Densities of Solids and Liquids
Density is an important intensive property of substance, and is defined as the ratio of the Mass of a sample to its
Volume (D = M/V). The SI unit of density is therefore the kg/m3, but densities are more commonly expressed in the
laboratory as g/cm3 or g/mL. [A note regarding units of volume—by convention, the volumes of solids are expressed
in cm3 but the volumes of gases and liquids are more commonly expressed in mL or L. 1 mL = 1 cm 3, and 1 L = 1
dm3.]
Any pure substance will always have the same density under identical conditions of temperature and pressure.
For solids and liquids, the density will change slightly as the conditions of temperature and pressure are varied
because the volume of the sample will exhibit a weak dependence on these variables. Most solids and liquids
expand slightly with increased temperature and contract slightly with increased pressure. Thus, for the most
exacting work, we must specify the conditions of temperature and pressure that any density measurement is
performed. However, because the volume of solids and liquids varies so slightly, we can
usually assume conditions close to standard conditions. The volume of samples of gas,
however, depends strongly on conditions of temperature and pressure, therefore the
density of a gas must be specified under the conditions measured.
Mixtures also exhibit the property of density, but the density of a mixture depends on
its composition. For example, pure Sulfuric Acid, H 2SO4, is a liquid with a density of 1.38
g/ . When Sulfuric Acid is diluted with Water, whose density is 1.00 g/ , the resulting
mL
mL
mixture will have a density between 1.00 g/mL and 1.38 g/mL, but the value will clearly
depend on the concentration of the solution. The more concentrated the solution, the
greater the density.
One consequence of density differences between substances is that substances with
lower densities will float on substances with higher densities, as shown. Oil floats on
Water because it has a lower density than Water. This principle is of practical use in
several applications, one of which is in determining the strength of the electrolyte solution
in your car battery. Before the days of “permanently sealed” batteries, it was necessary to
check the strength of your battery solution, which is a solution of Sulfuric Acid and Water.
When the solution became too weak (too little Sulfuric Acid) its density decreased. An
inexpensive “densitometer” consisted of a large bulb containing several plastic beads of
varying densities. The number of beads that would float on the battery solution would
indicate the density, and therefore the strength of the solution. If too few beads floated,
more Sulfuric Acid must be added to the battery fluid.
Discussion Questions:
1. If a substance has a density of X g/cm 3, what is its density in kg/m3?
2. Most substances are more dense in the solid phase than in the liquid phase. Water is an exception to this.
(As is the pure element Gallium). What unusual but easily observable property of Water shows that the liquid phase
is denser than the solid phase?