Johnny was playing in the yard when a bee stung his arm. To ease the pain, his mother gathered baking soda, vinegar, and water. In a spoon, she combined the baking soda and water until it was the consistency of a paste. She then smoothed the baking soda onto the sting area. Next, she slowly dropped vinegar onto the baking soda paste. Bubbles began to form, and eventually, Johnny's pain subsided. At which stage of the procedure did a chemical reaction occur?