Year 8 Science · Unit 1 · Lesson 10
Challenge Worksheet
Learning Goals
Find the errors
This paragraph contains exactly three errors. Underline each one and write the correction below.
A student’s explanation
The respiratory system exchanges gases with the environment. When you breathe in, carbon dioxide enters your body, and when you breathe out, oxygen leaves it. The lungs then pump oxygen out to every cell in the body. Cells release carbon dioxide as a waste, and the blood carries it back to the lungs. The lungs and the blood each do their own job, so trouble in the lungs could never harm the rest of the body.
1. Write the three corrections.
Plan an investigation
Investigation: testing breathed-out air
Limewater is a clear liquid that turns milky when carbon dioxide bubbles through it. The more carbon dioxide in the air, the sooner it turns milky. A class wants to test this question: “Does air breathed out contain more carbon dioxide than the air in the room?” They have two capped test tubes of limewater fitted with delivery tubes, two clean air-sample bags, and a hand pump. A student exhales into one personal sample bag away from the limewater; the detached bag is then connected to the pump so the collected air can be pushed through one test tube. Room air from the second bag is pumped through the other test tube. Wear eye protection, have the teacher handle the limewater, and never blow by mouth through tubing connected to limewater.
Variables
2. One group pumped the exhaled-air bag hard and fast but pumped the room-air bag slowly. Explain why this makes their test unfair.
| Air bubbled through | Trial 1 | Trial 2 | Trial 3 | Mean |
|---|---|---|---|---|
| Air breathed out | 35 | 28 | 32 | |
| Room air (hand pump) | still clear at 300 | still clear at 300 | still clear at 300 | still clear |
Source: made-up results for this worksheet. Both kinds of air went through at the same steady rate; each test ended at 300 s.
3. Complete the Mean column (to one decimal place). Write a conclusion for the investigation. Then explain where the extra carbon dioxide in breathed-out air came from, tracing its path through the body and naming both systems.
4. Sam has anaemia: his blood has fewer red blood cells than normal, so each litre of his blood can carry less oxygen. His lungs are healthy. In a cross-country race he tires quickly and feels short of breath. His friend says: “Your lungs are fine, so your oxygen supply must be fine.” Evaluate the friend’s claim. Explain Sam’s symptoms using system interaction, and predict how his heart rate in the race would compare with a healthy runner’s, with a reason.
Find the errors
1. (i) The gases are swapped: oxygen enters when you breathe in and carbon dioxide leaves when you breathe out. (ii) The lungs do not pump; the heart pumps blood, and the circulatory system carries the oxygen to cells. (iii) The systems interact: trouble in the lungs means less oxygen enters the blood, so cells all over the body are affected. [3 marks: 1 per correction]
Plan an investigation
Independent: the type of air (breathed out or room air). Dependent: the time for the limewater to turn milky. Controlled (any two): the volume of limewater; the same size of test tube; the rate of bubbling; the length of time bubbled; the same delivery-tube depth.
2. The rate of bubbling was not controlled. Pumping one bag faster pushes more air, and so more carbon dioxide, through the limewater each second, so the exhaled-air sample would turn milky sooner even if the two air samples had the same carbon dioxide concentration. [2 marks: 1 names the uncontrolled variable, 1 explains its effect]
Go Deeper
3. Mean: (35 + 28 + 32) ÷ 3 = 31.7 s. Conclusion: air breathed out contains more carbon dioxide than room air, because it turned the limewater milky in about 32 s while room air had not turned the limewater milky by 300 s. Path: cells release carbon dioxide as a waste; the circulatory system carries it in the blood to the lungs; the respiratory system passes it from the blood into the air in the lungs, and it is breathed out. [5 marks: 1 mean, 1 conclusion with data, 1 cells make it, 1 blood carries it, 1 lungs remove it]
Show What You Know
4. Marking guide: 1 respiratory system still works (air reaches the lungs and oxygen enters the blood normally); 1 circulatory problem (fewer red blood cells, so each litre of blood carries less oxygen to the muscles); 1 symptoms linked (muscle cells get too little oxygen, so he tires; he feels breathless because his body needs more oxygen than his blood can deliver); 1 prediction with reason (his heart rate would be higher than a healthy runner’s, because the heart pumps faster to move more blood each minute to make up for the lower oxygen in each litre); 2 justified judgement (the claim is wrong: oxygen supply to cells needs gas exchange and transport working together, so a problem in either system reduces it; 1 if no reasoning). [6 marks]
Evidence limit: 300 s was the stopping time, not a measured time for room air to turn the limewater milky. These results support a higher carbon dioxide concentration in breathed-out air under the controlled conditions, but do not establish an exact concentration ratio. A calibrated carbon dioxide sensor would be needed for a quantitative comparison.
Common error to watch for: saying Sam’s lungs “must be damaged” because he is short of breath. His lungs are healthy; the problem is how much oxygen his blood can carry after gas exchange.