C
Students will explore what happens to the level of carbon dioxide in their breath following exercise. They will use a respirometer with bromothymol blue to measure how much carbon dioxide is in their breath at rest and after doing 2 minutes of exercise.
● 1 respirometer pack (1 plastic 2-litre bottle, 6 bungs, 6 pieces of plastic tubing, 6 pieces of flexible tubing, oneway valve)
● bromothymol blue indicator solution, prepared as per the Technician notes on page 107
● 0.1 M sodium hydroxide solution
● 6 pipettes
● indicator charts
● stopclock
● 5 empty, washed, plastic 2-litre colatype drink bottles
● 2 test tubes
● test tube rack
● 500 cm 3 measuring cylinder
● safety goggles optional:
● skipping rope
● video clip of Zumba
®
or CD of fast dance music
● gym bench/step stool/sturdy box/stair of height 0.25
–0.3 m
● If anyone begins the activity and starts to feel unwell they should stop immediately.
● Any student (such as those with heart/lung problems) not able to take part in school PE/ games lessons may need to be excused from taking part in the physical part of this activity but can take on a time-keeper or data recording role.
● Identify students with asthma so they can have their inhaler close at hand and use it if required.
● Teachers who have not used a respirometer before will need training by an experienced colleague and should familiarise themselves with how it works.
● 0.1 M sodium hydroxide solution is a skin irritant and is extremely hazardous to the eyes.
Technicians should add an irritant label to the sodium hydroxide, before it is given to students to use. See the chemical safety sheets available on the In the Zone website.
● Bromothymol blue is low hazard but skin contact should be avoided. See the chemical safety sheets available on the In the Zone website.
● Make sure it is clear that students only breathe out into the apparatus and not breathe in via the tube. The non-return valve must be fitted to ensure that the students cannot breathe in the solution by accident.
● Students should wear safety goggles at all times.
● Ensure students carry out the activities in a suitable place, clear of any obstruction.
● Healthy competition is encouraged but be aware of and discourage excessive competition between students as it can lead to over-exertion and possible fainting or injury.
Under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 UK:England And Wales License you can copy, share and adapt the materials as much as you like, as long as it is not for commercial use. Where material is owned by a third party, certain restrictions may apply that you have to comply with. In particular, where a copyright line is included on a photograph you must not modify, adapt or remove that photo from its context.
Ages
16
–19
88

C
1 Assemble the respirometefr as shown in the diagram below. Practise using the respirometer so that you are familiar with it and can help students use it if necessary.
2 The flexible plastic tubing should be disinfected or sterilised before and after each use.
3 Carbon dioxide is acidic and will gradually turn the bromothymol blue solution from blue to green. If a known amount in moles of sodium hydroxide is added before any carbon dioxide then you can get an estimation of how much carbon dioxide is in one breath (from calculation
2 on the next page), because two moles of sodium hydroxide are required to neutralise one mole of carbon dioxide.
CO
2
+ 2NaOH → Na
2
CO
3
+ H
2
O
4 Have two test tubes containing bromothymol blue solution for students to use for colour comparisons. If you then add a drop of a weak acid (such as lemon juice or vinegar) to one tube, they can see the colour change from blue to green at pH 7 and to yellowy-green at pH 6. They will be looking for the green colour to show when the carbon dioxide in their breath has neutralised the sodium hydroxide added to the indicator solution. Students can also refer to the indicator charts supplied in the kit box. The charts are for universal indicator but both indicators change to green at pH 7.
5 You will need enough bromothymol blue solution to refill the respirometers three times (at 500 cm 3 each time) per group. See the Technician notes on page 107.
6 Students will use pipettes to add 5 cm 3 of 0.1 M sodium hydroxide solution to the bromothymol blue solution in the respirometer. This will turn it blue. It will take 0.01 g or 5.0 cm 3 of carbon dioxide to neutralise the indicator/sodium hydroxide solution.
Assembling the respirometer.
7 Students should practise breathing in through their nose and out through their mouth.
While they are seated and relaxed, they breathe out into the respirometer containing the
500 cm 3 bromothymol blue solution and 5 cm 3 0.1 M sodium hydroxide solution. They should use normal breaths and should not increase the volume of air they breathe out at each breath.
8 Students will be measuring the carbon dioxide given out before exercise, after lowintensity aerobic exercise such as steps, and after more high-intensity anaerobic exercise such as squats, which must be deep and performed as fast as possible.
9 See the Student sheet on page 91 for the method for obtaining evidence.
It should take more breaths to neutralise the solution at rest than after aerobic exercise (since more CO
2
is produced when exercising aerobically than at rest). The same is true for anaerobic exercise. However, students’ comparisons of aerobic and anaerobic exercise may differ and this could be discussed. The percentage of carbon dioxide in the breath is likely
Ages
16 –19
89

C to increase with increasing exercise intensity, however the volume of expired air (in this case volume in one breath) may also change. Furthermore, for anaerobic exercise there may be a delay in increased carbon dioxide production as lactate is buffered in the blood and then carbon dioxide is produced. Another factor is whether students exercising at high intensity actually reach the anaerobic threshold.
During anaerobic respiration, hydrogen ions are formed from the dissociation of lactic acid. In the blood this is buffered by sodium hydrogen carbonate to give sodium lactate and carbonic acid, which produces carbon dioxide and water:
Lactate
–
H + + NaHCO
3
→ Na lactate + H
2
CO
3
H
2
CO
3
→ H
2
O + CO
2
To calculate how much carbon dioxide is required to neutralise the solution:
Amount of NaOH in solution = 0.1 mol dm –3 × 0.005 dm 3 (i.e. 5 cm 3 of sodium hydroxide solution in respirometer)
= 0.0005 moles
1 mole of CO
2
neutralises 2 moles of NaOH (CO
2
+ 2NaOH → Na
2
CO
3
+ H
2
O)
Therefore: number of moles of CO
2
=
0.0005
2
= 0.00025 moles
1 mole of CO
2
occupies 22.4 litres, so 0.00025 moles occupy 5.6 cm 3 . This is the volume of carbon dioxide required to neutralise the sodium hydroxide added to the respirometer.
So, the volume of carbon dioxide in each breath =
5.6 cm 3 total number of breaths where students count the total number of breaths required to change the colour of the indicator from blue to green (i.e. to neutralise the sodium hydroxide solution). Results would be expected to be similar to those shown below, shown for 5 breaths in a period of 25 seconds. Students calculate these values in Lesson 3: Teacher sheet page 99, Student sheet page 101.
Breaths per minute = total number of breaths (N) time to neutralise solution in s
= 5/25 × 60 = 12
× 60
To neutralise the solution it takes 5.6 cm 3 of carbon dioxide.
Carbon dioxide in one breath=
5.6 cm 3 total number of breaths
=
5.6 cm 3
5
=
1.12 cm 3
Volume of carbon dioxide in 1 minute = breathing rate × volume of carbon dioxide in each breath
= 12 × 1.12 cm 3 = 13.44 cm 3
Ages
16 –19
90