ITM 207 Lecture Notes - Lecture 13: Disinfectant, Chemical Substance, Oxygen
9 May 2018
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Chapter 9
Glass examination:
- Physical properties vs. chemical properties:
o The forensic scientist must constantly determine those properties that impart
distinguishing characteristics to matter, giving it a unique identity
o Physical properties:
▪ Such as weight, volume, color, boiling point, and melting point describe a
substance without reference to any other substance
o Chemical property:
▪ Describes the behavior of a substance when it reacts or combines with
another substance
- Introduction:
o Matter is everything that has a mass and occupies space
o An element is the simplest substance known and provides the building block
from which all matter is composed
▪ All of the elements are listed by name and symbol in the periodic table
o Elements combine to form compounds
▪ EX. NaCl
o An atom is the basic particle of an element, and a molecule is the smallest unit
of a compound
o Matter can be classified according to the physical form it takes
▪ Solid:
• Definite shape and volume
▪ Liquid:
• Specific volume, takes the shape of its container
▪ Gas/vapor:
• Neither a definite shape nor volume
o Substances can change from one phase to another without forming a new
chemical species, matter is simply being changed from physical state to another
- Measurement system:
o Scientists throughout the world use the metric system of measurement
o The metric system has basic units of measurement for length, mass, and volume;
they are the meter, gram, and liter, respectively
o The following are common prefixes used in the metric system: deci, centi, milli,
micro, nano, kilo, and mega
- Important physical properties:
o Temperature is a measure of heat intensity, or the hotness or coldness of a
substance
▪ In science, the most commonly used temperature scale is the Celsius
scale
▪ The scale is derived by assigning the freezing point of water a value of 0
degrees Celsius and its boiling point a value of 100 degrees Celsius
o Weight:
▪ Is the force with which gravity attracts a body
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o Mass:
▪ Refers to the amount of matter an object contains independent of gravity
• The mass of an object is determined by comparison to the known
mass of standard objects
o Density:
▪ Defined as the mass per unit volume (D = M/V)
▪ Density is an intensive property of matter, meaning it remains the same
regardless of sample size
▪ It is considered a characteristic property of a substance and can be used
as an aid in identification
- Theory of light:
o Two models describe the behavior of light
▪ Light is described as a continuous wave
▪ Light is depicted as a stream of discrete energy particles
o When light passes through a prism, it is dispersed into a continuous spectrum of
colors
o Visible light ranges in color from red to violet in the electromagnetic spectrum
o The electromagnetic spectrum is the entire range of radiation energy from the
most energetic cosmic ways to the least energetic radio waves
o Waves are described in terms such as:
▪ Wavelength:
• The distance between two successive crests (or one trough to the
next trough)
▪ Frequency:
• The number of crests (or troughs) passing any one given point per
unit of time
• Frequency and wavelength are inversely proportional to one
another
o As electromagnetic radiation moves through space, its behavior can be described
as that of a continuous wave; however, once radiation is absorbed by a
substance, it is best described as discrete particles of light known as photons
- Refractive index:
o Light waves travel in air at a constant velocity until they penetrate another
medium, such as glass or water, at which point they are suddenly slowed,
causing the rays to bend
o The being of light waves because of a change in velocity is called refraction
o Refractive index:
▪ Is the ratio of velocity of light in a vacuum to that in the medium under
examination
▪ EX. At 25 degrees Celsius, the refractive index of water is 1.3333
▪ This means that light travels 1.333 times faster in a vacuum than it does
in water
▪ Like density, refractive index is an intensive property and will serve to
characterize a substance
find more resources at oneclass.com
find more resources at oneclass.com
- Refractive light method:
o Crystalline solids:
▪ Have definite geometric forms because of the orderly arrangement of
their atoms
▪ These solids refract a beam of light in two different light-ray components
▪ This results in double refraction
▪ Birefringence is the numerical difference between these two refractive
indices
o Not all solids are crystalline in nature. For example, glass has a random
arrangement of atoms to form an amorphous or non-crystalline solid
- Glass composition:
o Glass is hard, brittle substance that is composed of silicon oxides (sand) mixed
with various metal oxides
o Sodium carbonate (soda) is normally added to the sand to lower its melting
point
o Calcium carbonate (lime) is also added to the sand mix to prevent the glass from
dissolving in water
o Foesi aalsts usuall eaie soda-lie glass used to manufacture most
window, bottle glass, and light bulbs
o Soda-lead:
▪ Is use in fine table ware and are objects
o Borosilicate:
▪ Is heat resistant, like Pyrex
o Silica:
▪ Is used in chemical ware
o Tempered glass:
▪ Is stronger than normal glass due to rapid heating and cooling of the glass
surfaces
▪ Core of the glass becomes compressed much more quickly than the outer
layers, over and over again
▪ Ultra-fine series of invisible stress lines develop
▪ It shatters along these many tiny stress lines into small, safer pieces,
uniform in size, edges smooth
▪ Used for car side and rear windows
o Laminated glass:
▪ Found in car windshields has a layer of plastic between two pieces of
ordinary window glass
▪ Two outside layers of glass may crack, but the inside vinyl layer remains
intact, holds the pieces together
- Glass fragments:
o For the forensic scientist, the problem of glass comparison is one that depends
on the need to find and measure those properties that will associate one glass
fragment with another while minimizing or eliminating other sources
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find more resources at oneclass.com
Document Summary
Important physical properties: temperature is a measure of heat intensity, or the hotness or coldness of a substance. It is considered a characteristic property of a substance and can be used as an aid in identification. Is the ratio of velocity of light in a vacuum to that in the medium under examination: ex. For example, glass has a random arrangement of atoms to form an amorphous or non-crystalline solid. Is use in fine table ware and are objects: borosilicate, silica: Is used in chemical ware: tempered glass: Flotation method: this is a rather precise and rapid method for comparing glass densities. Glass immersion method: this (cid:373)ethod is (cid:271)est used to dete(cid:396)(cid:373)i(cid:374)e a glass f(cid:396)ag(cid:373)e(cid:374)t"s refractive index. If the suspe(cid:272)t"s shoes a(cid:374)d/o(cid:396) (cid:272)lothi(cid:374)g a(cid:396)e to (cid:271)e e(cid:454)a(cid:373)i(cid:374)ed fo(cid:396) the p(cid:396)ese(cid:374)(cid:272)e of glass fragments, they should be individually wrapped in paper and transmitted to the laboratory.
