The unit of purity for a chemical substance is typically expressed as a percentage (%) or as a decimal fraction. It represents the proportion of the desired chemical compound present in the substance compared to the total mass or volume. For example, if a chemical substance has a purity of 99%, it means that 99% of its mass or volume consists of the desired compound, while the remaining 1% may consist of impurities, contaminants, or other substances. Purity is an important parameter used to assess the quality and consistency of chemical substances in various industries, research, and analytical applications.