How to Actually Run the Reaction in a Bag Lab
The standard setup uses a Ziploc bag, baking soda, and vinegar, or alternatively calcium chloride and sodium bicarbonate. The goal is to capture a gas-producing reaction in a sealed environment so you can observe mass conservation, gas evolution, and temperature change simultaneously. It sounds simple. It is not always straightforward in practice. Here is what the standard procedure looks like, along with the chemical details and what students should report back. Part A: Baking Soda and Vinegar
You start by measuring out roughly 3 grams of sodium bicarbonate (baking soda) into a small piece of tissue paper or a gelatin capsule. This acts as a packet you can drop in without spilling. Then you measure about 20 mL of white vinegar (5% acetic acid) using a graduated cylinder and pour it directly into the bag before sealing. Slide the tissue packet inside, seal the bag almost completely leaving a small gap to push out excess air, and then seal it fully. Shake or invert the bag to start the reaction. The balanced equation is: NaHCO3 + HC2H3O2 NaC2H3O2 + H2O + CO2
Students should report that the bag inflates from carbon dioxide gas, the bag feels cold to the touch, and no mass is lost because everything remains sealed. The endothermic nature of the reaction is what causes the temperature drop. Most lab manuals accept that the reactants are sodium bicarbonate and acetic acid, the products are sodium acetate, water, and carbon dioxide, and the evidence of a chemical change includes gas production, temperature change, and a smell difference. Part B: Calcium Chloride and Sodium Bicarbonate This version uses about 5 grams of calcium chloride (CaCl2) and 3 grams of sodium bicarbonate (NaHCO3), both placed in separate compartments or wrapped separately so they do not mix until the bag is sealed and shaken. Water is added, usually around 10 to 15 mL. The reaction produces carbon dioxide gas and a precipitate of calcium carbonate.
Get the Full Details
The equation is: CaCl2 + 2NaHCO3 CaCO3(s) + 2NaCl + H2O + CO2 The bag inflates and you will see a white solid forming inside. This one is slightly exothermic compared to the vinegar reaction, so the bag may feel warm rather than cold. Students often mix these two up on their answer sheets because both produce gas. The distinguishing factor is the temperature change direction and the precipitate in the calcium chloride version.
I ran this lab for about eight years in a high school setting, and the most consistent failure point was the bag not inflating properly with the baking soda and vinegar combination. The issue usually comes down to the vinegar amount being too low or the seal not being tight enough. When I had a bag that only puffed up minimally, I found that increasing the vinegar to 25 mL and making sure the seal was wiped clean and dry along the zipper track before closing it solved the problem. A microscopic gap from residue lets CO2 escape slowly enough that the bag appears flat even though the reaction is happening. Another thing nobody warns you about: the tissue packet wrapping the baking soda needs to be small and thin. Thick layers of tissue paper slow down the dissolution of the bicarbonate dramatically, which means the reaction takes much longer and the temperature change is spread out over a longer period. Students reading the thermometer miss the peak because it happened gradually. Using a single layer of thin tissue or a pre-made gelatin capsule gives you a much sharper result and a clearer temperature reading. For the answer key portion, here are the typical questions and expected responses:
What type of reaction occurred? Both are double displacement reactions followed by a decomposition step where carbonic acid breaks into water and carbon dioxide. Is mass conserved? Yes. The total mass before and after remains the same within measurement tolerance, typically within 0.1 to 0.3 grams depending on your balance precision. What evidence indicates a chemical change? Gas production, temperature change, and in the calcium chloride version, precipitate formation.
Which reaction is exothermic and which is endothermic? The vinegar and baking soda reaction is endothermic. The calcium chloride and baking soda reaction leans exothermic. If you need to calculate the theoretical yield of CO2 from the baking soda and vinegar reaction, you use 3 grams of NaHCO3 divided by its molar mass of 84.01 g/mol to get roughly 0.0357 moles. Since the molar ratio is 1:1, you produce 0.0357 moles of CO2, which at STP occupies about 0.8 liters. A standard quart-size bag has an internal volume of roughly 1 liter, so the math checks out and the bag should visibly inflate under ideal conditions. The main limitation of this lab is that it is fairly qualitative. If you want quantitative rigor, you need a precise balance, careful measurement of reagents, and a way to ensure zero gas leakage. Most classroom setups do not achieve that level of control, so the answer keys reflect expected observations rather than exact numerical agreement. Some instructors prefer the calcium chloride version for this reason because the precipitate gives a second observable product beyond just gas, making the chemical change harder to dispute.
For the answer key document itself, you generally want to include the chemical equations, the classification of each reaction type, the observed physical changes, and the mass conservation verification. Make sure students show their work on the stoichiometry portion because that is where most point deductions happen.
