Final answer:
Compounds form in a fixed ratio of the elements that compose them, exemplified by water (H₂O) and sodium chloride (NaCl). The law of definite proportion ensures that a compound's composition is always consistent.
Step-by-step explanation:
Compounds are a type of chemical structure that form in a fixed ratio. This fixed proportion means that a given compound, regardless of the amount, will always contain the same elements in the same ratio. For example, water (H₂O) is a compound that consists of two hydrogen atoms bonded to one oxygen atom, and this ratio is constant. Similarly, sodium chloride (NaCl) is made up of one sodium atom for every chlorine atom. The law of definite proportion, also known as the law of constant composition, supports this characteristic, stating that the elemental composition of a compound is always the same.
Understanding the fixed nature of a compound's composition is essential when learning about chemical reactions, as it helps to predict the outcomes of reactions and to balance chemical equations. When naming compounds and writing their chemical formulas, one must know the elements within the compound and how many atoms of each element are combined, as this will dictate the chemical formula. Compounds have distinct properties that may be very different from the elements that compose them.