Measuring carbon dioxide from plant debris provides an opportunity for an inquiry-based experiment aimed at 14-15 year olds. Similar experiments are done by soil scientists and ecologists in their efforts to understand the global carbon cycle

An image of the world made from plant debris

Source: Petmal/iStock

Plant debris could help students to understand the global carbon cycle

Chemistry educators worldwide advocate inquiry-based practical work in the secondary school chemistry curriculum. However, good contextualised scientific inquiries for chemistry students are scarce.1 Here I describe an inquiry-based experiment done by Secondary 4 chemistry students (aged 14-15 years) in Hong Kong. This investigation can provide students with an excellent opportunity to understand part of the global carbon cycle.2,3

The carbon dioxide problem 

The purpose of this investigation is not for students to arrive at a single, 'correct' answer but to develop inquiry skills. Box 1 illustrates the problem. Students are seldom aware that bacteria and fungi feed on the dead remains of plants and, via respiration, release CO2 to the atmosphere. Leaf litter decomposition is a key process in the global carbon cycle. Each year, the mass of CO2 released by decomposing leaves is approximately equal to that released by animal and plant respiration. 

In a trial of the experiment, working with teachers, we divided a class into groups of three-four students based on their abilities. After about 30 minutes, we collected their plans for evaluation. The next day, we invited the students to present their plans to the rest of the class. This is done to turn assessment activities into a teaching-learning activity. Twenty minutes were allocated for each presentation. Each group needed to answer questions raised by other students. After the presentations, students did experiments according to their plans. The problem has been designed to be an open-ended investigation so students should be allowed to try out their own plans. 

Common procedural errors and limitations

We found that our students had the following common procedural errors or limitations in their experimental designs: 

  • some students did not recognise that CO2 is present in the air even without the decomposing leaves. They did not realise, therefore, that they had to determine how much more CO2 is produced as a result of the leaf decomposition, and a control set-up was needed;
  • many students wanted to use lime water to absorb the CO2 released by dead leaves and determine the mass of CO2 by weighing a bottle of lime water before and after a measurement period. This method does not work because lime water does not absorb CO2 efficiently;
  • some students used sodium hydroxide solution to absorb the CO2 released by dead leaves and then did a back titration with hydrochloric acid to calculate any excess sodium hydroxide. But sodium hydroxide reacts with CO2 to form sodium carbonate. Therefore, the resulting solution would be a mixture of carbonate ions and hydroxide ions. During the back titration, hydrochloric acid would react with both carbonate ions and hydroxide ions. (The composition of the hydroxide-carbonate mixture could be found by the double indicators method.4 Alternatively, barium chloride solution may be added to the mixture to precipitate the carbonate ions before the back titration is done.5 Unfortunately, these two methods are beyond Secondary 4 students.)  

Sample procedure

Here we present a sample procedure for teacher information. It should not be distributed to students as a cook-book style experiment.

Materials (per group of three-four students):

  • Safety goggles; 
  • Two airtight containers (at least 28cm × 21cm × 6cm); 
  • 10 dry dead leaves (same type of leaf);
  • Soda lime (1.0-2.5mm granular size, ca 40g, hazard warning label = corrosive); 
  • Deionised or distilled water; 
  • Two glass Petri dishes (at least 9cm diameter); 
  • Measuring cylinder (10cm3);
  • Spatula; 
  • Access to balance (±0.001g); 
  • Oven, and oven gloves. 

Pre-lab (by technician)

  • Put a thin layer of granulated soda lime in the bottom plate of a glass Petri dish. Find out the mass of soda lime used. This is approximately half of the mass of soda lime to be used by one group of students. Estimate the total amount of soda lime needed by the whole class. 
  • To inactivate soda lime, put the estimated total amount of soda lime in a beaker and dry it in an oven at 105°C for 24 hours. Place the soda lime in a desiccator immediately upon removal from the oven. 

Experimental details for students

(Put on your safety goggles and wear lab coat and gloves.) Obtain two airtight containers. Label one container as 'experimental' and the other as 'control'. Obtain eight to 10 leaves (use the same type of leaf and if the leaves are large in size, reduce the number of leaf). Record the total mass of leaves. Put the leaves into the experimental container. 

Fill the top plates of the two glass Petri dishes half-full with distilled water. Place one plate of water into the experimental container and the other plate into the control. Using a marker, label the bottom plates of the two Petri dishes as 'experimental' and 'control'. Weigh each plate separately and record its exact mass. Using a spatula, spread out a thin layer of the oven-dried soda lime granules in the plates. (CAUTION: Soda lime is corrosive. Do not touch it with your bare hands.) Weigh each plate separately and record the exact mass of soda lime used. Place the plates containing soda lime into the experimental and control containers. Using a measuring cylinder, carefully add 5.0cm3 distilled water to the soda lime in the experimental and control containers to activate the soda lime. Immediately seal the containers and place them in the location specified by your teacher for one week.

After one week, open the experimental and control containers and remove the two soda lime dishes. Dry the soda lime at 105°C in an oven for 24 hours. Find out the mass of CO2 absorbed by re-weighing the two soda lime dishes. 

Notes for teachers

Risk assessments should be done in advance by the teacher. Teachers should also note that: 

  • solid sodium hydroxide alone is not satisfactory to absorb CO2 in this experiment because it absorbs water vapour from the air and puddles of concentrated (and corrosive) sodium hydroxide solution will be formed. A lot of heat will also be released when sodium hydroxide solution is formed;
  • absorption of CO2 using solid calcium hydroxide alone is not efficient because CO2 must be dissolved in water before it can react with calcium hydroxide. Soda lime is more efficient because sodium hydroxide is added to calcium hydroxide to absorb moisture. The moisture in soda lime granules is not visible when the water content is less than 20 per cent. Because CO2 is chemically bound but the moisture is not, soda lime can be dried and weighed before and after a measurement period to determine the amount of CO2 absorbed. Since soda lime is inefficient for CO2 absorption unless moisture is available, this sample procedure allows water to evaporate from a dish to increase humidity. You should remind your students that soda lime is corrosive and so they should not touch it with their bare hands. I recommend the use of granular (1.0-2.5mm) soda lime. You may use other granular sizes, but it should be small enough for a large surface-to-volume ratio and large enough to prevent losses of fine particles during drying and handling.  

Sample calculations and results 

Although soda lime is a variable mixture of sodium hydroxide and calcium hydroxide, you do not need to know the exact percentages of these chemicals to calculate the mass of CO2  absorbed. You may guide your students to apply the following chemical concepts to solve this problem.  

Absorption of CO2 occurs by: 

Ca(OH)2 + CO2  → CaCO3 + H2 O  (i) 

2NaOH + CO2  → Na2 CO3 + H2 O  (ii) 

For every mole of CO2 that is reacted with Ca(OH)2 in the soda lime, one mole of H2 O is formed that is subsequently evaporated during oven-drying. Thus, the increase in mass of dried soda lime measured before and after the experiment is not equal to the mass of CO2 absorbed.  

From equation (i), if one mole (44g) of CO2  has been absorbed, the increase in mass = molar mass of CaCO3- molar mass of Ca(OH)2 = 100 - 74 = 26g. The mass of CO2 absorbed by Ca(OH)2 is proportional to the increase in mass after the experiment. Thus, 

Mass of CO2 absorbed/44g = Increase in mass/26g

Mass of CO2 absorbed = increase in mass × 44g/26g 

= increase in mass × 1.69

Similarly, from equation (ii), if one mole (44g) of CO2 has been absorbed by two moles of NaOH in the soda lime, the increase in mass 

= molar mass of Na2 CO3

- the mass of two moles of NaOH 

= 106 - (2 × 40)

= 26g.  

Similarly, from equation (ii), if one mole (44g) of CO2 has been absorbed by two moles of NaOH in the soda lime, the increase in mass 

Like Ca(OH)2, the mass of CO2 absorbed by NaOH = increase in mass × 1.69. Thus, the relative amounts of Ca(OH)2 and NaOH in a sample of soda lime is not important in this experiment. Some chemical suppliers also add potassium hydroxide to soda lime, but the mass of CO2 absorbed by potassium hydroxide can also be obtained by multiplying the increase in mass by 1.69.  

