Substitution reaction. Three test tubes showing the result of adding halobutanes to silver nitrate solution. Adding a 1-halobutane to the nitrate results in a nucleophilic substitution reaction. The halogen dissociates from the butane, and forms a precipitate of a silver halide. The precipitates seen here are (left to right) silver chloride, silver bromide and silver iodide. The time taken for the precipitate to appear may be used as a comparative measure of the ease with which the substitution occurs.