The Answer: The balloon leaks and the temperature went down.
If the oxygen leaks from the balloon, it is going to become smaller. Also, when gases are heated, they expand and when they are cooled, they contract. This is called thermal expansion.
Explanation:
Oxygen gas leaked from the balloon as a result, the balloon left half filled after short period of time.
When gas leaked then number of molecules become less inside the balloon. Thus, the space occupied by rest of the molecules decreases which results in decreasing the size of balloon.
Thus, balloon becomes half in size as number of molecules decreased.
