Expansion of gas when heated

The following experiment shows clearly how will the gas acts, when it is heated. Apparatus------- 1. A steel plate 2. An empty bottle whose mouth is completely covered by a balloon 3. Hot water Now we will keep the empty bottle on steel plate. Then hot water is added to plate. Now the balloon which was tied to the mouth of bottle, starts becoming bigger as if someone is blowing air into it ! Reason behind this _--------- We think bottle is empty as it is not having anything inside. But we are highly mistaken. Though there are no visible things inside the bottle , there is AIR inside the bottle Now this air is now trapped inside the bottle as its mouth is covered by a balloon. Now when we put hot water to the plate, it similar to the action of heating the bottle. We are heating the bottle means we are heating the trapped air inside. This air gets expand when it is heated. As there is no other way for it to occupy the space, it has to go inside the balloon by itself. Hence balloon gets swollen up by itself!