Paul Peter Urone; Roger Hinrichs

Which weighs more, a ton of feathers or a ton of bricks? This old riddle plays with the distinction between mass and density. A ton is a ton, of course; but bricks have much greater density than feathers, and so we are tempted to think of them as heavier. (See Figure 11.4.)

Density, as you will see, is an important characteristic of substances. It is crucial, for example, in determining whether an object sinks or floats in a fluid. Density is the mass per unit volume of a substance or object. In equation form, density is defined as

ρ = \frac{m}{V},

11.1

where the Greek letter $ρ$ (rho) is the symbol for density, $m$ is the mass, and $V$ is the volume occupied by the substance.

Density

Density is mass per unit volume.

ρ = \frac{m}{V},

11.2

where $ρ$ is the symbol for density, $m$ is the mass, and $V$ is the volume occupied by the substance.

In the riddle regarding the feathers and bricks, the masses are the same, but the volume occupied by the feathers is much greater, since their density is much lower. The SI unit of density is ${kg/m}^{3}$ , representative values are given in Table 11.1. The metric system was originally devised so that water would have a density of $1 {g/cm}^{3}$ , equivalent to $10^{3} {kg/m}^{3}$ . Thus the basic mass unit, the kilogram, was first devised to be the mass of 1000 mL of water, which has a volume of 1000 cm³.

Substance	$ρ (\times 10^{3} {kg/m}^{3}$ or $g/mL)$	Substance	$ρ (10^{3} {kg/m}^{3}$ or $g/mL)$	Substance	$ρ (10^{3} {kg/m}^{3}$ or $g/mL)$
Solids		Liquids		Gases
Aluminum	2.7	Water (4ºC)	1.000	Air	$1 . 29 \times 10^{- 3}$
Brass	8.44	Blood	1.05	Carbon dioxide	$1 . 98 \times 10^{- 3}$
Copper (average)	8.8	Sea water	1.025	Carbon monoxide	$1 . 25 \times 10^{- 3}$
Gold	19.32	Mercury	13.6	Hydrogen	$0 . 090 \times 10^{- 3}$
Iron or steel	7.8	Ethyl alcohol	0.79	Helium	$0 . 18 \times 10^{- 3}$
Lead	11.3	Petrol	0.68	Methane	$0 . 72 \times 10^{- 3}$
Polystyrene	0.10	Glycerin	1.26	Nitrogen	$1 . 25 \times 10^{- 3}$
Tungsten	19.30	Olive oil	0.92	Nitrous oxide	$1 . 98 \times 10^{- 3}$
Uranium	18.70			Oxygen	$1 . 43 \times 10^{- 3}$
Concrete	2.30–3.0			Steam $(100º C)$	$0 . 60 \times 10^{- 3}$
Cork	0.24
Glass, common (average)	2.6
Granite	2.7
Earth’s crust	3.3
Wood	0.3–0.9
Ice (0°C)	0.917
Bone	1.7–2.0
Silver	10.49

Table 11.1 Densities of Various Substances

A pile of feathers measuring a ton and a ton of bricks are placed on either side of a plank that is balanced on a small support.

Figure 11.4 A ton of feathers and a ton of bricks have the same mass, but the feathers make a much bigger pile because they have a much lower density.

As you can see by examining Table 11.1, the density of an object may help identify its composition. The density of gold, for example, is about 2.5 times the density of iron, which is about 2.5 times the density of aluminum. Density also reveals something about the phase of the matter and its substructure. Notice that the densities of liquids and solids are roughly comparable, consistent with the fact that their atoms are in close contact. The densities of gases are much less than those of liquids and solids, because the atoms in gases are separated by large amounts of empty space.

Take-Home Experiment Sugar and Salt

A pile of sugar and a pile of salt look pretty similar, but which weighs more? If the volumes of both piles are the same, any difference in mass is due to their different densities (including the air space between crystals). Which do you think has the greater density? What values did you find? What method did you use to determine these values?

Example 11.1

Calculating the Mass of a Reservoir From Its Volume

A reservoir has a surface area of $50.0 {km}^{2}$ and an average depth of 40.0 m. What mass of water is held behind the dam? (See Figure 11.5 for a view of a large reservoir—the Three Gorges Dam site on the Yangtze River in central China.)

Strategy

We can calculate the volume $V$ of the reservoir from its dimensions, and find the density of water $ρ$ in Table 11.1. Then the mass $m$ can be found from the definition of density

ρ = \frac{m}{V} .

11.3

Solution

Solving equation $ρ = m / V$ for $m$ gives $m = ρ V$ .

The volume $V$ of the reservoir is its surface area $A$ times its average depth $h$ :

\begin{array}{l} V & = & Ah = (50.0 {km}^{2}) (40.0 m) \\ = & [(50.0 k m^{2}) {(\frac{10^{3} m}{1 km})}^{2}] (40.0 m) = 2 . 00 \times 10^{9} m^{3} \end{array}

11.4

The density of water $ρ$ from Table 11.1 is $1 . 000 \times 10^{3} {kg/m}^{3}$ . Substituting $V$ and $ρ$ into the expression for mass gives

\begin{array}{l} m & = & (1 . 00 \times 10^{3} {kg/m}^{3}) (2 . 00 \times 10^{9} m^{3}) \\ = & 2.00 \times 10^{12} kg. \end{array}

11.5

Discussion

A large reservoir contains a very large mass of water. In this example, the weight of the water in the reservoir is $mg = 1 . 96 \times 10^{13} N$ , where $g$ is the acceleration due to the Earth’s gravity (about $9.80 {m/s}^{2}$ ). It is reasonable to ask whether the dam must supply a force equal to this tremendous weight. The answer is no. As we shall see in the following sections, the force the dam must supply can be much smaller than the weight of the water it holds back.

Photograph of the Three Gorges Dam in central China.

Figure 11.5 Three Gorges Dam in central China. When completed in 2008, this became the world’s largest hydroelectric plant, generating power equivalent to that generated by 22 average-sized nuclear power plants. The concrete dam is 181 m high and 2.3 km across. The reservoir made by this dam is 660 km long. Over 1 million people were displaced by the creation of the reservoir. (credit: Le Grand Portage)

11.2 Density

Calculating the Mass of a Reservoir From Its Volume

Strategy

Solution

Discussion