What is a hood used for in the chemistry laboratory?
I assume by hood you mean a 'fume hood'? If so, it is used to keep the scientist safe.
Fume hoods are used in the lab to prevent scientists being exposed to dangous fumes. The fume hood will suck the harmful fumes away from the scientist and out of the building usually via a tall chimney on the roof. The hood is just like an extractor fan in a kitchen.
- Fume hood image - wikimedia commons