Which of the following substances cannot be broken down by chemical means?

• A. Oxygen (O_2)
• B. Gold (Au)
• C. Water (H_2O)
• D. Carbon dioxide (CO_2)

Answer: (B)

Verified step by step guidance

1. Understand the concept of chemical decomposition: substances that can be broken down by chemical means are called compounds, while substances that cannot be broken down further by chemical reactions are elements.
2. Identify the nature of each substance given: Oxygen (O_2) is a molecule made of two oxygen atoms, but oxygen itself is an element; Water (H_2O) is a compound made of hydrogen and oxygen atoms; Carbon dioxide (CO_2) is a compound made of carbon and oxygen atoms; Gold (Au) is a pure element.
3. Recall that elements are the simplest forms of matter and cannot be chemically decomposed into simpler substances, whereas compounds can be broken down into their constituent elements by chemical reactions.
4. Since Gold (Au) is an element, it cannot be broken down by chemical means, unlike water and carbon dioxide which are compounds and can be decomposed chemically.
5. Therefore, the substance that cannot be broken down by chemical means is Gold (Au).