Density
How much mass fits in each cm³
1. General properties: mass, radius and metallic property
2. Same size, different feel
You pick up two balls of exactly the same size. One feels much heavier.
Pick the right answer
Predict: what is different about the heavier ball?
- A. More mass is packed into the same volume
- B. It takes up more space
- C. Gravity pulls only on that ball
الشرح
The two balls have the same size, so the same volume, and gravity pulls on both. The heavier one has more mass packed into the same volume: it has a higher density.
3. Mass per unit volume
Density
The mass of a unit volume of a substance.
density = massvolume
Example: an aluminum block has mass 27 g and volume 10 cm³. Its density is 27 ÷ 10 = 2.7 g/cm³.
4. Pack in more mass
Keep the volume at 10 cm³ and raise the mass. Then try changing the volume.
mass 30 g
volume 10 cm³
density 3 g/cm³
5. Your turn
Fill in the blanks
A 5 cm³ piece of metal has mass 39.5 g. Its density is _____ g/cm³.
الشرح
density = mass ÷ volume = 39.5 ÷ 5 = 7.9 g/cm³.
Pick the right answer
Box A and box B are the same size. Each holds 100 marbles of the same size, but B's marbles are heavier. Which box has the greater density?
- A. B
- B. A
- C. Equal: same number of marbles
الشرح
Density = mass ÷ volume. The boxes have the same volume and the same number of marbles, but B's marbles are heavier, so B has more mass in the same volume and the greater density.
6. Summary
| Sample | Mass | Volume | Density = mass ÷ volume |
|---|---|---|---|
| aluminum block (step 2) | 27 g | 10 cm³ | 2.7 g/cm³ |
| metal piece (step 4) | 39.5 g | 5 cm³ | 7.9 g/cm³ |
- Density is the mass of a unit volume: density = mass ÷ volume, in g/cm³.
- Same volume, more mass → higher density.
- Next in Chapter 1: why the density of transition metals rises from Sc to Cu.
Can you explain why two balls of the same size can have different densities?