Chemistry Regents Practice Test 2025 - Free Chemistry Regents Practice Questions and Study Guide

The substance that cannot be broken down by a chemical change is oxygen. As a pure element, oxygen is made up of only one type of atom, meaning it cannot be decomposed into simpler substances by chemical means. During a chemical change, elements can react with one another to form compounds, but a pure element like oxygen remains unchanged and cannot be split into anything simpler.

The other substances listed are compounds or elements that can be chemically altered. Mercury (option B) is a pure element, so it retains its identity and cannot be broken down into simpler substances either, but the question specifically regards substances that can be broken down chemically, which does not apply to pure elements. Carbon dioxide and water are compounds made up of different elements, and they can be chemically decomposed into their constituent parts under certain conditions. In summary, while both oxygen and mercury are not able to be broken down chemically, pure elements like oxygen serve as the most straightforward example for this question.