Year 7 Science · Unit 2 · Lesson 10
Challenge Worksheet
Learning Goals
Find the mistake
A student wrote this answer
"Dissolving salt in water is definitely a chemical change. I know this because when you add salt to water you can no longer see the salt — it completely disappears. Since the original substance is gone and you cannot see it anymore, a new substance must have been made. This is the same reason burning wood is a chemical change, so dissolving must also be chemical."
1. Identify the key mistake(s) in the student's reasoning.
2. Write a corrected version of the student's argument that uses the correct test for chemical change.
3. Explain why the student's comparison with burning wood is not valid. What is actually different about burning compared to dissolving at the particle level?
1. Breaking glass is irreversible — you cannot easily put a broken window back together. Yet it is classified as a physical change. Explain why, using the idea of "new substance" and what happens at the particle level.
2. A classmate argues: "Rusting must be a physical change because the iron is still there — iron oxide still contains iron." Evaluate this argument. Is it correct, partly correct, or wrong? Justify your answer with scientific reasoning.
Wrap Up
In one sentence, what was the main idea of this lesson?