Global Atmospheric Change
Finding the Carbon in Sugar
Most of the fuels we use come from dead plant or animal matter. The origin of fuel wood, of course, is obvious. However, all fossil fuels also are derived from decomposed organisms that have been buried at high temperatures and pressures for millions of years. The energy in these fuels was captured from the sun during photosynthesis by plants, some bacteria and algae.
When something burns, it combines rapidly with oxygen in a reaction that releases energy. Most of this energy is given off in the forms of light and heat. Other things are given off at the same time. Carbon dioxide, once trapped by green plants during photosynthesis, is formed again and released. Water, also essential for photosynthesis, is released as well. In addition, most fuels produce substances such as smoke and soot, and other gases like methane and carbon monoxide, when they are burned. Some fuels, such as natural gas, burn much more cleanly than others, such as coal. However, all fossil fuels release carbon back into the atmosphere during combustion.
All living things are made out of molecules containing carbon. Plants take in carbon as carbon dioxide from the air. During photosynthesis, plants make energy-rich molecules, such as sugars, that have carbon as a backbone. Plants and all other organisms use these simple molecules to provide energy and raw materials to manufacture other substances necessary for life. We can see the evidence of the carbon in sugar as a black residue that appears when the sugar begins to burn.
The formula for table sugar (sucrose) is: C12H22O11.
Session 1: What happens when something burns?
Have the following materials ready: large beaker or tempered glass bowl, candle, matches and several wet paper towels folded together to make a mat larger than the opening of the beaker or bowl.
Direct students’ attention to the materials you have gathered. Light the candle and ask, What is happening to the candle? After students answer that it is burning, ask, What do you think it means to burn something? Are we seeing a physical change in the candle or a chemical change? Remind students that a chemical change produces substances different from the ones that originally were present. Chemical changes usually give off or take in energy.
Ask students to predict what might happen if the candle is covered with the beaker. After students respond, place the lighted candle on the wet towels and cover it with the container. Fold the edges of the towels around the lip of the container to create a seal.
Have students observe what happens to the candle. The flame will become smaller until it finally extinguishes (this usually takes less than a minute). Ask, What happened to the candle? Did it run out of material to burn? Do you think it ran out of something else? Help students understand that the candle used as much oxygen gas (one of the gases in air) as was possible.
Lift the container slowly and have students observe the other substances present: smoke and condensed water vapor on the sides of the container. Let them examine the candlewick. Ask, What can we see or feel that was produced by the burning candle? (heat, water, smoke, charred wick). What was used by the burning candle? (melted wax and the wick as fuel, oxygen gas from air).
Session 2: Sugar as fuel
Have each Materials Manager collect a candle, a square of aluminum foil, a wet paper towel and one or more copies of the student sheet. Students should clear all papers and place their candles on the wet toweling in the center of their work areas.
Let students create a “testing spoon” by forming the foil into a spoon-like shape with a long handle. The bowl of the spoon should be made of only one layer of foil.
When students have completed their spoons, have one person from each group measure about 1/2 teaspoon of sugar into the spoon.
Have the students in each group predict what will happen when they heat the sugar over a lighted candle. They should record their predictions on their student sheets.
Light the candles (which should be placed on the wet paper towels) for each group. Direct each principal investigator to hold the bowl of the “spoon” over the candle flame. Other group members should observe and record what happens to the sugar. (It will become liquid and turn amber-colored. This is caramel, similar to the topping used for desserts like flan and custard. Finally, the sugar will burn and become blackened.)
Ask, What happened to the sugar? Help students recognize that the sugar underwent a physical change (solid to liquid) and a chemical change (burning of liquid sugar). Also ask, Where did the carbon in the sugar come from? Lead students to understand that the carbon was taken from air as carbon dioxide during photosynthesis. Have students examine the bottom of the spoon. Ask, Where did that carbon come from?
Keywords: carbon cycle | carbon monoxide | CO2 | combustion | Earth | ecology | energy | environment | fossil fuel | fuel | life science | radiation | solar energy | sun | temperature | lesson
- Moreno, N., Tharp, B., and Dresden, J. (2011) The Science of Global Atmospheric Change Teacher’s Guide. Baylor College of Medicine: Houston. ISBN: 978-1-888997-75-0.
- Illustration © Baylor College of Medicine\M.S. Young.
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Funded by the following grant(s)
My Health My World: National Dissemination
Grant Number: 5R25ES009259
The Environment as a Context for Opportunities in Schools
Grant Number: 5R25ES010698, R25ES06932
Foundations for the Future: Capitalizing on Technology to Promote Equity, Access and Quality in Elementary Science Education