Which of the following containers holds a pure substance?

• A. A container filled with air
• B. A container filled with sand and iron filings
• C. A container filled with saltwater
• D. A container filled with distilled water (H_2O)

Answer: (D)

Verified step by step guidance

1. Understand the definition of a pure substance: it is a material that has a constant composition and distinct chemical properties throughout, consisting of only one type of particle or compound.
2. Analyze each container's contents: air is a mixture of gases, sand and iron filings is a heterogeneous mixture of solids, saltwater is a homogeneous mixture (solution) of salt dissolved in water.
3. Recognize that distilled water (H_2O) is composed of only one type of molecule, with no other substances mixed in, making it a pure substance.
4. Conclude that among the options, only the container filled with distilled water meets the criteria of a pure substance.
5. Summarize that mixtures (air, sand with iron filings, saltwater) are not pure substances because they contain more than one component, whereas distilled water is a pure substance.