A "chemical" is a substance made up of atoms and molecules that can be found in nature or created in a laboratory. Chemicals can be simple, like water (H2O), or complex, like proteins that make up living organisms. They play a crucial role in everything around us, from the air we breathe to the food we eat. Chemicals can undergo changes through reactions, resulting in new substances. For example, when vinegar (acetic acid) is mixed with baking soda (sodium bicarbonate), they react to produce carbon dioxide gas. Understanding chemicals helps us in various fields, including medicine, environmental science, and engineering.