Which of the following substances can be broken down into simpler substances by chemical means?

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

Answer: (D)

Verified step by step guidance

1. Understand the difference between elements and compounds: Elements are pure substances that cannot be broken down into simpler substances by chemical means, while compounds are substances made of two or more elements chemically combined and can be broken down into simpler substances.
2. Identify each substance given: Oxygen gas (O_2) is an element composed of oxygen atoms, Gold (Au) is an element, Carbon (C) is an element, and Water (H_2O) is a compound made of hydrogen and oxygen atoms.
3. Recognize that elements like O_2, Au, and C cannot be chemically decomposed into simpler substances because they are already in their simplest form as pure elements.
4. Understand that water (H_2O), being a compound, can be broken down into its constituent elements (hydrogen and oxygen) by chemical means such as electrolysis.
5. Conclude that among the substances listed, only water (H_2O) can be broken down into simpler substances by chemical means.