Final answer:
An enzyme can enter a chemical reaction and come out unused and unchanged because it acts as a catalyst that is highly specific for its substrates. Unlike an enzyme, an acid or alkaline undergoes a change when participating in reactions, and a steroid typically functions as a signaling molecule rather than in catalysis.
Step-by-step explanation:
The substance that can enter a chemical reaction and emerge unused and unchanged is an enzyme. Enzymes are specialized proteins that act as catalysts in biochemical reactions, effectively speeding up the reaction without being consumed by it. They have the remarkable ability to be highly specific for their substrates, which are the organic reactants they act on. For example, salivary amylase is an enzyme that hydrolyzes only its specific substrate, amylose. This level of specificity is due to the unique three-dimensional structure of enzymes that allows them to bind only to certain molecules, facilitating the reaction of those molecules. In contrast, an acid or alkaline can participate in reactions by donating or accepting protons, and thereby they are changed in the process. A steroid, on the other hand, is generally a signaling molecule and is not involved in catalysis.