Which of the following is a pure substance?

• A. Water
• B. Granite
• C. Sand
• D. Air

Answer: (A)

Verified step by step guidance

1. Understand the definition of a pure substance: A pure substance is a material that has a constant composition and has consistent properties throughout the sample. It consists of only one type of particle.
2. Identify the composition of each option: Water (H₂O) is a compound made of hydrogen and oxygen atoms bonded together. Granite is a mixture of minerals, sand is a mixture of different particles, and air is a mixture of gases.
3. Recognize that a compound like water is considered a pure substance because it has a definite and fixed composition, unlike mixtures which can vary in composition.
4. Consider the properties of water: Water has consistent properties such as boiling point, freezing point, and density, which are characteristic of pure substances.
5. Conclude that water is the only option that fits the definition of a pure substance, as it is composed of a single type of molecule, H₂O, throughout.