Year 8 Science · Unit 2 · Lesson 3
Challenge Worksheet
Learning Goals
Find the mistake
A student wrote this answer
"Water is a mixture because it is made of hydrogen and oxygen. You can separate it by boiling, the water evaporates and leaves the hydrogen and oxygen behind. This is the same as evaporating salt water to get salt back. Because you can use a physical process to change water, it must be a mixture just like salt water."
1. Identify at least two mistakes in the student's answer. List each one clearly.
2. Rewrite the student's explanation correctly. Your version must explain what water actually is and what happens when it boils.
3. Why do you think students often confuse water (a compound) with salt water (a mixture)? What two ideas are getting mixed up?
1. In the boxes below, describe in words what the particle diagram for each substance would look like. Then write a one-sentence justification explaining how the diagram shows which category it belongs to.
| Substance | Describe the particle diagram (what shapes, how arranged, bonded or not?) | Justify the classification |
|---|---|---|
| Iron (Fe) Element |
||
| Carbon dioxide (CO₂) Compound |
||
| Air Mixture |
2. A scientist discovers a new substance. Its particle diagram shows groups of three circles: always two small green circles bonded to one large purple circle. The ratio never changes. (a) Classify this substance and justify your answer. (b) Would it be possible to separate the green and purple particles using filtration? Explain why or why not.
Wrap Up
In one sentence, what is the single most important question to ask when classifying a substance from its particle diagram?