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.
A) What type of gas is it?
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.
B) The gas is heavy
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.