Year 7 Science · Unit 2 · Lesson 4
Challenge Worksheet
Learning Goals
Find the mistake
A student wrote this answer
"Salt water is a pure substance because it looks the same everywhere — if you scoop it from the top or the bottom of the glass, it looks identical. Also, it tastes consistently salty throughout, which proves all the particles are the same. My dad buys 'pure sea salt' at the shops so I know salt is pure. Because it is uniform, salt water must be a pure substance."
1. Identify the mistake(s) in the student's answer. There is more than one error — find them all.
2. Write the correct version: explain what salt water actually is and why "looks the same everywhere" does not make something a pure substance.
3. Explain why this kind of mistake — confusing "looks uniform" with "is pure" — is so easy to make. What everyday experience leads students to this error?
1. A chemist says a substance is pure if and only if it has a fixed, consistent melting point. Use this idea to explain whether seawater from Bondi Beach would pass the "pure" test. Give a specific reason.
2. Draw a simple particle diagram in the box below that shows the difference between distilled water (pure substance) and saltwater (homogeneous mixture). Label each diagram and explain what each diagram shows about the particles.
Distilled water (pure substance)
Saltwater (homogeneous mixture)
Wrap Up
In one sentence, what was the main idea of this lesson?