Sharon,
You have given a lovely example of discovery learning, of slowly allowing a process to happen and to let the children wonder and explore.
Depending on their understanding (since this is about how acids react with carbonates, which is very much 2nd level — see this article which gives the chemical reactions https://www.scienceofcooking.com/eggs/naked-egg-experiment.html), they may come up with additional questions that they could investigate.