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Density

المؤلف:  Max M. Houck، Jay A. Siegel

المصدر:  Fundamentals of Forensic Science

الجزء والصفحة:  p441-442

2026-08-19

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Density

The density of an object is its mass divided by its volume. D=M/V For example, if a cubic block of wood weighs 250 g and is 10 cm on each side, its volume would be 1000 cm3 (10 cm × 10 cm × 10 cm) and its density would be 250 g/1000 cm3, or 0.25 g/cm3. Note that the number obtained for the density is dependent on the units of mass and volume. Most objects can be easily weighed on a common balance. If an object has a regular shape such as a cube, whose volume can be easily measured, then its density is easily determined. If the object has an irregular shape, but is fairly large, its volume can be measured by displacement of a liquid. A liquid, such as water, is put into a container and its volume is determined and then the object is added. The volume of the liquid is then measured and then the volume of the liquid alone is subtracted, leaving the volume of the object. Recall the old tale about the thirsty bird who could not reach a small amount of water in a deep hole. It dropped pebbles into the hole, thus raising the level of the water until it could reach it. The rocks displaced the water and caused it to rise in the hole. This is shown graphically in Figure 1. If an object is irregular in shape and too small to measure its volume by displace ment, then indirect methods must be used. The easiest way to compare the densities of two small pieces of glass is by the sink/float method. In this method, two miscible liquids (liquids that can mix with each other) are chosen such that one is less dense than glass and the other is denser. Bromoform and bromobenzene are often used, although they are both toxic, so caution must be observed. A tube is filled with a mixture of the two liquids and then tiny pieces of the glass are inserted. The particles are allowed to settle in the liquid mixture and then a determination is made to see if they all settle at the same level. Then the liquids are heated slightly using a hair dryer. Heat decreases the density of liquids, so the glass particles should then move lower. If they all move to the same spot, then they have the same density. It should be noted that a glass object, such as a headlight will exhibit slight variations in density within the object. The sink/float method of density determination is very sensitive and it may sometimes occur that two pieces of glass may show slight differences in density but may actually arise from the same object. Using several particles from each source may minimize this problem.

FIGURE 1 Measurement of volume by displacement of a liquid. A measured quantity of liquid is put into a container. Then an object is added. The object must be insoluble in the liquid and denser than the liquid. The object sinks to the bottom and displaces its own volume of liquid. The volume of the liquid is now measured again. The difference between the two volumes is the volume of the object.