6.3 Density
If you take equal volumes of different substances and weigh them on a balance, you will find that each of them has a different mass. One centimetre cube of wood may weigh only 0.7 g, but one centimetre cube of iron will weigh 8.0 g. Why is this so?
All substances are composed of molecules. The molecules of different substances differ in size and mass. The intermolecular spacing is also different. The mass of equal volumes of various substances is the mass of the total number of molecules present in that volume.
lightbulbDensity of a substance is defined as its mass per unit volume.
Density = Mass / Volume
ρ = m / V ......... (6.2)
The SI unit of density is kg m⁻³. Another unit also in use is g cm⁻³.
| Substance | Density (kg m⁻³) |
|---|---|
| Air | 1.3 |
| Petrol | 800 |
| Water | 1000 |
| Concrete | 2400 |
| Aluminum | 2700 |
| Steel | 7800 |
| Lead | 11400 |
| Gold | 19300 |
| Osmium | 22600 |
Architects and engineers take special care of the density of building materials to be used in designing and constructing roads, bridges, and buildings. The density of building material is essential for estimating the strength required in foundations and supporting pillars.
Example 6.1
The length, breadth, and thickness of an iron block are 3 cm, 2 cm, and 2 cm respectively. Calculate the density of iron if the mass of the block is 94 g.
Solution:
Length = 3 cm, Breadth = 2 cm, Thickness = 2 cm Mass = 94 g Density = ?
Volume = Length × Breadth × Thickness = 3 cm × 2 cm × 2 cm = 12 cm³
Density = Mass / Volume = 94 g / 12 cm³ = 7.8 g cm⁻³
Thus, density of iron = 7800 kg m⁻³.