Is smoke heavier than air, and if so, why are smoke alarms installed on the ceiling?

Public Comments

1. heat makes smoke rise so the safest place to be in always low to the floor
2. smokes goes up cuz its hot , and hot air is so lighter than the air on the normal thermal conditions thus they put smoke alarms on the ceiling .
3. Smoke is heavier but the air is lighter so air goes down and the smoke is up which that why you crawl on your leg very low to the ground because that is where most of the clean air is and that's is why the are put on the ceiling because the smoke will rise not sink but since the room or area is closed off it will cover the area which from ceiling almost to the floor
4. Heat makes the smoke rise.
5. here's a smart-assed answer. (Sorry couldn't resist) if you put them on the floor you'd trip over them