Surprise—your balloon swelled enormously, but the weight actually dropped.
This result is especially confounding if you happen to be familiar with the law of conservation of mass: In any closed system, mass is neither created nor destroyed by chemical reactions or physical transformations. In short, the mass of the products of a chemical reaction must equal the mass of the reactants.
Did you really just violate the law of conservation of mass? You might be dying to know what’s going on, but wait, weight—why not figure it out for yourself?
The answer is below…but to avoid a spoiler, skip down to the Going Further section before reading on.
Alright, here’s the answer: Besides the chemical reaction, the only thing that changed in your sealed system was the volume. When you added the baking soda to the vinegar, the two combined to make carbon-dioxide gas, which inflated the balloon.
The expansion of the balloon changed the weight of your sealed flask because you and your entire experiment are submerged in a fluid: air.
Just like water, air is a fluid, and fluids buoy up objects. The upward buoyant force on any submerged object is equal to the weight of the fluid displaced by that object—this is known as Archimedes’ principle. By increasing the volume of your sealed flask, you cause it to displace more air, increasing the buoyant force on it and reducing its weight. Here's the thing to remember: Scales measure weight, not mass. The mass stayed the same due to the law of conservation of mass, but because of buoyancy, the weight went down!