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Blue Vitriol, also called Blue Stone, is the common name for Copper(II) sulfate pentahydrate (CuSO₄·5H₂O). It is a bright blue crystalline salt that is widely recognized in inorganic chemistry. The blue color of this compound comes from the presence of five molecules of water of crystallization attached to each formula unit of copper sulfate. When heated strongly, it loses this water and turns into a white powder called anhydrous copper sulfate, which is often used as a test for moisture.

Copper sulfate has several important laboratory and industrial uses. In laboratories, anhydrous CuSO₄ is employed as a drying agent and as a test for the presence of water. In the Biuret test, Cu²⁺ ions from copper sulfate are used to detect proteins. In agriculture, Blue Vitriol is used as a fungicide and pesticide, and in the preparation of Bordeaux mixture for protecting plants from fungal infections. It is also used in electroplating, dyeing, and as a mordant in textile industries.

Looking at the other options: Silver nitrate (AgNO₃) is used for halide detection but is not known as Blue Vitriol. Potassium dichromate (K₂Cr₂O₇) is an orange crystalline salt used as an oxidizing agent, and Potassium chromate (K₂CrO₄) is yellow in color and also used in analytical chemistry. Neither of these chromium compounds are referred to as vitriols.

Different “vitriols” are known by color and metal content: Green Vitriol is ferrous sulfate (FeSO₄·7H₂O), White Vitriol is zinc sulfate (ZnSO₄·7H₂O), while Blue Vitriol specifically refers to copper sulfate pentahydrate. This distinct blue salt remains one of the most important compounds in inorganic chemistry due to its color, structure, and wide range of applications.