| FIGURE 2.6 The density of mercury (13.6 g/mL) compared with the density of water (1.000 g/mL). 1 mL mercury balances 13.6 mL water. Mercury is one of the heaviest liquids known. |
We have said that chemists determine the properties of matter, particularly those properties that help identify the composition of a sample. We can measure the mass and volume of a sample, as was done for several samples of iron with results shown in Table 2.7. However, neither their masses nor their volumes show that all the samples are iron, but all the samples do have the same ratio of mass to volume, as is shown in the far right column. This ratio is called density.
| Density = | mass
volume |
| Sample | Volume (cm3) | Mass (g) | Density (g/cm3) |
|---|---|---|---|
| A | 1.05 | 8.25 | 7.86 |
| B | 25.63 | 201.5 | 7.862 |
| C | 90.7 | 713 | 7.86 |
| D | 0.02471 | 0.1942 | 7.859 |
All samples of the same kind of matter under the same conditions have the same density. Density is a physical property that characterizes and identifies a particular kind of matter (see Figure 2.6). Table 2.8 lists the densities of some common solids and liquids under normal conditions. The densities of solids are usually given in grams per cubic centimeter (g/cm3), the densities of gases in grams per liter (g/L), and the densities of liquids in grams per milliliter (g/mL). Recall from Table 2.1 that 1 mL = 1 cm3. Using these units, the density of water is given as 1.000 g/mL at 4°C. Based on the information in Table 2.8, we can make some basic observations. The densities of most metals are greater than that of water. The densities of liquids vary; some are less dense than water, whereas others are more dense. For example, the density of gasoline is about 30% less than that of water, and the density of chloroform is about 50% greater.
Densities vary with temperature. For example, the density of water at 4°C is 1.000 g/mL and at 80°C is 0.9718 g/mL; the density of oxygen is 1.43 g/L at 0°C and 1.10 g/L at 80°C. Except for water, the densities in Table 2.8 are given at 0°C.
| FIGURE 2.6 The density of mercury (13.6 g/mL) compared with the density of water (1.000 g/mL). 1 mL mercury balances 13.6 mL water. Mercury is one of the heaviest liquids known. |
| Metals (g/cm3) | Other solids (g/cm3) | Liquids (g/mL) | |||
|---|---|---|---|---|---|
|
|
2.70 |
|
1.85 |
|
1.49 |
|
|
19.32 |
|
0.86 |
|
0.791 |
|
|
1.74 |
|
0.24 |
|
0.67 |
|
|
13.59 |
|
3.51 |
|
1.000 |
|
|
0.97 |
|
1.59 | ||
Density is a conversion factor that relates mass to volume. If you know two of the three quantities (mass, volume, and density), you can calculate the third.
Often, particularly in discussing fluids, specific gravity is reported rather than density. The specific gravity (sp gr) of a substance is the ratio of its density to that of a reference substance:
| Specific gravity = | density of a substance density of a reference substance |
Generally, water is the reference substance for comparing solids and liquids, and air is the reference substance for comparing gases.
A value of specific gravity must state the temperature at which the densities were measured. Specific gravity has no units, because the density units cancel in its calculation. For example, we calculate the specific gravity of benzene at 20°C as follows:
