Cold air is more dense than hot air. When I filled a 9 centimeter diameter balloon with cold air the mass was 1 gram and when I measured the mass of the same size balloon with hot air it was 0.5 grams. When molecules are cooled they move closer together and when they are heated up they move farther apart. Because of this more molecules can fit into a balloon when the air going in is cold than when the air going in is warm. (Please explain this to me)

Sorry this might not be great but when the air is cold the molecules are dense which means they are much closer together and hit each other but when they air is hot the molecules aren’t as dense which means they are more spread out. so as the cold air molecules are closer together, more of them could fit inside the balloon but with hot air since the molecules are so far apart not as many can fit in as they take up more as they spread and hit the side of the balloon