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.
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