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Narrative
Bromine is added to two test tubes containing pentane. One test tube is covered with black paper. Both tubes are irradiated with light from a slide projector. The red color of bromine disappears from the tube exposed to light. After the light is turned off, the paper is removed. The tube not exposed to light retains the color of bromine. The vapor above each tube is tested with pH paper. The test tube which lost the color of bromine shows the presence of an acidic gas - hydrogen bromide.

Discussion
Light initiates this reaction by generating bromine atoms (free radicals) from bromine molecules. The bromine atoms react with pentane to form pentane radicals which can propagate the reaction by reaction with bromine molecules or can terminate the reaction by reacting with bromine atoms. The overall reaction is the substitution of a hydrogen atom for a bromine atom. With pentane three monobrominated isomers are possible.