Which of the following substances can be broken down by a chemical change?

• A. Oxygen gas (O_2)
• B. Water (H_2O)
• C. Gold (Au)
• D. Nitrogen gas (N_2)

Answer: (B)

Verified step by step guidance

1. Understand the concept of a chemical change: it involves breaking chemical bonds in a substance to form new substances with different chemical compositions.
2. Analyze each substance to determine if it can be broken down into simpler substances by a chemical change:
3. Oxygen gas (O_2) is an elemental molecule composed of two oxygen atoms; it cannot be broken down into simpler substances by chemical means because it is already an element.
4. Water (H_2O) is a compound made of hydrogen and oxygen atoms chemically bonded; it can be broken down into hydrogen gas and oxygen gas through chemical processes such as electrolysis.
5. Gold (Au) is a pure element and cannot be chemically broken down into simpler substances, and nitrogen gas (N_2) is also an elemental molecule that cannot be broken down chemically.