Chloropicrin ($\text{CCl}_3\text{NO}_2$), also known as nitrochloroform or tear gas, is obtained when chloroform ($\text{CHCl}_3$) reacts with concentrated nitric acid ($\text{HNO}_3$). In this reaction, the hydrogen atom of chloroform is replaced by the nitro group ($-\text{NO}_2$) from nitric acid. The chemical equation is: $\text{CHCl}_3 + \text{HNO}_3 \xrightarrow{\Delta} \text{CCl}_3\text{NO}_2 + \text{H}_2\text{O}$