We did an organic functional group analysis lab. we tested an alkane, an alkene, and an aromatic ring with Br2 in DCM, and then tested the resulting...

We did an organic functional group analysis lab.we tested an alkane, an alkene, and an aromatic ring with Br2 in DCM, and then tested the resulting solutions with litmus paper.The question is:why is litmus paper used in this circumstance?