|MadSci Network: Physics|
The reason warm water is more buoyant than cold is that its molecules have more energy and are moving around more. This makes warm water less dense and it will rise above denser, colder water. The main source of error in your experiment would be getting exactly the same mass of water into the balloons to make it a fair test. Also the balloons themselves may differ very slightly in mass or ability to retain water but this effect would be too small to be noticeable. If you repeat the experiment with two containers, one with warm and one with cold water, you should see the balloon filled with room temperature water float in the cold but sink in the hot. Why not try it ?
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