How Jews died in the gas chambers?

Answer 1

1. Those considered unfit for work - the old, the young and visibly pregnant women - were killed as soon as practical on arrival.
2. Those selected for work were gassed when they became too weak to work.

Answer 2

A chemical that reacted with air (called Zyklon-B) was poured through slots in the roof of the gas chamber after the Jews were herded inside. After 1-2 minutes, the gas would infiltrate the chamber and kill all who breathed it. It was a quick death, usually taking only 2-5 minutes to kill the occupants.