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Characteristics of Group 17 of the periodic table.

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To Catch a Thief

To Catch a Thief

License: CC BY-NC 3.0

Today, DNA is the gold standard for criminal evidence. But often there is no DNA at the scene of a crime. Then, old-fashioned fingerprint analysis may be used. A skilled analyst can identify an individual from a single good print because no two people have identical fingerprints. But you have to be able to see the prints to analyze them.

The Back Story

  • Fingerprints are often present at a crime scene, but they may not be visible. These fingerprints are called latent prints.
  • When people touch objects with their fingers, they may leave behind traces of skin oils. The oil soutline the pattern of whorls on their fingers.
  • One method for bringing latent prints into view is called iodine fuming. Iodine is a halogen element in Group 17 of the periodic table. Iodine undergoes sublimation, and this is the basis of iodine fuming.
  • You can see how iodine fuming is done and how it makes latent fingerprints visible by watching this video: 


Show What You Know

Learn more about iodine fuming to reveal latent fingerprints at the link below. Then answer the questions that follow.

  1. In what state of matter does iodine occur at room temperature? What happens to iodine when it is heated?
  2. What chemicals in fingerprints interact with iodine vapor? What happens when this occurs?
  3. What is a fixative? How and why must a fixative be used on fingerprints revealed by iodine fuming?
  4. Why is it important to keep iodine crystals in a closed container, especially when heating them?
  5. Briefly describe another method of revealing latent fingerprints.

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