Open Cup and Sealed Bag in a Baking Soda Reaction
A group measures 5.0 g of baking soda and 40.0 g of vinegar. In Trial 1 they mix them in an open plastic cup on a balance. The balance reads 45.0 g before mixing and 43.6 g after the bubbling stops. The cup feels slightly cooler during the reaction. In Trial 2 they place the same amounts in a sealed zipper bag with a small balloon attached, press most extra air out before sealing, and record 45.4 g before mixing and 45.4 g after mixing. The balloon inflates. A student says Trial 1 proves that mass is not conserved in chemical reactions, while another says the missing 1.4 g turned into energy. The teacher asks them to reconcile the two trials using the law of conservation of mass and a particle model.
1. Explain the 1.4 g difference in Trial 1 and why Trial 2 shows no mass change. Use particle reasoning.
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Baking soda and vinegar react to form sodium acetate, water, and carbon dioxide gas. In Trial 1 the cup is open, so carbon dioxide escapes into the room. The final mass is lower by the mass of that escaped gas: 45.0 g - 43.6 g = 1.4 g. The atoms are not destroyed; they are rearranged into new products, and some leave the open cup as gas.
In Trial 2 the bag is sealed and the balloon captures the carbon dioxide. Because no gas escapes, the total mass stays 45.4 g. The particle model explains this: the same atoms are present before and after, just bonded differently. The 1.4 g is escaped carbon dioxide, not missing matter.
2. A student claims the missing 1.4 g was converted to energy. Evaluate this claim for this reaction.
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The claim is not supported. In a chemical reaction, mass is conserved. Energy is released or absorbed as bonds break and form, and there is a tiny mass equivalent to that energy, but for a reaction this size the mass change is far too small to measure on a balance that reads to 0.1 g, let alone account for 1.4 g. The 1.4 g difference is explained by carbon dioxide gas escaping from the open cup.
When the system is sealed, the mass remains 45.4 g. That is direct evidence against the idea that 1.4 g of matter became energy. The student is misapplying the mass-energy relationship; it does not explain ordinary chemical mass loss in an open container.
3. Design one modification to Trial 1 that would let the group test conservation of mass without using a balloon, and predict the result.
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Use a closed, rigid container, such as a flask with a stopper, or seal the reaction inside a thick zipper bag. Measure the mass of the container plus reactants before mixing, then mix without opening the system, and measure again after the reaction. Keep the same container and the same amounts of reactants so the comparison is fair.
Prediction: the total mass will remain the same, within the balance's precision, because the carbon dioxide cannot escape. If a flexible closed container is used, it may expand as gas is produced, but the total mass will not change. The key is that the system must be closed so gas cannot leave.