What is meant by a 'Substance' in Science?

A 'Substance' is a pure form of matter that consists of only one single type of particles (atoms or molecules) and cannot be separated into other kinds of matter by any physical process.

For example, pure iron, gold, salt (NaCl), and distilled water are all 'substances'. You cannot use a physical filter or magnet to pull apart the hydrogen and oxygen from pure water.

Pure substances are further classified into two categories:

1. Elements: Made of only one type of atom. They cannot be broken down further by any chemical reaction. (Examples: Oxygen gas O₂, pure Gold Au, Iron Fe).
2. Compounds: Made of two or more different types of atoms that are chemically bonded together in a fixed ratio. (Example: Water H₂O is a compound. It is a pure substance because every single molecule in the glass is exactly H₂O).

• A Substance has a fixed, uniform chemical composition. (E.g., Pure Sugar).
• A Mixture is formed when two or more pure substances are physically mixed together, but NOT chemically bonded. (E.g., Sugar dissolved in water, or a handful of soil). You can easily separate a mixture using physical methods like evaporation.