Why is a Compound a Pure Substance but Not a Mixture?

Mixtures and compounds are both combinations of different substances. However, they differ in their chemical and physical composition and properties. This article will explore these differences to clarify why a compound is a pure substance while a mixture is not.

Mixtures: Physical Combinations

Mixtures are formed when two or more substances are physically combined but retain their individual properties. Unlike compounds, a mixture does not create a new substance with unique properties. Instead, the substances can be separated by physical means such as filtration, distillation, or chromatography.

Example: Saltwater is composed of sodium chloride (NaCl) and water (H2O). Each substance retains its original properties, meaning that the salt and water can be separated by physical processes without altering their chemical identities.

Compounds: Chemical Combinations

Compounds, on the other hand, are formed when two or more elements combine chemically to create a new substance with unique properties. A compound has properties that are distinct from its constituent elements, and it can only be separated into its elements through chemical reactions.

Example: Water (H2O) is a compound formed from the chemical combination of hydrogen and oxygen. It exhibits properties that are different from pure hydrogen or pure oxygen, such as its boiling point and density.

Key Differences Between Mixtures and Compounds

Composition: Compounds have a fixed composition, whereas mixtures have a variable composition. Properties: Compounds have uniform properties, while mixtures can have varied properties depending on the components. Separation: Compounds can only be separated into their elements through chemical reactions, while mixtures can be separated by physical means such as filtration, distillation, or chromatography.

Why is a Compound Considered a Pure Substance?

A compound is considered a pure substance because its composition is fixed, and it has a specific chemical formula. This means that the compound maintains consistent physical and chemical properties throughout the sample. For example, water (H2O) is a compound formed from hydrogen and oxygen in a 2:1 ratio, and it has unique physical and chemical properties such as its boiling point and density.

In contrast, a mixture is a combination of two or more substances that are not chemically bonded. The components of a mixture can vary in proportion and retain their individual properties. For instance, a salad is a mixture of various ingredients like lettuce, tomatoes, and cucumbers, each maintaining its own characteristics.

Conclusion

In summary, compounds are pure substances due to their consistent composition and predictable properties, while mixtures are not pure substances because they consist of multiple components that can vary in ratio and retain their individual properties.