Which property can be used to identify a pure substance as an element?

• A. It has a variable composition.
• B. It cannot be broken down into simpler substances by chemical means.
• C. It can be separated into components by physical processes.
• D. It reacts with acids to produce hydrogen gas.

Answer: (B)

Verified step by step guidance

1. Understand the definition of an element: An element is a pure substance that cannot be broken down into simpler substances by chemical means.
2. Recognize that variable composition is a characteristic of mixtures or compounds, not elements, since elements have a fixed composition.
3. Recall that physical processes can separate mixtures but cannot break down elements into simpler substances.
4. Note that reacting with acids to produce hydrogen gas is a chemical property of some metals but does not define an element universally.
5. Conclude that the key identifying property of an element is that it cannot be chemically decomposed into simpler substances.