1.3 Density
Although mass and volume are both extensive properties, their ratio is an important intensive property called density (d). Density is defined as mass per unit volume and is usually expressed in grams per cubic centimeter (g/mL). As mass increases in a given volume, density also increases. For example, lead, with its greater mass, has a far greater density than the same volume of air, just as a brick has a greater density than the same volume of Styrofoam. At a given temperature and pressure, the density of a pure substance is a constant:
density = mass/volume
Density is a physical property that relates the mass of a substance to its volume. Density is defined as mass per unit volume of a substance. Density is reported as g/ml or g/cc. The mathematical form of density d= m/v. Physical meaning of density refers to the compactness of the substance.
Density of a substance depends on temperature. Specially for liquids and gases, when temperature increases, volume expands and therefore density decreases. For most substances solid state is denser than liquid and liquid is more dense than gas. Water is an exception. Ice is less dense than liquid water. A less dense substance floats on a denser liquid.
Density also depends on pressure. An allotrope of carbon Diamond can be converted to graphite at a very high pressure. Although they contain the same atoms of carbon but they do not share the same density.
If two liquids are mixed together and they are immiscible, liquid with lower density is always on top of liquid with higher density. In other words, substance with higher density always sinks in substance with lower density and vice versa.
There are two different ways density can be determined in Lab. 1) dimensional method and 2) displacement method.
In dimensional method, volume of the regularly shaped object is determined applying the standard formula. Mass of the object is determined using analytical balance. Then mass is divided by volume to determine the density.
Example:
Determine the density of a unknown metal cube that has side length 3.00cmand mass is 310.672g.
Volume of the metal cube 3.00*3.00*3.00= 27.0 cm3
Density of the metal= 310.671/27.0= 11.5 g/cm3
In displacement method some water is taken in the graduated cylinder and then the irregular shaped object is dropped in the cylinder. Volume of water in the cylinder rises. The difference in volume between initial and final volume of graduated cylinder is the volume of the cylinder.
In the above example, volume of the liquid (96.0-90.0)ml =6.0 ml.
If the mass of the sample 16.9050g, the density of the sample = 16.9050/6.0 ml= 2.8 g/ml.
Table 1.1 lists the density of common substances. Pure water, for example, has a density of 0.998 g/mL at 25°C. Notice that corn oil has a lower mass to volume ratio than water. This means that when added to water, corn oil will “float.” Example 3 shows how density measurements can be used to identify pure substances.
Table 1.1 Densities of Common Substances
https://pba.pbslearningmedia.org/resource/arct15-sci-densitylab/density-lab/
Play with different substances including “Gold, lead, Foam and find the density of unknown.
1) put the object on balance,
2) read the volume level in the beaker
3) drop the object in beaker
4) read the new volume in beaker
5) determine the difference in volume , equal to the volume of the substance.
6) determine density
