Let me be real: I’ve taught middle school science in classrooms where chaos was the norm—behavior issues, no designated lab space, and not nearly enough time. The idea of pulling off a lab felt exhausting. That’s why I started creating science labs that are simple, engaging, and realistic for everyday classrooms.
This Conservation of Mass lab is one of those gems. It’s straightforward, budget-friendly, and eye-opening for students. Even better? You don’t need a fancy lab or delicate equipment—just Ziploc bags, baking soda, vinegar, and a scale.
If you’re nervous about managing labs, I’ve got your back. I wrote this with you in mind. Keep reading to get all my tips!
Need the student worksheet that goes with this lab? Grab it here!
2 Ziploc bags (quart or sandwich size)
2 small plastic cups (or test tubes)
Baking soda
Vinegar
Measuring cups/spoons
Digital scale (as sensitive as you can find)
Paper towels
Goggles (recommended)
You’ll run two trials—one in an open system and one in a closed system—to show how gases affect the measured mass during a chemical reaction. Here’s the gist:
Place vinegar in the Ziploc bag.
Carefully add baking soda to a small cup and nestle it upright inside the bag (don’t mix yet).
Seal the bag completely for Trial #1 (Closed System), and leave it open for Trial #2 (Open System).
Measure and record the mass before the reaction.
Tip the baking soda cup to mix the reactants and start the fizz!
Wait for bubbling to stop, then record the mass again.
Compare your data and discuss: Where did the mass go?
Cup in the bag = finesse required. It takes some coordination to get the cup in the bag without spilling. Make sure you have a cup that fits easily and a student who’s steady enough to pull it off.
Exact measurements don’t really matter. The goal is to observe change in mass, not worry about being precise. That said, larger amounts will make the mass change more visible, so if you want that wow factor, go big—but…
Beware the pop! My first “sealed” bag exploded. Oops. Gas buildup is real. Try squeezing excess air out before sealing, or be prepared to hear a loud pop! mid-lesson.
Equipment limits = teachable moments. Both of my trials ended up showing a 1-gram loss—even in the sealed bag. I suspect my scale isn’t sensitive enough, or maybe gas just isn’t measurable in this setup. But honestly? That led to a great class discussion. Students were still blown away by the visible bubbling and gas production. That reaction in the sealed bag is gold. Use it to spark curiosity and introduce terms like “closed system” and “chemical change” in a real, memorable way.
This lab hits on some heavy-hitter concepts—conservation of mass, reactants and products, chemical reactions, and open vs. closed systems—but in a way that feels hands-on and exciting.
Even if the data doesn’t come out textbook-perfect, you’ve got so many jumping-off points for discussion:
Why did we lose mass in the open bag?
Why didn’t we lose (much) in the sealed one?
What do we think happened to the gas?
How does this apply to real-life reactions?
The science is solid. The setup is manageable. The learning is real.
