Law of Constant Proportions Explained

The Law of Constant Proportions, also known as the Law of Definite Proportions, is a fundamental principle in chemistry. It states that a specific chemical compound always contains the same elements in the same fixed ratio by mass, regardless of its source or method of preparation.

Application to Chemical Compounds

This law directly applies to pure chemical compounds. For example, water (H₂O) will always consist of hydrogen and oxygen atoms combined in a fixed mass ratio of approximately 1:8, whether it's sourced from a tap, a river, or synthesized in a lab.

Exclusion of Mixtures and Elements

• Mixtures of substances do not adhere to this law because their components can be present in variable proportions. For instance, air is a mixture of gases, and the percentage of nitrogen and oxygen can vary slightly.
• Elements are pure substances made of only one type of atom; the law describes the composition of compounds formed from elements.

Conclusion

Therefore, the Law of Constant Proportions is exclusively applicable to pure chemical compounds, affirming their definite composition.

The correct option is 1. Pure chemical compounds.