When you dropped the baking soda packet into the vinegar, the vinegar reacted with it to create carbon dioxide gas. When there was too much gas inside the bag, the seal broke and made a good pop!
In my first trial of this experiment, I didn’t have white vinegar on hand, only a small amount of apple cider vinegar, so I tried using half the amount called for and a different kind. I also didn’t pay much attention to the temperature of the water. I tried out two different kinds of quart-size bags. The first kind was a standard sandwich bag, but none of them were air-tight; they weren’t even water-tight! I tried two different generic store brands (Walmart and Wegmans), and the results were the same: the air escaped from the hole too fast to break the seal.I also tried quart-sized slide-zip bags, but rather than being too loose with the ingredients, the seal was too tight and didn’t budge! I had a real Goldilocks situation on my hands here.Finally, I tried Great Value brand quart-sized freezer bags with a zip top, and these worked perfectly for me with the experiment as written. I had bought white vinegar and paid better attention to the water temperature.
I could try the slide-zip bags with the full amount of white vinegar to see what results I get.See what happens when you change your variables around a bit, like I did. Change vinegar amounts, or try different kinds or sizes of bags.