Chemistry Regents Practice Test

Which substance cannot be broken down by a chemical change?

1. Oxygen

2. Mercury

3. Carbon Dioxide

4. Water

The correct answer is: Mercury

The correct answer is that mercury cannot be broken down by a chemical change because it is an element. Elements are pure substances that cannot be decomposed into simpler substances through chemical reactions. Each element is made up of only one type of atom, and no chemical change can separate the atoms in an element into simpler forms. In contrast, the other substances listed—oxygen, carbon dioxide, and water—are compounds or molecules that can undergo chemical changes to separate into their constituent elements or simpler compounds. For example, water can be electrolyzed to produce hydrogen and oxygen gases, and carbon dioxide can be broken down into carbon and oxygen. Thus, mercury stands apart as an element that remains unchanged through any chemical process.