Year 9 Science · Unit 2 · Lesson 20
Challenge Worksheet
Learning Goals
1. Write the three parts of an if–then–because hypothesis. Explain what the “because” part adds that a simple prediction does not.
2. Use the general formulas CₙH₂ₙ₊₂ and CₙH₂ₙ to write the formulas of octane (an alkane with 8 carbon atoms) and propene (an alkene with 3 carbon atoms). Which of the two can act as a monomer, and why?
3. Identify four errors in the answer. Correct each one using chemistry from a different part of the unit.
Find the mistakes: a student’s answer
“The solution to plastic pollution is simple: replace all plastics with biodegradable materials. Once we do this, microplastics will disappear from the ocean, because the new materials will break down. Metals and ceramics are better than plastics for every use, because they are harder and last longer. The energy transition just means replacing fossil fuels with solar power, and then we will not need crude oil at all.”
| Mistake | What the student wrote | Correct chemistry, and the unit topic it comes from |
|---|---|---|
| A | ||
| B | ||
| C | ||
| D |
4. PET is made when two different monomers join, and a water molecule is given off at every link. A revision sheet says: “The monomers join by addition polymerisation to make PET.” Name the error in the polymerisation type and correct it. Then name the unit idea that explains each separate example in the table.
| Example | Unit idea that explains it |
|---|---|
| Crude oil is separated into fractions before cracking | |
| A littered bottle breaks into flakes on a sunny beach | |
| The flakes end up in the tissue of fish |
Depth study pilot: cross-linking in slime
A student made slime by adding borax solution to PVA glue. Borax forms cross-links between the PVA chains. She changed the concentration of the borax solution and measured how far a 10 g blob of slime stretched before it broke. She did three trials at each concentration.
| Borax (%) | Trials 1, 2 and 3 | Average |
|---|---|---|
| 1 | 48, 52, 50 | 50 |
| 2 | 35, 37, 21 | |
| 3 | 24, 22, 23 | 23 |
| 4 | 15, 14, 16 | 15 |
Source: sample class results.
5. One result at 2% is an outlier. Identify it. Calculate the average at 2% with the outlier and again without it, and write the better value in Table 1.
With outlier cm
Without outlier cm
6. Show the results in Table 1 as a line graph, with borax concentration on the horizontal axis.
7. Describe the trend and explain it using cross-links. Do the results support the idea that more cross-links make a polymer less stretchy? Give one reason to be cautious.
8. Another student writes: “If the PVA concentration increases, then the film will hold a bigger load, because a higher concentration makes the polymer chains longer.” Explain what is wrong with the “because” part, and rewrite it correctly.
9. Design a depth study on a material from this unit. Give your question, a hypothesis in if–then–because form, the variables, a short method with repeats, and how you will process the results. Explain how your study links at least two parts of the unit.