When copper sulphate ($\text{CuSO}_4$) reacts with potassium cyanide ($\text{KCN}$), initially a yellow precipitate of cupric cyanide $\text{Cu}(\text{CN})_2$ is formed. This cupric cyanide is unstable and decomposes to give white cuprous cyanide ($\text{CuCN}$) and cyanogen gas ($\text{C}_2\text{N}_2$). $2\text{Cu}^{2+} + 4\text{CN}^- \rightarrow 2\text{Cu}(\text{CN})_2 \rightarrow 2\text{CuCN} + (\text{CN})_2$ Cuprous cyanide ($\text{CuCN}$) dissolves in excess of $\text{KCN}$ to form a soluble complex, potassium tetracyanidocuprate(I). $\text{CuCN} + 3\text{KCN} \rightarrow \text{K}_3[\text{Cu}(\text{CN})_4] \rightleftharpoons 3\text{K}^+ + [\text{Cu}(\text{CN})_4]^{3-}$