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Carbon dioxide (or CO2) is very important for life on Earth: it is found all around us in the air we breathe, it is one of the most important greenhouse gases, and it is fundamental in photosynthesis. Through a practical demonstration, this activity introduces students to CO2 showing them that it is heavier than air, it can be used to extinguish flames and that it can sometimes have fizzy reactions!
Students conduct and interpret an experiment (individually or in small groups) to learn about carbon dioxide (CO2).
Carbon dioxide
Carbon dioxide (CO 2 ) is not only one of the most important greenhouse gases, it is found all around us: in the air (0.0388 vol%) we breathe; in the air we exhale (4 vol%). It is also found in fizzy drinks; in cakes, which rise thanks to the CO 2 produced by baking powder; and when organic compounds such as paraffin, paper, wood or petroleum are burned. Geysers can be driven by carbon dioxide, as well as steam. In liquid form, it is used in fire extinguishers and as a refrigerant in the food industry (for example to store and transport ice cream).
Image: Close-up to one of the geysers in "El Tatio" in San Pedro de Atacama, Chile. Credit: AndresGottlieb
In high concentrations, CO 2 can become dangerous for humans and other animals, but it is also a source of life: during photosynthesis, plants use CO 2 and light to produce sugar, starch, fats, and proteins, as well as the oxygen we need to survive. *A volume per cent or vol% is the number of cubic centimetres of a substance (usually oxygen or carbon dioxide) contained in 100 ml of another substance (for example blood).
Note: The amounts of carbon dioxide produced in these activities are not high enough to be dangerous.
CO 2 properties
Carbon dioxide is colourless. At low concentrations, the gas is odourless. At higher concentrations it has a sharp, acidic odour. At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m 3 ; this is about 1.67 times more than air. CO 2 is present in very small quantities in the air, which makes it hard to keep as one layer. Gases don’t stay still but move around in the air. CO 2 then doesn’t remain on the lower parts of the atmosphere even though it is heavier than air.
Detection of CO 2 with lime water
The lime water test to detect CO 2 was developed by chemist Joseph Black (1728–1799). Lime water is a solution of calcium hydroxide (slaked lime). When carbon dioxide is bubbled through it, a solid precipitate of calcium carbonate is formed. Calcium carbonate is chalk or limestone, and it is this that makes the lime water cloudy.
calcium hydroxide + carbon dioxide -> calcium carbonate + water. Ca(OH) 2 (aq) + CO 2 (g) CaCO 3 (s) + H 2 O(l)
The first six steps are common to both activities A and B– then you have two options as to how to proceed.
Blow up a balloon and let the air out again to make the rubber more elastic.
Use the funnel to fill the balloon with a packet of baking powder (20 g) or five crushed effervescent tablets.
Pour 2-3 cm of water into the bottle.
Cover the bottleneck with the balloon and tip the baking soda / effervescent tablets into the bottle. You may need to hold the balloon onto the bottleneck to prevent it from slipping off.
Shake the bottle lightly. The balloon fills with a gas that is produced in the effervescence.
When the balloon has stopped inflating, twist it shut so that no gas can escape and pull it off the bottle.
Ask students to describe what is happening. Ask them to share their hypotheses about what caused the balloon to fill. Discuss the hypotheses before going to part A.
Put some lime water into a beaker.
Place a drinking straw into the mouth of the balloon, and slowly and carefully release the gas from the balloon into the lime water. The lime water will become cloudy.
Repeat step 7 with clear water.
Ask students to draw the two different beakers and illustrate their observations. Why is the water clear and the lime water cloudy? What could cause the clouds in the lime water?
Discuss the answers with the students. What conclusion can we make? How does this relate to the hypothesis discussed in the first part of the activity?
Image credit: Chemol
Safety note: if you get lime water into your eyes, rinse them immediately with water. See also the general safety note.
The gas in the balloon is CO 2 . Where did our CO 2 come from? Both baking powder and effervescent tablets contain sodium bicarbonate (NaHCO 3 ) and a solid acid (such as citric acid crystals or monocalcium phosphate). In contact with water, sodium bicarbonate and the acid react with one another, ultimately forming water and CO 2 . This gas is what forms the bubbles when a fizzy tablet dissolves. In our daily lives, it is also what makes cakes rise.
Hold the mouth of the balloon into a beaker and let the gas flow out. You cannot see anything, but we will see whether anything has happened. Put the beaker to one side.
Light a tea light candle and use a pair of tongs to place it in a second, empty, beaker (alternatively, you could stick a wooden skewer into the wax and use that to lift the tea light candle into the beaker). It should continue to burn.
Now place the tea light in the first beaker, which contains the gas from the balloon. The candle should stop burning because the gas (CO 2 ) will choke the flame. Ask students to draw the experiment and note their observations. Discuss their notes. What caused the flame to extinguish in the first beaker?
Repeat steps 1-7 to collect more CO 2 in a beaker. Now pour the invisible contents of this beaker into yet another empty beaker. Place a burning tea light into this beaker. What happens? Again, the flame is extinguished, showing that we were able to pour the gas from one beaker to another, as though it were a liquid.
Ask students to draw the experiment and note their observations. Discuss the results with the students. What conclusions can we draw from this experiment?
This experiment demonstrates that CO 2 is heavier than air. Ask students what would have happened if CO 2 were lighter than air.
Ask students if they can think of daily applications of this property. An example is the use of CO 2 in some fire extinguishers. Such extinguishers expulse CO 2 on fire to contain the flames and extinguish the fire. They work exactly as demonstrated in part B.
UK KS2: Year 5 Science - Properties and changes of materials: explain that some changes result in the formation of new materials, and that this kind of change is not usually reversible, including changes associated with burning and the action of acid on bicarbonate of soda. UK KS3 Science: Chemistry - Chemical reactions: reactions of acids with alkalis to produce a salt plus water.
The activity works best when it is done with small groups of 5 to 6 students. Teachers demonstrate each step throughout the activity and supervise the groups to follow the demonstration.
