FORENSIC GLASS ANALYSIS

A physical property describes a substance without reference to any other substance.

· weight · volume · color · boiling point · melting point

A chemical property describes the behavior of a substance when it reacts or combines with another substance.

Why is glass important to a crime scene?

· It can be lodged in a suspect’s shoes or clothes.

· Headlight glass can help identify a suspect's vehicle

What is glass made of and how is it made?

· Glass is made of silicon oxide and metal oxides

· Sand and metal oxides are melted and then cooled

Types of Glass

· Window and bottle glass are made of soda-lime, sand, and the following metal oxides: sodium, calcium, magnesium, and aluminum.

· Auto headlights and heat-resistant glass also have boron oxide.

· Tempered glass is stressed glass that is rapidly heated and cooled.

· Laminated glass is used as windshields and is made by sandwiching a piece of plastic between two pieces of window glass

How is a glass fragment analyzed?

· Pieces are put together like a puzzle

· Refractive index: velocity of light in a vacuum divided by the velocity of light in the medium

· density: the mass divided by the volume

A Lesson on Density

You come out of the grocery store and find that someone has broken the brake light on your car. You find a small shard of glass that does not match your brake light but appears to match a broken headlight on the car in the next space. You get a loose piece of headlight glass and the license number from the other car. Now, how could you determine whether the two pieces of glass come from the same headlight? Density may be the key.

BACKGROUND

Density is the ratio of the mass of an object to the volume of the object. This ratio can be expressed as an equation, as shown below, where d is density, m is mass, and V is volume.

When an object is placed in fluid, the difference between its density and the fluid’s density determines whether the object will sink or float. One of three things will happen when an object is placed in fluid:

· If the density of the object is greater than the density of the fluid, the object will sink in the fluid, and the volume of fluid displaced (meaning the amount the fluid level raises when the object is dropped into it) will be the same as the volume of the object.

· If the density of the object is less than the density of the fluid, the object will float on the surface of the fluid and the volume of fluid displaced will have a weight equal to the weight of the object.

When the density of the object is the same as the density of the fluid, the object will neither sink nor float: it will remain suspended in the fluid, and the volume of fluid displaced will be equal to the volume of the object.

An object made of a certain material will have a density that is characteristic of that material. Therefore, density can be used to identify the material an object is made of. This has been known since ancient times: Archimedes, a Greek who lived in the third century BCE, discovered an important fact, now known as Archimedes’ principle, about buoyant force. Archimedes’ principle states that when an object is placed in a fluid, a buoyant force is exerted by the fluid on the object that is equal to the weight of the fluid displaced by the object. Archimedes’ principle can be stated in a very simple equation, shown below.

Buoyant force = weight of displaced fluid

Because of the buoyant force, a submerged object will have an apparent weight that is less than its weight in air. A floating object, on the other hand, appears to be weightless. This is because a floating object is less dense than the fluid that in floats in, so only part of its volume displaces the fluid, and the weight of the displaced fluid is the same as the weight of the entire object. Archimedes’ principle can be used in various interesting and creative ways, depending on the requirements of a particular sample, to determine the density of an object.

DENSITY DETERMINATION OF AN IRREGULARLY SHAPED OBJECT

The density of a relatively large but irregularly shaped object is most easily determined by using Archimedes’ principle. In this method, the weight of an object is compared with its apparent weight when it is submerged in a fluid. The equation for determining the density of an object submerged in a fluid by this method is shown below.

This equation can be used to determine the density of an object when it is difficult or impossible to accurately find the volume of the object. A piece of shattered glass with sharp edges and points is an example of the type of object for which this method of density determination is suitable.

The advantage of this method, as you can see by glancing at the equation, is that you don’t have to measure volumes at all: you need to know only the density of the fluid used and the weight of the object in air and in the fluid.

Water is often used as the fluid for this technique because its density of about 1.00 g/cm³ simplifies calculations, but any fluid can be used provided that it has a density that is less than the density of the object.

In the equation shown above, the weight difference divided by the density of the fluid yields the volume of the object. The equation below shows how the units in the equation above cancel to arrive at the same simple equation shown on the previous page, which is mass per unit volume.

DENSITY DETERMINATION OF A VERY SMALL OBJECT

To determine the density of a very small object such as a tiny piece of glass or plastic, it is often more convenient and accurate to place the object in a fluid of known density to determine whether the object sinks or floats. The density of the fluid can then be adjusted by adding another fluid, one that is miscible with the first fluid but has a different density, drop wise until the object being tested neither floats nor sinks. As described earlier, when this condition is reached, the density of the fluid and the density of the object are the same.

Accurate results require careful recording of the number of drops of fluid added, so you can calculate an accurate final density. Knowing the exact number of drops of each fluid added and the density of each fluid, you can calculate the final volume of the mixtures by using weighted averages.

In using weighted averages for these calculations, a “weight” is given to the density of each fluid based on how much of each was present in the mixture at the point where it achieved the same density as the glass sample. This simply means multiplying the density of each fluid by the number of drops that was present. Then you add the two weighted densities together and divide that by the total number of drops of fluid in the mixture: this weighted average will be the final density of the mixture.

For example, say you have glass sample that is suspended in a mixture. To make the mixture, you had added 30 drops of a fluid with a density of 2.50 g/mL, plus 5 drops of water, which has a density of about 0.998 g/mL.

First, you “weight” each density by the number of drops, and add them together:

(30 drops × 2.50 g/mL) + (5 drops × 0.998 g/mL) = 80.0 drops • g/mL

To get the weighted average, divide this weighted total by the total number of drops:

Because 2.29 g/mL is the final density of the mixture when the glass sample is suspended, 2.29 g/mL is also the density of the glass sample.

COMPARING DENSITIES OF TWO OBJECTS BY USING A COLUMN

Sometimes it is less important to find the density of an object than to simply find out if its density is the same as that of another object. Comparing the density of two objects can be performed in a single step by the use of a density gradient column. The column contains several fluids of progressively greater density.

Dropping the objects into the column and allowing them to come to rest is all that is required for the comparison. If the objects come to rest at the same position in the column, you know that they have the same density. Otherwise, you know the densities are different. Figure 1 below shows a diagram of such a column containing various materials that have settled to the layers that indicate their densities (shown in grams per milliliter).

Objects having different densities come to rest at different levels in a density column. Each object placed in the density column sinks until it reaches a point where its density is less than the density of the surrounding fluid. Notice that each object floats on the fluid that has a density greater than the density of the object.

Topic Questions

1. What is the mathematical definition of density?

2. State Archimedes’ principle.

3. An object placed in a fluid sinks and displaces a volume of fluid. What is the relationship between the volume of fluid displaced and the volume of the object?

4. If an object is submerged in a fluid, how is the weight of the object affected? Why?

5. Two pieces of plastic are placed in a density column. Piece A comes to rest close to the middle of the column, whereas piece B sinks to the bottom. What could you say about the densities of the two pieces?

6. Suppose that to determine the density of an object, you use the weight difference technique. What information is necessary for the determination to be successful? Explain.

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