Wait, Weight, Don't Tell Me!

1. Put on your safety goggles.
2. Attach a balloon to the end of the funnel.
3. Using the funnel, pour two level teaspoons (10 mL) of baking soda into the balloon (see photo below). (Make sure the funnel doesn’t clog, and all the baking soda passes through the neck of the balloon.)
   
4. Pour about 1/2 cup (120 mL) of vinegar into the bottle or flask.
5. Attach the balloon to the flask, making sure the neck of the balloon is snugly seated below the lip or collar of the flask. As you work, keep the balloon hanging off to the side of the flask so none of the baking soda falls into the vinegar—yet.
   

To begin, carefully put the sealed flask onto the scale and write down its starting weight.

You’re about to tip the balloon’s contents into the flask. What do you think will happen? Will the weight go up, down, or stay the same? Why?

Now that you have your guess, flip up the balloon to empty all of the baking soda into the flask (see photo below).

Stand back, watch the balloon grow, and watch the scale (see photo below).

Write down the final weight when the reaction is over.

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!

Consider possible explanations for the weight change: Did the balloon leak? Did something funny happen to the scale? What else might be going on? Plan an experiment to test your theory, gather equipment, and carry it out.

For an illuminating variation on the original experiment, try combining your chemicals while they’re sealed inside a 2-liter bottle. Getting things to mix only after you’ve sealed the bottle is an engineering design challenge unto itself. Caution: Do not exceed the recommended amounts of 1/2 cup (120 mL) vinegar and 2 teaspoons (10 mL) baking soda.

To confirm Archimedes’ principle, measure the volume of the balloon and use the known density of air (0.001225 g/cm³ at 15° C at sea level) to calculate exactly the weight of air displaced by your expanding balloon. Does the weight loss of your flask match the theoretical prediction?

This activity is meant to spark more experimentation. Having a variety of supplies on hand will allow for creative investigation into this phenomenon.

This idea was first introduced to us by visiting fellow Eleanor Duckworth of Harvard University.