Year 8 Science · Unit 3 · Lesson 7
Challenge Worksheet
Learning Goals
Explain it to a Year 5 student
Imagine your younger cousin thinks that if they pour saltwater through a coffee filter, the salt will get caught and they'll have clean fresh water. Use the sentence starters below to explain why this won't work. You must talk about particle size, what dissolving means, and what the right method would be.
Starter 1: "The basic idea of dissolving is that the salt particles…"
Starter 2: "The holes in a filter are much bigger than the dissolved salt particles, so when saltwater passes through…"
Starter 3: "Filtration only works when the solid particles are big enough to be trapped, for example…"
Starter 4: "To actually get the salt out of the water, you would need to use a different technique called… because…"
1. A student claims: "Dissolving salt in water is a chemical change because the salt disappears and can't be seen anymore." Write a scientific rebuttal that uses at least two pieces of evidence or reasoning from this lesson to show the student is wrong.
2. A mixture contains three substances: sand (insoluble), salt (dissolved), and a food dye (dissolved, coloured). Design a separation sequence with three steps to isolate all three substances separately. Name the technique for each step and explain what it achieves.
Wrap Up
In one sentence, what was the main idea of this lesson?