Year 7 Science · Unit 2 · Lesson 3
Challenge Worksheet
Learning Goals
1. Graphite, the “lead” in a pencil, is called an exception. (a) Which rule about conductors does it break? (b) Name one use that depends on this.
Find the error
Leo’s working
A stone has a mass of 52 g. The water level in a measuring cylinder was 40 mL before the stone went in and 60 mL after.
Line 1: Volume of the stone = 60 cm³.
Line 2: Density = volume ÷ mass = 60 ÷ 52 = 1.15 g/cm³.
Line 3: The density is more than 1.0 g/cm³, so the stone floats.
2. Each of Leo’s three lines contains one error. Rewrite each line correctly, with the right numbers.
Plan an investigation: does size change density?
Plan a fair test to find out whether a small, a medium and a large piece of the same plasticine have the same density. You have plasticine, an electronic balance, a measuring cylinder and water.
Hypothesis
Variables
Method
Results table
| Piece | Mass (g) | Volume (cm³) | Density (g/cm³) |
|---|---|---|---|
| Small | |||
| Medium | |||
| Large |
What results would support your hypothesis?
Read the data: heavy or dense?
Claim: “Heavy things sink and light things float.”
To test the claim, a class measured four objects and then put each one in a tub of water.
| Object | Mass (g) | Volume (cm³) | Density (g/cm³) | In water |
|---|---|---|---|---|
| A. Pine log | 30,000 | 60,000 | Floats | |
| B. Iron nut | 23.6 | 3.0 | Sinks | |
| C. Candle | 180 | 200 | Floats | |
| D. Glass marble | 5.0 | 2.0 | Sinks |
Source: sample class results.
3. Calculate the density of each object and write it in Table 1. Show one calculation here.
Example g/cm³
4. Use Table 1 to evaluate the claim. Suggest a better rule.
5. In 1912 the Titanic, a huge steel ship, hit an iceberg and sank, while the iceberg kept floating. Use density to explain (a) why the steel ship floated at first, (b) why the iceberg floats, and (c) why the ship sank once seawater flooded into it.
Question 1
(a) Most non-metals are insulators, but graphite is a non-metal that conducts electricity (1). (b) Batteries (1). [2 marks]
Find the error
2. Line 1: volume = 60 − 40 = 20 cm³ (1). Line 2: density = mass ÷ volume = 52 ÷ 20 = 2.6 g/cm³ (1). Line 3: the density is more than 1.0 g/cm³, so the stone sinks (1). [3 marks]
Plan an investigation: does size change density?
Hypothesis: a bigger piece will have the same density, because mass and volume increase together (1). Change: the size of the piece; measure: its density, from mass and volume; keep the same: type of plasticine, measuring cylinder, reading at eye level (1 if the change, measure and at least two controlled variables are all correct). Method: find each piece’s mass on a zeroed balance; find its volume by displacement; divide mass by volume (2: 1 for mass and displacement, 1 for dividing). Supporting results: all three densities are the same, or very close (1). [5 marks]
Read the data: heavy or dense?
3. A 0.5; B 7.9 (7.87); C 0.9; D 2.5 g/cm³. Two or three correct = 1; all four = 2. [2 marks]
4. Wrong: the log is the heaviest (30,000 g) but floats (1); the marble (5.0 g) and nut (23.6 g) are light but sink (1). Better rule: an object floats if its density is less than water’s 1.0 g/cm³, whatever its mass (1). [3 marks]
Question 5
Marking guide: 1 steel is denser than water (about 7.9 vs 1.0 g/cm³), so a lump sinks; 1 the hull was mostly air, so the ship’s average density was less than water’s; 1 ice (0.92 g/cm³) is less dense than water, so the iceberg floats; 1 ice is less dense because its particles form a crystal with large gaps; 1 flooding replaced air with water, so mass rose, volume stayed the same and average density rose above water’s; 1 at least two correct density values used. [6 marks]
Model answer: Steel (about 7.9 g/cm³) is denser than water (1.0 g/cm³), but the Titanic’s hull was mostly air, so its average density was less than water’s and it floated. Ice (0.92 g/cm³) is less dense than water because its particles form a crystal with large gaps, so the iceberg floats. Flooding replaced air with water, so the ship’s mass rose while its volume stayed the same; its average density rose above water’s and it sank.
Common error: dividing volume by mass.