Elements, Compounds, and Mixtures

Overview

Matter can be classified as pure substances or mixtures. Pure substances are elements or compounds, while mixtures contain two or more substances not chemically combined.

Key concepts

• An element is made of only one kind of atom (e.g., oxygen, gold).
• A compound is formed when elements combine chemically in fixed ratio (e.g., water).
• A mixture contains substances mixed physically and can be separated.
• Mixtures are homogeneous (uniform) or heterogeneous (non-uniform).
• Compounds have properties different from their elements.

Important terms / formulae

• Element: a substance of one kind of atom.
• Compound: a substance of two or more elements chemically joined.
• Mixture: two or more substances physically combined.
• Symbol: a short form for an element, e.g., O for oxygen.

Solved example or key process

Water (H₂O) is a compound made of hydrogen and oxygen in a fixed ratio. Although hydrogen burns and oxygen supports burning, water puts out fire. This shows a compound has properties different from its elements.

Important questions

1. Differentiate between an element and a compound.
2. How is a mixture different from a compound?
3. Give two examples each of homogeneous and heterogeneous mixtures.
4. Why does a compound have a fixed composition?

Quick revision

Elements have one type of atom; compounds are elements chemically joined in fixed ratios; mixtures are physical combinations that can be separated. Compounds differ in properties from their elements.