Year 7 Science · Unit 2 · Lesson 3
Apply Worksheet
Learning Goals
Fill the gap: measuring density
Choose the correct word or phrase from the word bank. Each is used at most once. Two are not needed: cross them out.
To find density, first measure the of the object on an electronic balance. If the object has flat faces and square corners, measure its length, width and height with a and multiply them to get its volume. A stone has an irregular shape, so we use : lower it into a that already holds water and see how far the water level rises. Because 1 mL of water takes up exactly 1 , the rise in mL equals the stone’s volume in cm³.
1. An object is weighed in a beaker. Why must the balance be zeroed with the empty beaker on it first?
Order the steps: finding the density of a stone
The stone’s mass has already been measured. Number the steps from 1 (first) to 5 (last).
| Order (1–5) | Step |
|---|---|
| Subtract the starting level from the new level to get the stone’s volume. | |
| Divide the stone’s mass by its volume. | |
| Tilt the cylinder and gently slide the stone in, so that no water splashes out. | |
| Pour water into a measuring cylinder and read the starting level at eye level. | |
| Read the new water level at eye level. |
Read the data: six materials
Data
| Material | Density (g/cm³) | Conducts heat well? |
|---|---|---|
| Copper | 8.9 | Yes |
| Aluminium | 2.70 | Yes |
| Iron | 7.87 | Yes |
| Plastic | 1.2 | No |
| Wood | 0.5 | No |
| Cork | 0.24 | No |
For comparison, water has a density of 1.00 g/cm³. Source: values from the lesson.
2. Which of the six materials in Table 1 would float in water? Explain how you know.
Read the data: the mystery lump
Scenario
A scientist is given a small, silvery-grey lump of metal with an irregular shape. Its mass is 157.4 g. When she lowers it into a measuring cylinder, the water level rises from 35.0 mL to 55.0 mL. The lump does not dissolve in water.
| Substance | Density (g/cm³) | Melting point (°C) | Soluble in water? |
|---|---|---|---|
| Aluminium | 2.70 | 660 | No |
| Iron | 7.87 | 1538 | No |
| Gold | 19.30 | 1064 | No |
| Table salt | 2.16 | 801 | Yes |
| Sugar | 1.59 | 186 (breaks down) | Yes |
Source: values from the lesson.
3. Calculate the volume and then the density of the lump.
Density g/cm³
4. Which substance in Table 2 is the lump most likely to be? Use two pieces of evidence.
5. Can the scientist be certain of her answer from these measurements alone? Name one more property from Table 2 she could measure, and the result that would support her answer.
6. A company is designing a light saucepan for bushwalkers to carry. Choose one material from Table 1 for the pan and one for the handle. Justify each choice using conductivity and density, and explain why you rejected one other material.
Fill the gap: measuring density
mass; ruler; displacement; measuring cylinder; cm³. Not used: thermometer, weight.
Question 1
So the balance reads only the object’s mass, not the beaker’s as well (1). [1 mark]
Order the steps: finding the density of a stone
Reading down the table as printed, the correct step numbers are 4, 5, 2, 1, 3. In order: pour in water and read the starting level; slide the stone in gently; read the new level; subtract to get the volume; divide the mass by the volume.
Read the data: six materials
2. Wood and cork (1). Their densities (0.5 and 0.24 g/cm³) are less than water’s 1.00 g/cm³ (1). [2 marks]
Read the data: the mystery lump
3. V = 55.0 − 35.0 = 20.0 mL = 20.0 cm³ (1); d = 157.4 ÷ 20.0 (1) = 7.87 g/cm³ (1). [3 marks]
4. Iron (1). Its density matches iron’s 7.87 g/cm³ and is far from aluminium and gold; it does not dissolve, which rules out salt and sugar (1). [2 marks]
5. No: another substance not in the table could have a similar density, and measurements have small errors (1). Measure its melting point: about 1538 °C would support iron (1). [2 marks]
Question 6
Marking guide: 1 pan material that conducts heat (aluminium best; copper or iron accepted); 1 conducting explained (it carries heat from the flame to the food); 1 density used: aluminium (2.70) is far less dense than copper (8.9) or iron (7.87), so a pan of the same size is lighter; 1 handle of wood, plastic or cork, because it is an insulator and will not burn the hand; 1 a rejected material with a reason (copper conducts well but is over three times denser, so heavy; wood would not conduct heat to the food); 1 at least two correct values quoted. [6 marks]
Model answer: The pan should be aluminium. It conducts heat from the flame to the food, and at 2.70 g/cm³ it is much lighter than a copper pan (8.9 g/cm³) of the same size. The handle should be wood: it is an insulator, so it will not burn your hand. I rejected copper because, although it conducts heat very well, it is more than three times denser than aluminium (8.9 ÷ 2.70 = 3.3).
Common error: reading the top water level as the volume.