You could let the air out of the balloon while it is under the water with a container filled with water upside down over it. And measure the water displacement.
Answer:
An object has a negative charge if it's atoms have more electrons. On the other hand if an object had more protons, then it would have a positive charge.
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Carbon is used to reduce the oxide of aluminium to get the metal, in case condensation happens during this process.
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There the picture is the answer from the internet
