Ms. Pitt set up a classroom demonstration to show what happens when a sample of zinc metal is added to a flask containing hydrochloric acid (HCl). The diagram shows what the students in the classroom observed during the demonstration. The students made two other observations: The flask felt warmer after adding the sample. The sample became smaller over time.Explain why the change to the balloon is evidence of a chemical reaction. Captionless Image The balloon filled up with a gaseous substance. This gas is a new substance formed by the reaction between zinc, which is a solid, and HCl, which is a liquid. The formation of a new substance is evidence of a chemical reaction . The balloon filled up with HCL. No new substance formed by the reaction between zinc, which is a solid, and HCl, which is a liquid. There is no formation of a new substance and no evidence of a chemical reaction new substance is evidence of a physical reaction.