Which characteristic is shared by all pure substances?

• A. They exist only as solids at room temperature.
• B. They can be separated into simpler substances by physical means.
• C. They have a fixed and definite composition.
• D. They are always composed of more than one type of atom.

Answer: (C)

Verified step by step guidance

1. Understand the definition of a pure substance: A pure substance is a form of matter that has a constant composition and distinct chemical properties.
2. Recall that pure substances have a fixed and definite composition, meaning their chemical makeup does not vary regardless of the sample size or source.
3. Recognize that pure substances can exist in different states (solid, liquid, gas) at room temperature, so the statement about existing only as solids is incorrect.
4. Know that pure substances cannot be separated into simpler substances by physical means; separation requires chemical methods if the substance is a compound.
5. Understand that pure substances can be elements (composed of one type of atom) or compounds (composed of more than one type of atom chemically combined), so the statement about always having more than one type of atom is not true for all pure substances.