What class of chemical compounds must contain hydrogen?

The class of chemical compounds that must contain hydrogen is organic compounds. Organic chemistry is fundamentally defined by the presence of carbon and hydrogen atoms. While many organic compounds also contain other elements like oxygen, nitrogen, sulfur, and halogens, the inclusion of hydrogen is a characteristic feature that helps classify them as organic.

In contrast, inorganic compounds can exist without hydrogen; for instance, many salts, oxides, and minerals are classified as inorganic and do not contain hydrogen as part of their fundamental structure. Salts, specifically, are made up of cations and anions, often lacking hydrogen in their formulas. Minerals are solid inorganic substances found naturally in the earth and may or may not include hydrogen. Therefore, the defining aspect of organic compounds is their hydrogen content, making them a distinct class of chemical compounds.