1. Definition:

* A pure substance is a single type of matter with a fixed composition and uniform properties throughout.

2. Characteristics:

* Fixed Composition: The ratio of elements in a pure substance is always the same. For example, water (H₂O) always has two hydrogen atoms and one oxygen atom.

* Uniform Properties: All parts of a pure substance have the same physical and chemical properties. This means the melting point, boiling point, density, etc., are constant.

* Cannot be Separated by Physical Means: You can't use simple techniques like filtration, evaporation, or magnetism to separate a pure substance into different components.

3. Examples:

* Elements: Gold (Au), Oxygen (O₂), Carbon (C)

* Compounds: Water (H₂O), Salt (NaCl), Sugar (C₁₂H₂₂O₁₁)

4. How to Test:

* Melting and Boiling Points: A pure substance will have a specific, sharp melting and boiling point.

* Density: The density of a pure substance will be consistent.

* Chemical Reactions: Pure substances will react predictably with other substances.

* Spectroscopy: Techniques like infrared spectroscopy can identify the unique chemical bonds present in a pure substance.

5. Distinguishing from Mixtures:

* Mixtures are combinations of two or more substances that are not chemically bonded. They can have variable compositions and properties. You can often separate components of a mixture using physical methods.

* Examples of mixtures: Saltwater, air, sand and water.

Important Note:

* It's difficult to find perfectly pure substances in everyday life. Many materials are very close to pure but contain tiny traces of impurities.

Let me know if you'd like more detail on any of these points!