Key Characteristics:
* Fixed Chemical Composition: The substance is made up of only one type of molecule or one type of atom. For example, pure water is always H₂O, pure gold is always Au.
* Uniform Properties: All parts of the pure substance have the same physical and chemical properties. This means the substance will have a consistent melting point, boiling point, density, color, etc.
* Cannot be Separated by Physical Means: Pure substances cannot be broken down into simpler substances by physical methods like filtration, evaporation, or magnetism.
Examples of Pure Substances:
* Elements: Gold (Au), Oxygen (O₂), Carbon (C)
* Compounds: Water (H₂O), Table Salt (NaCl), Sugar (C₁₂H₂₂O₁₁)
Important Note:
It's crucial to understand that pure substances are not always "pure" in the everyday sense. For example, drinking water can contain dissolved minerals and gases, making it a mixture. However, in the scientific context, it's still considered pure water as long as the majority of the molecules are H₂O.
Contrast with Mixtures:
Mixtures, on the other hand, are composed of two or more different substances that are not chemically bonded. They can be separated by physical means, and their properties vary depending on the composition.
In summary, the defining feature of a pure substance is its fixed chemical composition and uniform properties, which makes it distinct from mixtures.
