Potassium cyanide - KCN
Very pure potassium cyanide is a purely white chemical, but commercial samples, even general lab grade material, frequently have a faint yellow color as shown in this picture. This is due to the ease of aerial oxidation of cyanide, which leads to formation of brown polymeric species. For this reason, potassium cyanide must be stored in tightly sealed bottles such that no air and moisture can reach the chemical. Solutions of potassium cyanide in water also tend to discolor when allowed to stand for a prolonged time.
Potassium cyanide is very toxic. A dose of 500 mg may be sufficient to kill a healthy, grown up person. Potassium cyanide hence is a chemical which must be used with great care when experiments are done with it. It readily reacts with acids, giving very toxic hydrogen cyanide gas. Especially when the solution is warm, this easily escapes from the solution as bubbles of gas.