Matter is anything that has mass and occupies space. It exists in different physical states – solid, liquid, and gas – and is made up of tiny particles called atoms and molecules. Based on composition, matter can be classified into two broad categories: pure substances and mixtures. ^[1-4]

A pure substance is a type of matter that has a fixed composition and uniform properties throughout. It consists of only one type of particle—either an element or a compound—and cannot be separated into simpler substances by physical methods such as filtration or evaporation.

Properties ^[1-6]

1. Fixed Melting and Boiling Points

Because pure substances have a consistent composition, they melt and boil at specific temperatures. This property helps distinguish them from mixtures, which usually have a range of melting or boiling points.

Example: Pure water boils at 100°C and freezes at 0°C at standard atmospheric pressure.

2. Uniform Composition and Appearance

A pure substance looks the same throughout and is composed of identical particles—either atoms (in elements) or molecules (in compounds). It has no visible layers or differences in texture, color, or phase.

Example: A piece of pure aluminum has a consistent silvery appearance and density, no matter where you examine it.

Example: A piece of pure aluminum has the same silvery appearance and density throughout, no matter where you cut or examine it.

3. Consistent Chemical Behavior

Since a pure substance contains only one type of particle, it behaves predictably during chemical reactions. Its reactivity does not change unless it is altered or contaminated.

Example: Pure hydrogen gas (H₂) consistently reacts with oxygen (O₂) to form water (H₂O), releasing energy.

4. Cannot Be Separated by Physical Methods

The components of a pure substance cannot be separated using physical techniques such as filtration, evaporation, or decantation. This is because the atoms are either identical (in elements) or chemically bonded (in compounds).

Example: Water cannot be split into hydrogen and oxygen through boiling. It requires a chemical process like electrolysis.

Types

Pure substances can be classified into elements and compounds, both having a definite composition and specific chemical properties. However, they differ in their fundamental structure. [1-6]

1. Elements

An element is a pure substance made up of only one kind of atom. These atoms can exist as single atoms (e.g., helium, He) or as molecules formed from identical atoms (e.g., oxygen, O₂). Elements cannot be broken down into simpler substances by either chemical or physical methods.

All known elements are listed in the Periodic Table, each represented by a unique chemical symbol.

Examples of Common Elements

2. Compounds

A compound is a pure substance formed when two or more different elements combine chemically in a fixed ratio. The atoms in a compound are bonded together, and the resulting substance often has very different properties from the individual elements.

Compounds can be broken down into their elements by chemical reactions, but not by physical processes.

Examples of Common Compounds

Pure Substance vs. Mixtures

Here is how pure substances differ from mixtures: [1-6]

In conclusion, pure substances have a fixed composition and distinct properties, making them essential building blocks in both chemistry and everyday life.