A chemical reaction occurs when mixing water and alka seltzer together. The key hint is that there are bubbles which signify there is gas formation. A chemical reaction normally occurs when there is a temperature change, a color change, or gas formation. Remember that adding hot water, adding food coloring, or stirring vigorously to create bubbles does not mean that the reaction is a chemical reaction. It must solely be a result of the two chemicals combined and not out of human interference. Hope this helps!