What could develop in the stomach if it were not protected from the stomach acid

Which class of molecules functions as chemical signals? hormones water carbohydrates proteins

Light waves travel than sound waves.

When the chemicals iron sulfide (fes) and hydrochloric acid (hcl) are combined, bubbles appear from the mixture. 1. does the appearance of bubbles indicate a physical or chemical change? 2. why do the bubbles indicate this change? 3. what property is this?