Basic Chemistry is a story mission in Far Cry 4. It is only available if you choose to side with Sabal.
In the mission, you have to destroy the Rochan Brick Factory located in the mid-west of Kyrat. First you have to break the cables holding the smokestack in place, and then destroy it with explosives. Then you can enter the factory, kill the soldiers inside and place more explosives.
After you escape from the factory, you can detonate the explosives.