for the exams. The interconnection between macro, micro and symbolic levels of teaching plays a key role in chemistry education.2 With no microscopic understanding of concepts, many students are unable to form a bond with the subject and therefore don’t pursue it further.
In this article, I would like to show you how Johnstone’s triangle (figure 1) can be applied to teach or learn gas laws. The experiments described in this article are inexpensive, easy to perform and yet very effective and hence teachers can easily replicate them in the classroom.
Activity 1: Charles’s Law
Charles’s Law is named after Jacques Alexandre César Charles, a French scientist. It states that “The volume occupied by a fixed amount of gas is directly proportional to its absolute temperature if the pressure remains constant.”
Materials required: Two glass beakers, plastic syringe, hot water and cold water.
Procedure: Seal the tip of the syringe and record the volume of air inside the syringe at room temperature. Dip the syringe into a beaker containing hot water. Observe and record the changes in the volume of air inside the syringe after three minutes. Then transfer the same syringe to a beaker containing cold water and record the difference in the volume of air after three minutes.
Johnstone’s approach
(i) Macroscopic: The students will observe that the volume of air inside the syringe increases in hot water and decreases in cold water. (ii) Microscopic: Due to the increase in the temperature, the kinetic energy of the gas molecules also increases and they move faster. As a result of this rapid movement, the molecules hit the plunger more often, pushing it up, thus the volume of gas increases when the temperature increases and vice-versa. During this entire process, the pressure inside the syringe remains constant because of the movement of the plunger. When the pressure starts to increase inside the syringe (with the molecules moving faster and hitting the sides more often), the plunger is pushed up until the pressure exerted by the gas molecules inside the syringe is the same as the pressure exerted by the water molecules outside the syringe, thus keeping the pressure constant. Therefore, the volume of gas molecules trapped inside the syringe increases when in hot water and decreases in cold water. (iii) Symbolic: The relationship between volume (V) and temperature (T) of the gas is represented by,
V α T
The volume of the gas is directly proportional to the temperature of the gas (measured in Kelvin) at constant pressure.
Activity 2: Boyle’s Law
Robert Boyle studied the relationship between pressure and volume in gases in the 1660s. He found that the volume of a gas changes with changes in the pressure. As the pressure increases, volume decreases and vice versa. His law states, “Pressure of a given mass of gas is inversely proportional to the volume at a fixed temperature for a given gas.”
Materials required: Balloon, large plastic syringe, thread.
Procedure: Inflate a balloon slightly so that it fits inside the large syringe. Remove the plunger from the syringe and drop the air-filled balloon inside the syringe so that it is close to the tip. Then seal the tip of the syringe and push the plunger back into the syringe. Push the plunger as far inside the syringe as it will go. Ask your students to observe the change in the balloon size.
Johnstone’s approach
(i) Macroscopic: The size of the balloon decreases when we push the plunger into the syringe, because pressure increases inside the syringe and therefore the volume of the gas inside the balloon decreases as shown in figure 3. (ii) Microscopic: When we press the plunger down, the pressure on the gas molecules inside the syringe increases. As the pressure increases, the amount of force on the gas molecules also increases and acts on the balloon. Thus, the volume of space taken by the gas molecules inside the balloon decreases and the balloon shrinks in size. The more densely the molecules are packed, more often they will collide with each other and create pressure inside the balloon. Hence, if the volume of gas is less, then the collisions and pressure of the gas will be greater and vice versa. (iii) Symbolic: The pressure of gases is indirectly proportional to the volume of the gases. If P, V, T are the pressure, volume and temperature of gas respectively, then at constant T (measured in Kelvin),
V ∝ 1/P
Activity 3: Gay Lussac’s Law
Gay Lussac, a French chemist and physicist, discovered this phenomenon while building an ‘air thermometer’ and the law states that, “The pressure of a gas of fixed volume is directly proportional to its absolute temperature.”
Materials required: Conical flask, boiled egg, matchbox and a piece of paper.
Procedure: Take a conical flask and place a boiled egg at the mouth of the flask. Try to push the egg into the flask without breaking it. Did you manage it? Now light a piece of paper and put it inside the flask. Now place the boiled egg at the mouth of the flask. Ask your students to observe the changes that take place for the next two minutes.
Johnstone’s approach
(i) Macroscopic: The students will observe that the egg is sucked into the flask. (ii) Microscopic: The kinetic energy of the gas molecules inside the flask increases when a burning paper is placed in it (temperature increases). Few gas molecules will escape the flask and the remaining molecules will hit the walls of the flask with greater force (figure 4a). When the boiled egg is placed at the mouth of the flask, the fire is extinguished due to excess carbon dioxide. Then the distance between the molecules trapped inside the flask will become smaller and the energy will decrease because the temperature inside the flask will come down. As a result, the pressure inside the flask will become so low that the atmospheric pressure is enough to push the egg into the flask (figure 4c). Remind your students that air always flows from a region of higher pressure to a region of lower pressure. Since the pressure inside the flask is low, air from outside tries to rush into the flask, pushing the egg inside along with it. Hence, lowering the temperature resulted in a decrease in the pressure of the air inside the flask. Therefore, pressure is directly proportional to temperature at constant volume. (iii) Symbolic: The law can then be expressed mathematically as
P α T
where, P is the pressure of the gas, and T is the temperature of the gas (measured in Kelvin).
After completing all three activities, the class time can be devoted to a discussion of the application of gas laws in everyday life. Use everyday household items like cycle-pump, carbonated drink can, aerosol spray bottle, fire extinguishers and other materials to explain their working principle associated with the gas laws. New learning experiences with syringes, balloons and other materials will help students think and feel like scientists while performing experiments and investigating the given situation.
References
- Johnstone, A. H. (1993). The development of chemistry teaching: A changing response to changing demand. Journal of Chemical Education, 70(9), 701-705.
- Gilbert, J. K., Treagust D. F. (2009). Multiple representations in chemical education, Chapter.1, Introduction: Macro, submicro, and symbolic representations and the relationship between them: Key model in chemical education. Springer Netherlands, 4, pp 1-8.
The author received his PhD (chemistry) from Annamalai University, Tamil Nadu. He continues his research as a postdoctoral fellow at the Indian Institute of Science (IISc), Bengaluru. Currently, he is working as a senior researcher at Prayoga Institute of Education Research, Bengaluru. His research interests include experiential learning, flipped classroom and assessment of learning outcomes. He can be reached at athavan@prayoga.org.in.
