Year 7 Science · Unit 2 · Lesson 8
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Learning Goals
Odd one out
Each row gives a rule. Use the rule to circle the odd one out, then give your reason.
| # | Rule and group | Odd one out and reason |
|---|---|---|
| 1 | Which is not an alloy? steel · bronze · copper · brass | |
| 2 | Which is a pure substance? air · distilled water · seawater · tap water | |
| 3 | Which is not one of the gases in air? nitrogen · argon · sodium chloride · oxygen |
Compare two
Complete the table to compare pure copper with brass. One box is done for you.
| Feature | Pure copper | Brass |
|---|---|---|
| What it is made of | Copper atoms only | |
| Pure substance or mixture? | ||
| Which is harder? | ||
| One use |
Cause and effect: from iron to steel
Fill the two empty boxes. Each box is caused by the one before it, and row 2 carries on from the end of row 1.
1. A student says: “Brass must be a compound, because it is made by melting copper and zinc together.” Do you agree? Explain, using the correct scientific terms.
Read the data
Scenario: a stuffy classroom
A class of 28 students keeps its doors and windows shut for a double lesson. A sensor measures the percentage of carbon dioxide in the air of the room. At 10:35 the teacher opens the windows.
| Time | 9:00 | 9:30 | 10:00 | 10:30 | 10:50 |
|---|---|---|---|---|---|
| Carbon dioxide (%) | 0.04 | 0.09 | 0.14 | 0.19 | 0.06 |
Source: made-up sensor readings for this worksheet.
2. By how much did the percentage of carbon dioxide rise between 9:00 and 10:30? Give a reason for the rise.
3. Use Table 1 to explain why air must be a mixture and cannot be a compound.
4. An engineer is designing a coin that will be handled millions of times. Should it be made of pure copper or of bronze (copper and tin)? Justify your choice using at least two properties, and explain with atoms why the alloy has those properties.
Odd one out
1. Copper: it is a pure metal (one element); the others are alloys. 2. Distilled water: it is only H₂O; the others have other substances mixed in. 3. Sodium chloride: it is a solid salt dissolved in seawater, not a gas in air. [3 marks: 1 per row, with reason]
Compare two
Made of: brass is copper and zinc. Pure or mixture: copper is a pure substance (an element); brass is a mixture (an alloy). Harder: brass is harder; copper is softer. Use: copper for electrical wires; brass for trumpets or door handles. [4 marks: 1 per row]
Cause and effect: from iron to steel
Box 1: the small carbon atoms sit in the gaps between the layers of iron atoms (1). Box 2: the layers of iron atoms snag on the carbon atoms instead of sliding over each other (1). [2 marks]
Question 1
Disagree (1). Melting the metals together mixes their atoms but does not chemically join them, so brass is a mixture (an alloy) (1). It has no fixed formula: the amount of zinc can change (1). [3 marks]
Read the data
2. 0.19 − 0.04 = 0.15 percentage points (1). The students breathe out carbon dioxide into the closed room (1). [2 marks]
3. The carbon dioxide changed (0.04 to 0.19, then 0.06), but it was still air (1). A compound has a fixed formula, so its make-up cannot change (1). A mixture’s parts are only physically combined, so their amounts can vary (1). [3 marks]
Question 4
Marking guide: 1 choose bronze; 1 bronze is harder than copper, so the coin will not dent or wear smooth; 1 bronze resists corrosion better than copper, so the coin keeps its surface and details; 1 in pure copper all the atoms are the same size in tidy layers that slide easily; 1 the tin atoms are a different size and sit among the copper atoms; 1 so the layers snag instead of sliding, which makes the alloy harder. [6 marks]
Model answer: Bronze. It is harder than pure copper, so the coin will not dent or wear smooth, and it resists corrosion better, so its details stay clear. In pure copper the atoms are all one size, so the layers slide easily; in bronze, tin atoms of a different size sit among them, the layers snag, and the metal resists denting.
Common error: calling an alloy a compound because it looks like one substance.