Year 9 Science · Unit 2 · Lesson 1
Challenge Worksheet
Learning Goals
1. Magnesium melts at 650 °C. Magnesium also burns in air with a bright white flame, leaving a white powder. Explain why the first is a physical property and the second is a chemical property.
Data: carbon in steel
Steel is iron with a little carbon added. Hardness (Brinell, HB) is found by pressing a hard ball into the surface: higher means harder. Elongation is how far a bar stretches before it breaks: higher means more ductile (easier to bend and shape).
| Carbon (% by mass) | 0.10 | 0.20 | 0.40 | 0.60 | 0.80 |
|---|---|---|---|---|---|
| Hardness (HB) | 95 | 111 | 149 | 201 | 229 |
| Elongation before breaking (%) | 28 | 25 | 18 | 12 | 10 |
Source: SAE J1397 estimated properties of hot-rolled carbon steel bars (grades 1010, 1020, 1040, 1060, 1080).
2. From 0.10% to 0.80% carbon, by how many HB does the hardness rise? By what factor does the elongation fall?
Hardness rises by HB
Elongation falls by a factor of
3. Use Table 1 to choose a steel for (a) a car body panel that must be pressed into a curved shape and (b) a chisel that must keep a sharp edge. Justify each choice with data, and name the trade-off you accept.
4. Read the student’s report below. Identify three errors in it and correct each one.
A student’s report
“Chef’s knife blade. Step 1: it must look attractive. Step 2: I chose gold, because gold does not react with water or food, which is a physical property. Step 3: gold is the densest candidate (19.3 g/cm³), so it will be the hardest, strongest blade.”
| Error in the report | Correction |
|---|---|
5. The lesson states that “no single material is best for everything.” Evaluate this claim, using at least two different materials and a specific application for each. Include evidence from Table 1, and explain what happens if an engineer ignores this principle.