Year 8 Science · Unit 1 · Lesson 10

The Respiratory System and Gas Exchange

Challenge Worksheet

Name

Date

Class

Learning Goals

Design a fair test to demonstrate that exhaled air contains more CO₂ than inhaled air.
Identify independent, dependent, and controlled variables in an investigation.
Evaluate the validity and limitations of a limewater-based experimental method.

Go Deeper

Design a mini-experiment

A scientist wants to test: "Does exhaled air contain more carbon dioxide than inhaled (atmospheric) air?" The scientist plans to bubble each type of air through limewater, a clear solution that turns milky white when CO₂ is present. Plan the full investigation below.

What I will change (independent variable)
What I will keep the same (controlled variables, list at least 3)
What I will measure or observe (dependent variable)
My prediction (use the word "because")
How would I know if my prediction is wrong?
One limitation of using limewater as a test

Show What You Know

1. A student sets up the limewater experiment but blows air through both tubes at different speeds. Explain why this would make the investigation unfair and what variable the student has failed to control.

Challenge 3 marks

2. The limewater test can show that CO₂ is present but cannot tell you exactly how much CO₂ is present. Suggest one improvement to the experimental method that would allow the scientist to measure the amount of CO₂ rather than just its presence.

Challenge 3 marks

Wrap Up

In one sentence, what was the main idea of this lesson?