Air molecules collide into the balloon with the same energy inside and outside the balloon. When the bottle is heated, the air molecules inside start moving faster. These molecules now collide into the balloon with more energy resulting in increased pressure. The increased pressure causes the balloon to expand.
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Answer:
Air molecules collide into the balloon with the same energy inside and outside the balloon. When the bottle is heated, the air molecules inside start moving faster. These molecules now collide into the balloon with more energy resulting in increased pressure. The increased pressure causes the balloon to expand.