- Any proportion
- Fixed proportion
- Random ratio
- Equal masses only
Compounds are chemical substances formed when two or more elements combine in fixed, definite proportions by mass, governed by the law of definite proportions (also called Proust’s law). This fundamental principle, established by Joseph Proust in the late 18th century, states that a pure chemical compound always contains the same elements combined in the same constant mass ratio, regardless of its source or preparation method. For example, water (H₂O) always has 8 grams of oxygen for every 1 gram of hydrogen—never varying proportions. Similarly, sodium chloride (NaCl) contains 23 grams of sodium to 35.5 grams of chlorine. This fixed composition distinguishes compounds from mixtures, where substances combine in any proportion without chemical bonding (like sand and salt). The law of definite proportions, along with the law of multiple proportions (elements can combine in different ratios forming different compounds—e.g., CO and CO₂), established stoichiometry—the quantitative relationship between reactants and products in chemical reactions. Understanding fixed proportions is essential in chemical analysis, synthesis, and industrial production, enabling precise calculations of reactant quantities and expected yields.
