I teach elementary school science and have been puzzled by one of my lessons. I place baking soda in an empty balloon. I place vinegar in a flask. I place the opening of the balloon over the opening of the flask so that the two substances do not mix. I weigh them on a scale. I then lift the end of the balloon so that the two substances mix. I move my hand back and watch the scale. Even though the balloon inflates as I capture the gas that is created, the weight of the substances goes down. This seems to run counter to the law of conservation of mass. Am I missing something? Why does the weight drop?
– Durran Fewkes, via the ask a science question page.
Hello Durran,
Interesting question.
The short answer for this is buoyancy.
Long answer:
When the flask and the balloon setup is placed, it is inside a fluid (air) medium. And as it occupies a certain volume, the setup would experience a buoyant force. (If the density of the whole setup is less (hypothetically) than that of the air, it will float way, of course. But in reality, the density is high, and so it sinks.) Now when the baking soda and vinegar in the flask reacts, the gas inflates the balloon. This increases the volume of the whole system, which makes it less dense compared to the previous state. As the density decreases, the buoyant force on it increases a bit, making it to weigh lighter than before.
The law of conservation of mass is not violated here as the total mass remains the same, while the weight (mass × acceleration due to gravity) of the system is the one that changes.
Hope it helps! :)
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