MCQ Detailed View

Silver nitrate (AgNO₃) is the chemical that produces black stains on skin and paper. This effect happens because silver nitrate is light-sensitive and decomposes when exposed to light. When it comes into contact with skin or organic materials like paper, it reacts with proteins and organic compounds. The silver ions (Ag⁺) present in AgNO₃ are reduced to metallic silver (Ag⁰), which appears as black or dark brown spots.

This reaction is the same principle used in early photography, where silver salts were essential in capturing images on photographic plates. Once silver nitrate stains the skin, the marks usually remain until the stained outer layer of skin naturally sheds, because the reaction occurs within the epidermis and cannot be washed off easily.

Looking at the other options:

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  Copper(II) sulfate (CuSO₄) is a blue crystalline solid, commonly called Blue Vitriol. It does not produce black stains.

  
  


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  Potassium manganate (K₂MnO₄) is green in color and not responsible for black stains on skin.

  
  


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  Acetic acid (CH₃COOH), the main component of vinegar, is a colorless liquid and does not leave permanent black marks.

  
  

Therefore, only AgNO₃ (Silver nitrate) is known for this characteristic property.

In laboratories, silver nitrate is used as a reagent for detecting halide ions, in antiseptic solutions, and in photographic processes. However, due to its staining effect and caustic nature, it is handled with care.

The black staining property of AgNO₃ makes it a recognizable and often-tested point in analytical chemistry MCQs, helping students connect chemical properties with practical laboratory experiences.