Chemistry has been definced as the science that is concerned with the composition, properties, and structure of matter and with the ways in which substances can change from one form to another. But this definition is too broad to be useful. Chemistry isn't the only science that deals with the composition and transformations of matter. Bullets are matter, and they're transformed by impact, a process studied by forensic physicists. Insect and microbe populations on a corpse tranform the corpse by various metabolic processes studied by forensic entomologists. Latent fingerprints are made excretions of the eccrine gland studied by biologists. Chemists are unique because they understand or explain everything, including the processes mentioned above, in terms of the properties of just over 100 kinds of atoms found in all matter, and the amazing variety of molecules that are created by forming and breaking bonds between atoms. So chemistry is defined by its approach, not its subject matter. Chemistry explains or understands any subject in terms of the properties of atoms and molecules.
Forensic science (often shortened to forensics) is the application of a broad spectrum of sciences to answer questions of interest to the legal system. So forensic chemistry deals with evidence that is based on molecular explanations; that would include
- different methods of fingerprinting, because molecular processes are used to develop the latent prints. For example, methyl cyanoacrylate polymerisation works because fingerprints catalyze polymerization of the molecule (shown in the figure), which forms white solid images of the latent print.
Superglue developed prints on a plastic bag 
- DNA evidence, since the polymerase chain reaction (PCR) is used to synthesize replica DNA molecules from a tiny sample
- detection of poisons by tests dependent on their chemical properties (molecular structure and tendency to form particular types of bonds). Thallium is detected by the famous green flame test for which it is named (the word comes from the Greek word meaning a green bud). Thallium poison was detected in soup by this method in the murder mystery, "The Documents of the Case". The thallium flame is similar to the color of the copper flame test shown below.
The thallium flame can be distinguised from copper by chemists who measure the entire spectrum. The thallium spectrum with its single line is compared below to the copper spectrum (both are sketches of typical spectra). The spectrum depends on the conditions under which it was obtained, and exact spectra are easily found.
Forensic chemistry would not include such things as The "Eureka" legend of Archimedes (287-212 BC) where he determined the density of a crown and found that it was not gold as claimed. Density does not require a molecular explanation, so it is usually considered a physical (not chemical) property. Forensic chemistry also would not include determination of time of death by studying insect infestation of a corpse (forensic entymology), or cause of death by studying organ pathology (biology or medicine). Subdisciplines are thus defined by the kinds of explainations they offer: Chemical explanations are in terms of the properties of atoms and molecules; biological explanations involve organisms, organs and cells; physical explanations involve objects understood at the macroscopic level in terms of forces, masses,anddensities, for example.
From ChemPRIME: 1.0: Prelude to Chemistry