Hello!
The observation that would provide the best evidence that a chemical reaction occurred is that The baking soda and hydrochloric acid combined, and bubbles formed.
When baking soda (NaHCO₃) and Hydrochloric Acid (HCl) combine, the following reaction happens:
NaHCO₃ + HCl → NaCl + H₂O + CO₂(g)↑
The gaseous Carbon Dioxide (CO₂) generated in this reaction is the responsible for the bubbles. The releasing of this gas is an evidence that a chemical reaction occurred between NaHCO₃ and HCl.
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