Law of Constant Proportion Explained

The Law of Constant Proportion, also known as the Law of Definite Proportions, states that a pure chemical compound always contains the same elements in the exact same proportion by mass, regardless of its source or method of preparation.

Applicability Analysis

• Ionic Compounds: These are specific chemical compounds formed by electrostatic attraction between ions. They adhere strictly to the Law of Constant Proportion because their constituent elements are always present in a fixed ratio. For example, sodium chloride (NaCl) always consists of sodium and chlorine in a 1:1 ratio by atoms, and specific mass proportions.
• Elements: While the law is primarily stated for compounds, elements themselves are pure substances. The law is about the composition *within* a substance. Elements represent the fundamental building blocks and do not combine in variable proportions within themselves. The law dictates how elements combine to form compounds.
• Compounds: This is the direct application of the law. All chemical compounds, whether formed naturally or synthetically, maintain a constant proportion of their constituent elements by mass. Water (H₂O) is always ~88.8% oxygen and ~11.2% hydrogen by mass.
• Mixtures: Mixtures consist of two or more substances that are not chemically bonded. The components of a mixture can be present in variable proportions. For instance, air is a mixture of gases like nitrogen, oxygen, and argon, and their proportions can vary. Saltwater is another example; the amount of salt dissolved in water can change. Therefore, the Law of Constant Proportion is NOT applicable to mixtures.

Conclusion

Based on the analysis, the Law of Constant Proportion defines the fixed composition of chemical compounds. Mixtures, by definition, do not have a fixed composition and thus do not follow this law.

The correct option is Mixtures.