Reactions at the Crime Scene
Forensic chemists use their chemical knowledge to identify substances at crime
scenes. But what if they're searching for evidence that isn’t so easy to find? In some
cases, forensic evidence isn't obvious: it might be invisible or have changed into
something else during a chemical reaction. Forensic chemists use their knowledge of
chemical reactions to reveal what used to be and what can no longer be seen.
Chemical reactions are all about substances turning into other substances: during a
chemical reaction, the atoms rearrange to form new substances with new properties.
The substances that participate in a chemical reaction are called reactants, and the
substances they turn into during the reaction are called products. The products have
different properties from the reactants, which means that anything about the products
can be different from the reactants, including color, phase, and smell. Forensic
chemists know a lot about which substances react with other substances and what
products they form during those reactions—and that knowledge can tell the chemists a
lot about what they've found at a crime scene and what might have happened there in
the past.
Fingerprints
Forensic chemists also use chemical reactions to gather information that nobody else
can see—for example, to make invisible fingerprints visible, detect certain poisons,
and determine whether alcohol was involved in traffic accidents. Choose a chapter
from the chapter menu to learn about different reactions found at crime scenes.
When forensic chemists search a crime scene for evidence, one of the first things they
look for is fingerprints that might tell them who has been in the area. Sometimes
fingerprints are easily visible—for example, if the person who made them had paint or
blood on his or her hands. But most fingerprints are invisible to the naked eye and are
made of sweat and oils from the human hand. These invisible fingerprints are called
latent fingerprints. Forensic chemists use a few different methods to make latent
fingerprints visible; one of them uses a chemical reaction and a surprising reactant:
super glue!
Latent fingerprints are formed when a person presses his or her finger on a hard
surface, such as a doorknob or window, leaving behind sweat and oils in a pattern that
follows the ridges of his or her fingerprint. The latent print contains proteins and
amino acids. Forensic chemists can use a reaction between those proteins and amino
acids and super glue, or cyanoacrylate, to make the print visible: the molecules in the
glue react with water and the proteins to form a new product, a thin layer of a sticky,
white substance that can be seen by the naked eye. The change in properties is
evidence that a new substance was formed through a chemical reaction. The atoms
have rearranged. Atoms can’t change into other types of atoms—they can only form
new combinations. The chemical reaction makes the latent print visible so that it can
be photographed and used later in the case or in court.
How does exposing fingerprints with super glue actually work? Gooey glue straight
from the tube won’t do the trick; it’s too thick and would glue right over the prints!
Instead, forensic chemists put the surface with the fingerprints inside a special
chamber, then boil the glue until it turns into a gas. When the gas comes in contact
with the latent fingerprints, it sticks to the lines of oil left by the fingers. As soon as
they make contact, the molecules in the glue react with the protein molecules in the
print: the atoms rearrange into new combinations. The product of this reaction is a
sticky white substance that is different from super glue—and because the reaction
took place only in areas where the finger left its oils, it forms the shape and pattern of
the fingerprint. The print is no longer invisible and can be photographed and used as
evidence.
Testing For Poison
One chemical reaction forensic chemists can use to find unseen evidence is called the
Marsh Test. The Marsh Test is a way of revealing the presence of a poison called
arsenic in the body of a victim or in samples of food or drink. Scientists have been
performing versions of the test for almost two hundred years and it’s still in use today.
The Marsh Test was invented by a chemist named James Marsh back in 1836—before
then, murder by arsenic poisoning was hard to trace because arsenic powder mixes
easily into food and drink and doesn’t smell or leave any trace in the body. In fact,
arsenic was often suspected in cases where kings and other leaders died mysteriously.
The Marsh Test changed all that: James Marsh found that when he exposed a sample
of the victim’s body tissues to zinc and sulfuric acid, any arsenic in the sample would
cause the atoms in the sulfuric acid to rearrange, and the two would form a gas called
arsine gas. The other reactants would rearrange to produce zinc sulfate and water. We
know that the atoms rearranged because atoms can’t change into other types of
atoms—they can only form new combinations. If there was no arsenic present in the
sample from the victim, the reaction wouldn’t produce any arsine gas. By looking for
that gas, Marsh could tell the difference between cases of arsenic poisoning and other
causes of death.
After the invention of the Marsh Test, arsenic poisoning became much less
common—once criminals knew they might not get away with their crime, they
became less likely to try it. Today, the Marsh Test isn’t the only way to test for
arsenic poisoning, but it’s still used in some cases.
Detecting Alcohol
Every year, thousands of people are involved in traffic accidents where one or more
driver is under the influence of alcohol. It’s not only dangerous to drive a car while
under the influence of alcohol—it’s also against the law. In most U.S. states, the legal
limit is 80 milligrams of alcohol for every 100 milliliters of blood in the body. One
way police catch drunk drivers is by using a breathalyzer, a small machine that uses
the amount of alcohol in a person’s breath to determine how much alcohol is in his or
her body. Breathalyzers use a chemical reaction to quickly tell the police whether a
driver can legally be behind the wheel. (Even with a blood alcohol level that’s below
the legal limit, driving with alcohol in your system is dangerous.)
Breathalyzers contain a substance called potassium dichromate, which is orange.
When potassium dichromate molecules come into contact with alcohol molecules, the
atoms in both substances rearrange to form a new product: chromium sulfate, which is
green. We know that the atoms have rearranged because atoms can’t change into other
types of atoms—they can only form new combinations. The more alcohol molecules
there are, the more green product will appear. Different colors on the orange-green
spectrum match up with exact blood-alcohol levels because each one has different
amounts of reactants (orange) and products (green)—so when a person breathes into
the breathalyzer, the police can tell by looking at the product of the reaction how
much alcohol is in his or her body, if any.
The invention of the breathalyzer has given police a reliable way to test drivers’ blood
alcohol levels. Police sometimes use breathalyzers to check drivers’ blood alcohol
levels even if they haven’t been in an accident. They also test whether drivers who
have been in an accident had alcohol in their bodies when the accident happened,
giving police and forensic teams a helpful tool for piecing together what might have
caused a crash. Breathalyzers are useful for preventing dangerous situations on the
road before they happen, as well as figuring out what happened in the past.