To find the amount of CO2 released by decomposing leaves, students need to subtract the mass of CO2 absorbed in the control container from the mass of CO2 absorbed in the experimental container. 

Example

Mass of dry dead leaves used = 3.073g 
Name of plant = Liquidambar formosana
Duration = seven days  
See Table 1 for the results to this example.
Therefore, the mass of CO2 released per gram of leaf per day = (0.059 - 0.008)g/3.073g × 7 days 

And finally

The mass of CO2 released depends upon factors such as the type of plant, the amount of bacteria and fungi, and the temperature. With Bauhinia purpurea leaves, for example, I found that 0.012g of CO2 was released per gram of leaf per day. In countries with cooler climates than Hong Kong, a treatment period longer than seven days may be needed. Students could do the following additional activities as extensions: 

  • compare the amounts of carbon dioxide produced by different types of plant; 
  • write a paper on the role of plants in global climate change; 
  • debate whether society should act now to halt global warming. 

Overall we have found that such practical activities set in a real-life context are of prime importance to chemistry students because they help to structure the learning process, and give purpose to learning chemistry.

Dr Derek Cheung is associate professor in the department of curriculum and instruction at The Chinese University of Hong Kong, Shatin, Hong Kong (e-mail: spcheung@cuhk.edu.hk).

Acknowledgements

I would like to thank the Quality Education Fund for financial support of this project (grant code 2003/0750). 

Box 1 

The carbon dioxide problem

Plant debris decomposes in the soil, releasing carbon dioxide. This is because bacteria and fungi break down dead leaves by the following chemical reaction: 

Cx (H2 O)y + x O2→ x CO2 + y H2 O + energy 

This reaction is one of the components of the global carbon cycle. Different types of plant material decompose at different rates. Green, leafy matter decomposes easily, but woody, stem debris takes longer. Low temperatures, dry conditions and flooding will also slow down decomposition. Unfortunately, human activities - especially burning of fossil fuels - have led to changes in the natural carbon cycle. The cycle is now out of balance. The amount of carbon dioxide in the atmosphere has increased from 280ppm to 370ppm over the past 140 years, resulting in global warming via the greenhouse effect. The slight warming of the Earth's surface may cause bacteria and fungi to decompose dead leaves more rapidly, releasing even more carbon dioxide into the atmosphere. This investigation will provide you with an opportunity to understand part of the carbon cycle dynamics. 

Your task

Suppose that you work as a chemist in an environmental protection company. Your challenge is to plan and do an investigation to determine the number of grams of carbon dioxide released by one gram of dead leaves per day. Submit the plan as group work by __(enter date)__. Your group will be presenting on __(enter date)___ in front of the company representatives. You will have 10 minutes to present your plan, followed by 10 minutes in which you will be expected to respond to queries.  

Your presentation needs to answer the following questions: 

  • how will you measure the rate of formation of CO2 for a particular type of leaf (eg, oak)? 
  • what variables will you need to keep constant in this investigation?
  • will the proposed procedure be feasible and safe?

After reviewing your experimental design, the teacher will discuss any safety precautions that are specific to your design. Obtain teacher approval before beginning any lab work.  

Submit a lab report in writing by __(enter date)__. Make sure you include the following information in your report: 

  • the purpose of your investigation; 
  • the actual method used for investigation; 
  • data and results; 
  • conclusion.

References

  1. V. L. Lechtanski, Inquiry-based experiments in chemistry. Washington, DC: ACS, 2000. 
  2. P. Buell and J. Girard, Chemistry fundamentals: an environmental perspective. Sudbury, MA: Jones and Bartlett, 2003. 
  3. J. W. Raich, Forest Ecol. and Manage., 1998, 107, 309. 
  4. D. A. Skoog, D. M. West, F. J. Holler and S. R. Crouch, Analytical chemistry: an introduction. Fort Worth: Saunders College, 2000. 
  5. D. J. Wink, S. F. Gislason and J. E. Kuehn, Working with chemistry: a laboratory inquiry program. New York: W. H. Freeman, 2005. 

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