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Crime scene investigation chemistry
What are some of the chemistry principles and methods in CSI(crime scene investigation)?
Chemistry is is really looking at how chemical reactions take place. In a CSI case it is about finding out what is at the scene and how long it has been there for. Comparing chemicals at a scene or amplifying them is important for gather evidence that can either show that someone or something was at a scene of a crime.
Biochemical reactions
In a crime scene there are a wide range. One classic example is the reaction of ninhydrine and amino acids. This is sometimes used to bring out the details of a finger print. Because finger prints contain tiny amounts of amonia this can reveal a finger print by producing a purple colour.
DNA reactions 
Amplifying DNA is also a classic chemical reaction. More specifically something called PCR or polyerase chain reaction. This techniques used enzymes to chemically increase a small amount of DNA into a much larger number of pieces.
References sources:
http://en.wikipedia.org/wiki/Ninhydrin
http://en.wikipedia.org/wiki/PCR
http://en.wikipedia.org/wiki/Forensic_chemistry
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Classic chemistry in CSI
DNA finger printing is also used in CSI, for example a sample of DNA can be cut up chemically using enzymes.
There are also a range of other basic principles in chemistry.
Sometimes knowing needing to know the pH of a sample is important for storage or preservation of materials.
Chemical analysis techniques like
Gas chromatography-mass spectrometry, or mass spectrometery are often used to identify chemicals in samples. These techniques can be very sensistive.
For example materials can be compared against a database of known substances.
Sometimes knowing how substances react to chemicals helps in investigations. This might include the way something gets worn out overtime due to corrosion, eg ozone, or chlorine might erode something. Knowing this can help estimate the time and sample has been exposed.