What is a primary standard in chemistry?

A primary standard in chemistry is defined as a highly pure substance that can be used to calibrate analytical instruments or to prepare standard solutions. For a substance to be considered a primary standard, it must meet several criteria, including stability, high purity, and solubility in water.

Option A accurately describes a primary standard as a water-soluble substance that is stable and available in pure form. This quality is crucial because in titration processes or when preparing solutions, a primary standard must reliably yield consistent results without chemical reactions that could alter its concentration or composition.

The other options do not meet the necessary criteria for a primary standard. For instance, a substance that causes oxidation reactions does not inherently imply any of the characteristics needed to serve as a primary standard. Similarly, a compound containing both carbon and oxygen does not automatically qualify as a primary standard, as it may not be pure or stable. Lastly, a mixture of various soluble substances cannot function as a primary standard since its composition may change over time, affecting the accuracy of any measurements taken. Thus, the description of a primary standard aligns perfectly with the qualities stated in Option A.