The Sealed Bag and the Open Cup
On St. Thomas, a fifth-grade team investigates a fizzing reaction. Group A puts 5.0 g of baking soda into a heavy zip-top bag, adds 30 mL of vinegar, squeezes out the extra air, seals the bag, and finds the sealed bag's mass is 78.4 g. After the fizzing stops and the bag is shaken, the sealed bag still reads 78.4 g. Group B uses the same amounts in an open plastic cup on the same balance: 78.4 g before mixing, 76.5 g after the fizzing stops. Group B's journal says, "The missing 1.9 g means the baking soda was used up and turned into heat." The class must decide what the two results, taken together, actually show about matter.
1. Using both groups' numbers, explain what happened to the 1.9 g and why Group A's sealed bag did not change mass. Then predict what Group A would see if their bag had a tiny pinhole leak.
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What the numbers show
- Both groups started with 78.4 g of the same materials, so the starting matter was identical.
- Group A's mass did not change because the gas made by the reaction (bubbles) stayed trapped inside the sealed bag. The gas has mass, so nothing left the system.
- Group B lost 1.9 g because the gas escaped into the room. The 78.4 g − 76.5 g = 1.9 g is the mass of gas that left the cup, not matter that vanished.
Why this counts as evidence for conservation of matter The two setups differ in only one way that matters: one is closed and one is open. The closed one keeps its mass; the open one loses exactly the mass that escaped as gas. That pattern - not the fizzing itself - is the evidence.
Prediction for a pinhole leak Group A's reading would slowly drop below 78.4 g as gas escaped through the hole, and the drop would be smaller than Group B's 1.9 g only if the hole let gas out slowly before the bag was massed. The final mass would depend on how much gas had escaped by then.
2. Evaluate Group B's claim that matter was "used up and turned into heat." Describe one change to Group B's setup that would let them observe where the missing mass went, and predict the result.
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Evaluating the claim The claim does not fit the evidence. If baking soda were truly turned into heat and disappeared, then Group A's sealed bag would also have lost mass, because the same reaction happened inside it. Group A's bag did not change mass. The only difference between the groups is whether gas could escape, so escaping gas - not disappearance - explains the 1.9 g.
A better test Stretch a balloon over the mouth of Group B's cup (or use a bottle with a balloon) and mass the whole thing before and after mixing.
Predicted result
- The balloon inflates as gas collects.
- The total mass of bottle + balloon + contents stays at 78.4 g after the fizzing stops, because the gas is now trapped.
- This shows the "missing" matter was gas all along - it was leaving the open cup, not being destroyed.
A claim only counts as supported if the evidence could have come out the other way, and here the sealed-bag result rules it out.
3. Group B repeats the test with 10.0 g of baking soda and the same 30 mL of vinegar, and the open cup again loses about 1.9 g. What does this tell the class about what controlled how much gas formed?
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What stayed the same despite adding more baking soda
- Doubling the baking soda did not double the gas; the mass loss stayed near 1.9 g.
- If baking soda were the ingredient that limited the reaction, adding more should have produced more gas and a bigger loss.
- So the vinegar - the amount that did not change - must have been used up first, and the extra baking soda was simply left over in the cup.
How to check this reasoning Run the reverse test: keep the baking soda at 5.0 g and double the vinegar. If the gas amount goes up, the vinegar is confirmed as the limiting ingredient; if it stays the same, the baking soda is limiting.
Why this is a fair conclusion, not a guess The class changed only one thing (baking soda) and measured one result (mass lost). The result did not change with the change, which is exactly the pattern expected when a different ingredient runs out first. Leftover powder visible in the cup after fizzing stops would be extra supporting evidence.