Which of the following ionic compounds is commonly used as a building material?

• A. CaCO_3 (limestone)
• B. MgO (magnesium oxide)
• C. NaCl (salt)
• D. Fe_2O_3 (iron oxide)

Answer: (A)

Verified step by step guidance

1. Understand the question: It asks which ionic compound is commonly used as a building material. This means we need to identify a compound known for its structural or construction applications.
2. Review each compound's common uses: - \(\mathrm{CaCO_3}\) (calcium carbonate) is found in limestone, which is widely used in construction. - \(\mathrm{MgO}\) (magnesium oxide) is used in refractory materials but less commonly as a primary building material. - \(\mathrm{NaCl}\) (sodium chloride) is common table salt, not used in building. - \(\mathrm{Fe_2O_3}\) (iron oxide) is rust, which is not a building material itself but related to iron corrosion.
3. Focus on \(\mathrm{CaCO_3}\): Limestone, primarily composed of calcium carbonate, is a natural stone used extensively in building due to its durability and availability.
4. Compare with other options: While \(\mathrm{MgO}\) and \(\mathrm{Fe_2O_3}\) have industrial uses, they are not typically classified as building materials in the same way limestone is.
5. Conclude that \(\mathrm{CaCO_3}\) (limestone) is the correct answer because it is the ionic compound commonly used as a building material.