Year 7 Science · Unit 2 · Lesson 13
Challenge Worksheet
Learning Goals
Find the mistakes
A student wrote this Unit 2 summary
"In Unit 2, I learned that all matter is made of particles. In a solid, the particles are far apart and move around freely, which is why a solid has a fixed shape. Elements and mixtures are both pure substances, but mixtures can be separated by physical means. When salt dissolves in water it is destroyed, so you can never get it back. To separate sand from water you would use evaporation, because sand is soluble."
(a) Find all four errors in the student's summary. For each one, identify the topic it relates to and write the correct version.
(b) Choose two of the errors you found. For each one, explain why this mistake is easy for a Year 7 student to make, what is the common confusion behind it?
1. A substance has a fixed boiling point of 100 °C, is made of molecules containing two hydrogen atoms bonded to one oxygen atom, and cannot be separated into simpler substances by physical means. Classify this substance fully, is it an element, compound, or mixture? Explain using at least three Unit 2 concepts.
2. A student dissolves blue copper sulfate crystals in water to make a blue solution, then heats it gently until all the water evaporates and blue crystals remain. Using Unit 2 vocabulary (solute, solvent, solution, evaporation), explain at the particle level what happens at each stage, and explain why this shows that dissolving is a physical change that can be reversed.
Wrap Up
What surprised you most in Unit 2? Why?