Balloon Expansion

from SCIENCE.COM by sanjaytikariya

1. Stretch the opening of the balloon around the open end of the bottle.
2. Set the bottle on the hot plate.
3. Give the bottle and balloon apparatus a few moments to get warm. This may take a while, but you won't want to take your eyes off of the balloon.
4. Once the bottle is warm, what happens to the balloon? Wow! It's filling with air. But how?

How does it work?

So what made the balloon begin to fill with air? We could be wrong here, but we're pretty sure that there isn't a tiny person in there blowing as hard as he can. Instead, you are seeing the expansion of air. Unlike water, which expands when it is cooled, air expands when it is heated. Since the bottle was full of air when you stretched the balloon over it, it only had one place to go when it began expanding… into the balloon!

DENSITY

Density

A pound coin floats in mercury due to the buoyancy force upon it.

A density column containing some common liquids and solids. From top: baby oil, rubbing alcohol (with red food coloring), vegetable oil, wax, water (with blue food coloring), and aluminum.

If the weight of an object is less than the weight of the displaced fluid when fully submerged, then the object has an average density that is less than the fluid and when fully submerged will experience a buoyancy force greater than its own weight. If the fluid has a surface, such as water in a lake or the sea, the object will float and settle at a level where it displaces the same weight of fluid as the weight of the object. If the object is immersed in the fluid, such as a submerged submarine or air in a balloon, it will tend to rise. If the object has exactly the same density as the fluid, then its buoyancy equals its weight. It will remain submerged in the fluid, but it will neither sink nor float, although a disturbance in either direction will cause it to drift away from its position. An object with a higher average density than the fluid will never experience more buoyancy than weight and it will sink. A ship will float even though it may be made of steel (which is much denser than water), because it encloses a volume of air (which is much less dense than water), and the resulting shape has an average density less than that of the water.

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