Here's why:
* Ninhydrin reaction: Ninhydrin is a chemical that reacts with amino acids to form a colored product called Ruhemann's purple. This reaction is highly sensitive and can detect even trace amounts of amino acids.
* Amino acids in skin: Skin contains a variety of amino acids, which are the building blocks of proteins. These amino acids are present in the various layers of skin, including the epidermis, dermis, and subcutaneous layers.
Therefore, when ninhydrin is applied to human skin, it reacts with the amino acids present, producing a purple stain. This reaction is commonly used in fingerprint analysis and forensic science to visualize fingerprints.
