A tube is filled with a mixture of the gases chlorine (yellow) and methane (colorless). The reaction takes place in the light: The products are chloromethane, dichloromethane, trichloromethane and tetrachloromethane, liquids which supernate, and hydrochloric acid (which colors bromothymol blue in yellow):. $CH_4 + Cl_2 \rightarrow CH_3Cl + HCl$ $CH_3Cl + Cl_2 \rightarrow CH_2Cl_2 + HCl$ $CH_2Cl_2+ Cl_2 \rightarrow CHCl_3 + HCl$ $CHCl_3 + Cl_2 \rightarrow CCl_4 + HCl$