What is meant by a pure substance? Give examples.

A pure substance is a form of matter that has a uniform and definite composition. It consists of only one type of particle, either atoms or molecules, and has consistent properties throughout the sample. Pure substances can be categorized into two types: elements and compounds.

Examples of Pure Substances:
Elements: These are substances that consist of only one type of atom. They cannot be broken down into simpler substances by chemical means.

Oxygen (O₂): A colorless, odorless gas that is essential for respiration.
Gold (Au): A yellow metallic element that is highly valued for jewelry and currency.
Compounds: These are substances formed when two or more elements are chemically bonded together in a fixed proportion. They have unique properties different from their constituent elements.

Water (H₂O): A compound made from hydrogen and oxygen, essential for all known forms of life.
Sodium Chloride (NaCl): Commonly known as table salt, it is composed of sodium and chlorine.
Key Characteristics of Pure Substances:
Uniform Composition: The composition of a pure substance is the same throughout the sample. For instance, a sample of pure water will always have the same ratio of hydrogen to oxygen.
Consistent Properties: Physical and chemical properties such as melting point, boiling point, density, and reactivity are uniform for pure substances. For example, pure gold always melts at 1064 degrees Celsius.
In summary, a pure substance is composed of a single type of particle with a fixed composition and distinct properties, whether it is an element like gold or a compound like water.